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In-situ growth of carbon nanotubes on ZnO to enhance thermoelectric and mechanical properties

In-situ growth of carbon nanotubes on ZnO to enhance thermoelectric and mechanical properties Journal of Advanced Ceramics 2022, 11(12): 1932–1943 ISSN 2226-4108 https://doi.org/10.1007/s40145-022-0657-4 CN 10-1154/TQ Research Article In-situ growth of carbon nanotubes on ZnO to enhance thermoelectric and mechanical properties a a a b,c Shengjie FAN , Tingting SUN , Meng JIANG , Shijia GU , a,b,* d,* b,c Lianjun WANG , Haixue YAN , Wan JIANG State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai 201620, China Institute of Functional Materials, Donghua University, Shanghai 201620, China School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK Received: July 5, 2022; Revised: August 16, 2022; Accepted: August 31, 2022 © The Author(s) 2022. Abstract: As a high-temperature thermoelectric (TE) material, ZnO offers advantages of non-toxicity, chemical stability, and oxidation resistance, and shows considerable promise as a true ready-to-use module under air conditions. However, poor electrical conductivity and high thermal conductivity severely hinder its application. Carbon nanotubes (CNTs) are often used as a reinforcing phase in composites, but it is difficult to achieve uniform dispersion of CNTs due to van der Waals forces. Herein, we developed an effective in-situ growth strategy of homogeneous CNTs on ZnO nanoparticles by exploiting the chemical vapor deposition (CVD) technology, in order to improve their electrical conductivity and mechanical properties, as well as reducing the thermal conductivity. Meanwhile, magnetic nickel (Ni) nanoparticles are introduced as catalysts for promoting the formation of CNTs, which can also enhance the electrical and thermal transportation of ZnO matrices. Notably, the −1 electrical conductivity of ZnO is significantly boosted from 26 to 79 S·cm due to the formation of dense and uniform conductive CNT networks. The lattice thermal conductivity (κ ) is obviously declined by the intensification of phonon scattering, resulting from the abundant grain boundaries and interfaces in ZnO–CNT composites. Importantly, the maximum dimensionless figure of merit (zT) of 0.04 at 800 K is obtained in 2.0% Ni–CNTs/ZnO, which is three times larger than that of CNTs/ZnO prepared by traditional ultrasonic method. In addition, the mechanical properties of composites including Vickers hardness (HV) and fracture toughness () K are also reinforced. This work IC provides a valuable reference for dispersing nano-phases in TE materials to enhance both TE and mechanical properties. Keywords: ZnO; carbon nanotubes (CNTs); in-situ grown; thermoelectric (TE); mechanical properties * Corresponding authors. E-mail: L. Wang, wanglj@dhu.edu.cn; H. Yan, h.x.yan@qmul.ac.uk www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1933 ZnO-based materials [19]. References [20–23] show 1 Introduction that the zT value of ZnO can be also significantly improved by doping Mn, Sn, Bi, and nickel (Ni). Thermoelectric (TE) materials can directly convert heat Incorporation of a nanosized second phase to the TE to electricity, holding an attractive prospect for alleviating matrix is another promising approach for improving the global energy crisis. The conversion efficiency of the zT of TE materials by increasing the phonon TE materials is evaluated by the dimensionless figure scattering. In particular, nanosized second phases can of merit (zT), which can be expressed by zT  S T/ , be introduced into the matrix by special methods, such where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the as hot deformation, melting spinning, and hydrothermal synthesis [24–26]. Another popular method for forming absolute temperature [1]. In the past decades, great advances have been made in the classical TE materials, a nanosized second phase within the TE matrix is the direct addition of nanomaterials, which has proven to such as Bi Te -based, PbTe-based, and GeTe-based 2 3 materials [2–4]. However, these TE materials exhibit be a reliable and controllable procedure. In the family poor durability when fabricated into devices and usually of nanomaterials, two-dimensional materials, such as contain toxic or rare elements on earth. In contrast, graphene and MXene, have ultra-high electrical oxide-based compounds have outstanding chemical and conductivity and are regarded as the ideal reinforcements thermal stability at elevated temperatures and an oxidizing [27,28]. For example, Chen et al. [29] reported that the atmosphere, and are abundant in raw materials, which  was remarkably reduced by the phonon boundary endow them greater practical application potential [5–7]. scattering at the interface between the ZnO matrix and To date, single-crystalline Na CoO and BiCuSeO rGO. Guo et al. [30] prepared the MXene/ZnO composite x 2−δ have achieved competitive zT of over 1.0 through material by using a cold firing process. The electrical strategies, such as doping, alloying, and nano-structuring conductivity was improved by 1–2 orders of magnitude [8–11]. However, it is worth noting that the development due to the addition of MXene. Apart from this, both the of n-type oxide TE materials lags far behind those of hardness and elastic modulus were increased by the p-type counterparts [12]. On the other hand, most 40%–50%. high-performance oxide TE materials have complex Carbon nanotubes (CNTs) have attracted much compositions or require harsh preparation conditions. attention serving as a one-dimensional nanosized second For example, SiTiO -based compounds need to be phase since their discovery due to their rather low annealed under a reduction atmosphere and high density, excellent electrical conductivity, and outstanding temperatures for a long time, which restricts their mechanical properties. They have been thus widely reproducibility and large-scale application [13]. used as a reinforcement in many fields [31–33]. For ZnO is a promising n-type candidate in high- example, Dreßler et al. [34] mixed AZO powders and temperature TE field due to its advantages of simple multi-walled CNTs by ultrasonic dispersion and preparation process, low cost, high thermal stability, mechanical milling, and then sintered the mixture by and non-toxicity [14]. Unfortunately, the zT value of the spark plasma sintering (SPS). The results indicated ZnO is currently much inferior to those of other oxide that the thermal conductivity decreased from 31.2 to −1 −1 materials due to its low electrical conductivity and 26 W·m ·K after the incorporation of multi-walled high thermal conductivity, which severely limits its CNTs. At the same time, the electrical conductivity applications [15,16]. Doping is considered as an effective was significantly increased. The similar results were route to boost the TE performance by adjusting the also observed in Ref. [35], in which single-walled electrical transport properties [17]. Ohtaki et al. [18] CNTs were used as a reinforcement. CNTs have shown prepared Zn Al O (AZO) samples by solid-state considerable promise for enhanced TE performance. 0.98 0.02 reaction followed by pressed into a pellet, and then However, it is extremely difficult to achieve a uniform sintered at 1400 ℃ in air. The results showed that a dispersion of CNTs in the TE matrix owing to the −4 −1 −2 power factor of (10–15)×10 W·m ·K was obtained, strong van der Waals force, which has been a and the zT value reached 0.24 at 1273 K. After that, Ga bottleneck, limiting their broad development in TEs. In was added to AZO as a second doping element, and the addition, traditional methods, such as long-time mechanical zT value increased further to 0.65 at 1247 K, which ball milling, are easy to destroy the perfect structure of produced the highest zT value ever reported for CNTs, as well as severely deteriorate the TE performance www.springer.com/journal/40145 1934 J Adv Ceram 2022, 11(12): 1932–1943 2. 2 Synthesis of catalyst precursors of the materials, thus severely degrading their strengthening effect [36–38]. The catalyst precursors were synthesized as follows: In this work, we design a new approach to synthesize First, appropriate amounts of Ni(NO ) ·6H O and ZnO 3 2 2 CNTs/ZnO TE composites, where CNTs are in-situ powders were dissolved in 50 mL deionized water by grown on the pristine ZnO powders by using a chemical vigorous stirring to yield a mass ratio of Ni:ZnO = vapor deposition (CVD) method catalyzed by Ni. The 1.5:98.5. At the same time, appropriate amount of SPS is employed to prepare bulk samples. The NaOH was dissolved in 50 mL deionized water morphologies and distributions of CNTs were regulated 2+ − according to the molar ratio of Ni /OH = 1:2 and by controlling the catalyst contents, and the effect of quickly poured into the previous mixture, as shown in CNTs on the properties of ZnO is fully investigated. Fig. 1. Subsequently, the precipitate was collected by Benefitting from the uniform distribution of CNTs, the vacuum filtration, and then vacuum dried at 70 ℃ for electrical conductivity of ZnO is greatly improved, 24 h. To adjust the content of CNTs, a series of while the thermal conductivity is decreased, contributing precursors with different mass ratios of Ni:ZnO = a high zT value of 0.04 at 800 K. An outstanding 1.75:98.25 and 2:98 were prepared. mechanical property is also obtained owing to the intact structure of CNTs. 2. 3 In-situ growth of CNTs The growth of CNTs was carried out under atmospheric 2 Materials and methods pressure in a CVD system (CVD-12III-3Z, Tianjin Zhonghuan Furnace Co., China). After loading 3 g 2. 1 Materials precursor powders into a corundum crucible and then into a quartz tube, the tube furnace was firstly heated The ZnO matrix was prepared by using commercial −1 to 450 ℃ at 10 ℃·min and held in argon (Ar; powders (> 99.99%, Shanghai Aladdin Biochemical −1 150 mL·min , > 99.99%, Shanghai Haoqi Co., Ltd., Technology Co., Ltd., China) without any other China) for 2 h to covert nickel hydroxide to nickel purification. Sodium hydroxide (NaOH, AR, Sinopharm oxide. Ar was then switched off, and hydrogen (H ; Chemical Reagent Co., Ltd., China) and nickel nitrate 2 −1 150 mL·min , > 99.99%, Shanghai Haoqi Co., Ltd., hexahydrate (Ni(NO ) ·6H O, AR, Sinopharm Chemical 3 2 2 China) was introduced to reduce nickel oxide to Ni at Reagent Co., Ltd., China) were used to synthesize 450 ℃ for 1 h. Subsequently, the furnace was heated the catalyst precursors. Commercial multi-walled CNTs −1 up to 550 ℃ at 10 ℃·min with the flowing of H to (95 wt%, Timesnano, China) were purchased for grow CNTs. Once the growth temperature was reached, comparison. Fig. 1 Schematic illustration of the synthesis process. www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1935 a mixture of Ar/CH (> 99.99%, Shanghai Haoqi Co., concentrations and mobility were measured by the Hall Ltd., China) was introduced to replace H at a flow rate measurement system (Lakeshore 8400 Series, HMS, −1 of 150/50 mL·min , and the reaction was maintained at USA) according to the van der Pauw method, and the the growth temperature for 3 h. Finally, CH was test temperature of Hall measurement is from 300 to switched off, and the furnace was cooled down to room 800 K. The measurement errors for Seebeck coefficient, temperature in an Ar atmosphere. For simplicity, the , and Hall measurement electrical conductivity, D, c composite powders synthesized with different contents were about 7%, 3%, 5%, 5%, and 7%, respectively. of catalysts (1.5:98.5, 1.75:98.25, and 2:98) were The Poisson’s ratio (γ) and Young modulus (E) were denoted as 1.5% Ni–CNTs/ZnO, 1.75% Ni–CNTs/ZnO, measured by the advanced ultrasonic material and 2.0% Ni–CNTs/ZnO, respectively. For comparison, characterization system (UMS-100, TECLAB, France). commercially available multi-walled CNTs were dispersed The Vickers hardness (HV) was determined by the micro-hardness testing machine (HV-1000, Shenzhen into ZnO with a mass ratio of 2.0% by ultrasonic dispersion, which was marked as 2.0% CNTs/ZnO. Lunjie Technology Co., Ltd., China) with a load (P) of 4.9 N for 10 s. The fracture toughness was To fabricate bulk samples, 1 g of the as-prepared () K IC powders was loaded into a graphite die with a diameter calculated according to the Anstis equation, as presented by Eq. (1): of 12 mm and sintered by the SPS instrument (Dr Sinter:2000, Japan) at 1100 K for 5 min under an axis 0.5  1.5 pressure of 80 MPa and an Ar atmosphere. KP  0.016 C (1) IC  HV  2. 4 Characterization where C is the half crack length. The modified small punch (MSP) method was adopted to calculate the flexural The crystal structures of all samples were characterized strength ( ) of CNTs/ZnO composite by Eq. (2) [39]: by the X-ray diffractometer (DX-2700, Dandong 3P Haoyuan, Instrument Co., Ltd., China) with a Cu Kα (λ =   (2)     1 ba 1.5418 Å) radiation source. The range of 2θ was from 2πt1( 1)ln       5° to 90°. The field emission scanning electron 4 b       microscope (FE-SEM; MAIA3, TESCAN, Czech where t is the thickness of the sample, and a and b are Republic) equipped with an energy-dispersive X-ray the radius of the hole of the mold and the radius of the detector (X-MAX 65T, Oxford, UK) was employed to tip of cylindrical punch, respectively. characterize the distributions of CNTs and morphologies of the composites. The transmission electron microscope (TEM; JEM-2100F, JEOL, Japan) was performed to 3 Results and discussion study the growth mechanism of CNTs and the interfaces between different phases. In addition, the contents of Figure 2(a) shows the X-ray diffraction (XRD) patterns in-situ grown CNTs were studied by the thermogravimetric of different ZnO composite powders. These samples analyzer (STA 449 F5, Netzsch, Germany). include three categories: (1) CNTs/ZnO composite powers The Seebeck coefficient and electrical conductivity synthesized by using commercial CNTs combined with were measured synchronously by using power conversion ultrasonic dispersion (named as 2.0% CNTs/ZnO); (2) efficiency measuring instrument (ZEM-3, Ulvac-Riko, ZnO powders with 2.0 wt% Ni catalysts grown on the Japan) from 300 to 800 K under the protection of surface (named as 2.0% Ni/ZnO); and (3) ZnO powders helium (> 99.99%, Shanghai Haoqi Co., Ltd., China). with in-situ grown CNTs using different loading contents () The thermal conductivity was calculated according of Ni catalysts. It is shown that the main patterns of all to   Dc , where D is the thermal diffusivity tested samples can be indexed to the wurtzite structure of by the laser flash analyzer (LFA457, Netzsch, ZnO (PDF#36-1451) [40]. Sharp diffraction peaks of Germany) from 300 to 800 K, c is the specific heat ZnO can be obviously seen in all samples, indicating capacity tested by the differential scanning calorimeter good crystallinity. However, characteristic peaks of (DSC; 204F1, Netzsch, Germany) from 300 to 800 K, CNTs cannot be detected, neither in the sample with and ρ is the density tested according to the Archimedes the addition of commercial CNTs nor the samples with method. The temperature dependence of carrier in-situ grown CNTs, which is probably ascribed to the www.springer.com/journal/40145 1936 J Adv Ceram 2022, 11(12): 1932–1943 relatively small amount of CNTs [41]. Furthermore, an additional small diffraction peak located at around 43° can be detected in the samples with in-situ grown CNTs or merely Ni catalysts. This peak is corresponding to the reaction of Ni and ZnO. After sintering into bulk samples, as presented in Fig. 2(b), the diffraction peaks still match well with the standard pattern of ZnO. Compared with those of powder samples, an impurity diffraction peak corresponding to Ni–Zn–C phase is observed at around 50° for the samples with in-situ grown CNTs, which is due to the amorphous carbon [42]. The contents of the in-situ grown CNTs were studied by the thermogravimetric analysis (TGA). In Fig. 2(c), it can be observed that the TGA curves of the CNTs/ZnO hybrid powders fluctuate below 380 ℃, which is mainly due to the reaction of amorphous carbon and Ni with oxygen. Afterwards, there is an obviously weight loss varying from 1.25 to 1.85 wt% in 380–550 ℃, which is ascribed to the decomposition of in-situ grown CNTs under an oxidizing atmosphere. As expected, the amount of CNTs in the composites increases with the increasing catalyst, which is related to the number of Ni nanoparticles that serve as the growth sites of CNTs. Figures 3(a)–3(d) display the SEM images of pristine ZnO powders and the representative states of catalyst precursors. After deposition, calcination, and reduction, it can be clearly observed from Fig. 3(d) that the Ni nanoparticles marked by red circles are uniformly distributed on the surface of ZnO powders, which is crucial for achieving uniform distribution of in in-situ grown CNTs. Figures 3(e), 3(f), and 3(g) show the SEM images of ZnO hybrid powders with CNTs in-situ grown at different amounts of Ni catalysts (1.5, 1.75, and 2.0 wt%), respectively. CNTs can be obviously observed in all samples, demonstrating the feasibility of our Fig. 2 (a) XRD patterns of different ZnO composite proposed method. More importantly, the in-situ grown powders, (b) XRD patterns of bulk ZnO composite samples, and (c) TGA curves of CNTs/ZnO hybrid powders. CNTs are uniformly distributed in the ZnO matrix, which is mainly due to the uniform distribution of Ni and length of the obtained CNTs with the increasing catalyst particles, as presented in Fig. 3(d). This is a catalyst content. This is because they are mainly prerequisite to enhance the TE performance of the determined by the size of the catalyst, growth time, and composites utilizing CNTs [43]. Simultaneously, the proportion of gas used [44–46]. In contrast, the CNTs amount of in-situ grown CNTs increases as the Ni introduced by ultrasonic dispersion have poor concentration is increased from 1.5 to 2.0 wt%, which dispersion in the matrix, which can be clearly observed is consistent with the TGA results (Fig. 2(c)). This is in Fig. 3(h). To further explore the microstructures of due to the formation of more catalyst nanoparticles the as-grown CNTs, the sample named as 2.0% with the increasing catalyst precursors, which provide Ni–CNTs/ZnO is analyzed by the TEM, as shown in more nucleation and growth sites for CNTs [44]. Figs. 3(i)–3(k). It was found that almost every CNT However, there is no significant change in the diameter www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1937 Fig. 3 SEM images of precursors: (a) pure ZnO, (b) Ni(OH) /ZnO, (c) NiO/ZnO, and (d) Ni/ZnO. (e–g) SEM images of CNTs/ZnO composite powders with in-situ grown CNTs synthesized using different mass ratios of Ni catalysts: 1.5, 1.75, and 2.0 wt%; (h) SEM image of CNTs/ZnO composite powders mixed with commercial CNTs by ultrasonic dispersion; (i–k) TEM and HRTEM images of 2.0% Ni–CNTs/ZnO sample; (l) selected area electron diffraction pattern; and (m–p) EDS elemental mappings of 2.0% Ni–CNTs/ZnO sample. has a nanoparticle wrapped around its top, indicating 3(p)) of 2.0% Ni–CNTs/ZnO sample confirm that the that the in-situ grown CNTs on ZnO follows the Ni nanoparticles are uniformly distributed in the matrix, “tip-growth mode” [47]. The high-resolution TEM which ensures the uniformity of the as-grown CNTs. (HRTEM) images show that in-situ grown CNTs are To further investigate the distribution states of CNTs multi-walled CNTs, and the interlayer spacing of in the sintered samples, we characterized the graphite layers is about 0.36 nm. In addition, the microstructures of the CNTs/ZnO bulk composites. As graphitic layers are not parallel to the axis direction for the sample with commercial CNTs added by and are disordered, revealing that the in-situ grown ultrasonic dispersion, serious agglomeration of CNTs CNTs have many edges and defects (Fig. 3(j)) [48]. can be clearly seen in Fig. 4(a). In contrast, the in-situ Furthermore, the interlayer space of lattice fringes for grown CNTs are not easy to be observed in the the nanoparticles is ~0.21 nm, which corresponds to as-sintered samples, which may be due to the short the (111) plane of Ni, implying that metallic Ni acts as length of the CNTs [50]. Another obvious phenomenon a catalyst for the growth of CNTs [49]. The HRTEM is that the crystal grain sizes decrease with increasing image performed in Area 2 (Fig. 3(k)) also reveals that the content of the as-grown CNTs (Figs. 4(b)–4(d)). the in-situ grown CNTs are tightly anchored on the This is due to the pinning effect of CNTs that limits the ZnO matrix, ensuring good interface connectivity. The growth of grains during the sintering [51]. The increased selected area electron diffraction pattern (Fig. 3(l)) grain boundaries can enhance the scattering of phonon, identifies the wurtzite ZnO, which agrees with the leading to an improvement of the TE performance. XRD results. More importantly, the energy-dispersive However, it should be noted that this effect will be spectroscopy (EDS) elemental mappings (Figs. 3(m)– deeply weakened if the CNTs agglomerate in the www.springer.com/journal/40145 1938 J Adv Ceram 2022, 11(12): 1932–1943 Fig. 4 SEM images of fractured surfaces of ZnO bulk composites: (a) 2.0% CNTs/ZnO, (b) 1.5% Ni–CNTs/ZnO, (c) 1.75% Ni–CNTs/ZnO, (d) 2.0% Ni–CNTs/ZnO, (e) high-magnification image of 2.0% Ni–CNTs/ZnO, and (f) line scanning. matrix, which is the case in Fig. 4(a). In order to better mainly due to the agglomeration of CNTs, which observe the distributions of in-situ grown CNTs in the prevents the excellent conductivity of CNTs from as-sintered bulk, further characterization on the 2.0% being effectively utilized [34]. Figure 5(b) shows the Ni–CNTs/ZnO sample by the high-magnification SEM temperature dependence of Seebeck coefficient. The is presented in Fig. 4(e). The CNTs marked with red negative values of all samples indicate an n-type circles remain uniformly distributed in the bulk material electrical transport behavior. For the samples with and well connected with the matrix. In addition, the in-situ grown CNTs, the values of Seebeck coefficient CNTs are mainly located on the grain boundaries, with show little change over the entire measurement a small amount on the surface of the ZnO matrix. The temperature range. With increasing the amount of elemental line scanning plotted in Fig. 4(f) further in-situ grown CNTs, the absolute values of Seebeck proves that the marked area in Fig. 4(e) is the CNTs. coefficient increase firstly and then decrease. In general, Figure 5(a) shows the electrical conductivity as a the Seebeck coefficient is inversely proportional to the function of temperature for all bulk samples. It can be carrier concentration (n) according to Eq. (3) [54]: noticed that the electrical conductivity decreases with 2/3 8π k  π increasing the temperature, indicative of the metallic Sm  T (3)  3n 3eh  conducting behavior [52]. Obviously, the electrical where k is the Boltzmann constant, h is the Planck’s conductivity of the samples with in-situ grown CNTs is significantly enhanced compared with that of the constant, and m is the effective mass of carrier. The main contribution of the Seebeck coefficient for sample without CNTs. Furthermore, by controlling the amount of catalyst (from 1.5 to 2.0 wt%), the electrical ZnO-based composites lines to carrier concentration. However, the 1.75% Ni–CNTs/ZnO sample presents conductivity increases with the increasing content of CNTs. On the one hand, the enhanced electrical the highest Seebeck value and keeps a high carrier conductivity results from the inherent high concentration. This may be ascribed to the change of conductivity of CNTs [53]. On the other hand, a good m when the CNTs are introduced into ZnO, which conductive network is established in the matrix owing should be further investigated. To better understand the to the homogeneous dispersion of in-situ grown CNTs, impact of in-situ grown CNTs on the electrical transport which facilitates the transport of electrons. In contrast, of ZnO composites, the temperature dependence of for the sample with commercial CNTs by ultrasonic carrier concentration and carrier mobility (μ ) for all technology, the electrical conductivity is much lower, samples are measured and analyzed. www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1939 As shown in Fig. 5(c), the carrier concentration is is plotted in Fig. 6(a). The values of total thermal found to increase as the temperature rises from room conductivity decrease with the increasing temperature. temperature to 800 K, which can be ascribed to the It is clearly observed that the introduction of in-situ intrinsic excitation [55]. Moreover, there is an apparent grown CNTs has a significant effect on reducing the increase in carrier concentration as the content of total thermal conductivity at low temperatures. For −1 −1 in-situ grown CNTs in the ZnO matrix increases. A example, it is suppressed from 25 to 19 W·m ·K at 19 −3 maximum carrier concentration of 1.66×10 cm at 300 K. However, this effect is diminished if the CNTs 800 K is obtained for the sample catalyzed by 2.0 wt% are agglomerated in the matrix, as shown in the sample Ni, exhibiting an increase of ~168% compared with with the direct addition of commercial CNTs. This that of the sample catalyzed by 1.5 wt% Ni (0.62× demonstrates the advantage of introducing CNTs 19 −3 10 cm ). It is noteworthy that the sample with through in-situ growth technology. Both phonons and commercial CNTs also has a relatively high carrier electrons can act as carriers of heat, and their concentration. In contrast, μ (Fig. 5(d)) is negatively H contributions to the total thermal conductivity are referred correlated with temperature, which is caused by the to as  and the electronic thermal conductivity ( ), L e scattering of charge carriers. In addition, all samples respectively. Furthermore,  is calculated according with in-situ grown CNTs possess higher μ compared to the Widemann–Franz law (Eq. (4)): with the sample with mechanically mixed CNTs, which    LT (4) is due to the high μ of CNTs and the formation of where L is the Lorenz number.  is obtained by conducting channel between the ZnO and CNTs. Moreover, the tight bonding of ZnO and in-situ CNTs subtracting  from the total thermal conductivity. As shown in Fig. 6(b), decreases with increasing the may reduce the interface potential barrier for electron  temperature, showing the same changing trend as that transport, demonstrating the benefits of uniform dispersion of CNTs [56,57]. of the total thermal conductivity. As expected, the The temperature-dependent total thermal conductivity sample with 2.0% Ni–CNTs/ZnO has the lowest lattice Fig. 5 Temperature dependence of (a) σ, (b) S, (c) n, and (d) μ for ZnO-based composites. www.springer.com/journal/40145 1940 J Adv Ceram 2022, 11(12): 1932–1943 thermal conductivity over the entire temperature range. repetitions, indicating good reproducibility of this This can be attributed to three factors: Firstly, the work. Moreover, the zT values of the ZnO composites addition of CNTs limits the growth of grains during the prepared by our method are comparable to those sintering process, as evidenced by the SEM images in reported in Refs. [59–61] by doping metal elements, Fig. 4, thereby providing more grain boundaries to such as Al and Ni, or introducing second phase, as scatter phonons; secondly, the rich interface formed compared in Fig. 6(d). between the CNTs and the ZnO matrix enhances the Excellent mechanical properties are the basis for scattering of phonons; and thirdly, the magnetic Ni realizing practical applications from materials to nanoparticles formed during the in-situ growth of devices. Although many materials currently possess CNTs can also act as phonon scattering centers [58]. outstanding TE properties, they cannot be used in Figure 6(c) shows the temperature dependence of  . modules due to the poor mechanical properties [62]. As When the amount of catalyst is improved from 1.5 to shown in Fig. 7(a), HV is improved when the CNTs 2.0 wt%,  rises owing to the enhancement of electrical introduced through in-situ grown technology. A maximum conductivity. However, this increase can be compensated value of 4.12 GPa is achieved for the sample with by the decrease of  . 2.0% Ni–CNTs/ZnO, which is 11.6% higher than that As a consequence, the zT values of samples with of the sample denoted as 2.0% Ni/ZnO and 38.7% CNTs obtained by in-situ technology are enhanced, higher than that of the sample with commercial CNTs. benefitting from the increase in electrical conductivity This improvement may be ascribed to the change in and the decrease in thermal conductivity, as shown in grain size [63]. The MSP method is adopted to measure Fig. 6(d). The highest zT of 0.04 at 800 K was  , which is similar to the 4-point bending method [39]. achieved in 2.0% Ni–CNTs/ZnO sample, which is a Compared with that of the 2.0% Ni/ZnO sample, the for 2.0% Ni–CNTs/ZnO is increased by 18.5%, two-fold increase compared to that of the 2.0% Ni/ZnO  sample and a three-fold increase compared to that of from 111.06 to 131.66 MPa, as shown in Fig. 7(b). At commercial CNTs. Besides, comparable TE performance the same time, the K for 2.0% Ni–CNTs/ZnO can IC 1/2 reach 1.55 MPa·m , which is 55% higher than that of of 2.0% Ni/ZnO sample was achieved after three Fig. 6 Temperature dependence of (a) total thermal conductivity ( ), (b)  , (c)  , and (d) zT values for ZnO-based total L e composites. www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1941 Fig. 7 Mechanical properties of ZnO-based composites: (a) HV and (b)  and K . IC the 2.0% CNTs/ZnO sample with mechanically mixed Declaration of competing interest commercial CNTs, suggesting that the evenly distributed CNTs in the matrix is vital to improve the K [64]. The authors have no competing interests to declare that IC Moreover, it is worth noting that the sample denoted as are relevant to the content of this article. 2.0% Ni/ZnO also possesses improved mechanical properties, which demonstrated that the remaining Ni References nanoparticles also can reinforce mechanical behaviors [65]. [1] Caballero-Calero O, Ares JR, Martin-Gonzalez M. Environmentally friendly thermoelectric materials: High 4 Conclusions performance from inorganic components with low toxicity and abundance in the earth. Adv Sustainable Syst 2021, 5: In summary, homogeneously dispersed CNTs are [2] Jia BH, Huang Y, Wang Y, et al. Realizing high successfully prepared in the ZnO matrix by in-situ thermoelectric performance in non-nanostructured n-type growth technology. The incorporation of uniformly PbTe. Energy Environ Sci 2022, 15: 1920–1929. dispersed CNTs into ZnO can simultaneously optimize [3] Jin Y, Wang DY, Qiu YT, et al. 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Commons Attribution 4.0 International License, which permits Thermoelectric properties of ZnO ceramics densified use, sharing, adaptation, distribution and reproduction in any through spark plasma sintering. Ceram Int 2020, 46: medium or format, as long as you give appropriate credit to the 5229–5238. original author(s) and the source, provide a link to the Creative [53] Zhang YC, Zhang QC, Chen GM. Carbon and carbon Commons licence, and indicate if changes were made. composites for thermoelectric applications. Carbon Energy The images or other third party material in this article are 2020, 2: 408–436. included in the article’s Creative Commons licence, unless [54] Nong J, Peng Y, Liu CY, et al. Ultra-low thermal indicated otherwise in a credit line to the material. 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In-situ growth of carbon nanotubes on ZnO to enhance thermoelectric and mechanical properties

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Springer Journals
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Copyright © The Author(s) 2022
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2226-4108
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10.1007/s40145-022-0657-4
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Abstract

Journal of Advanced Ceramics 2022, 11(12): 1932–1943 ISSN 2226-4108 https://doi.org/10.1007/s40145-022-0657-4 CN 10-1154/TQ Research Article In-situ growth of carbon nanotubes on ZnO to enhance thermoelectric and mechanical properties a a a b,c Shengjie FAN , Tingting SUN , Meng JIANG , Shijia GU , a,b,* d,* b,c Lianjun WANG , Haixue YAN , Wan JIANG State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China Engineering Research Center of Advanced Glasses Manufacturing Technology, Ministry of Education, Donghua University, Shanghai 201620, China Institute of Functional Materials, Donghua University, Shanghai 201620, China School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK Received: July 5, 2022; Revised: August 16, 2022; Accepted: August 31, 2022 © The Author(s) 2022. Abstract: As a high-temperature thermoelectric (TE) material, ZnO offers advantages of non-toxicity, chemical stability, and oxidation resistance, and shows considerable promise as a true ready-to-use module under air conditions. However, poor electrical conductivity and high thermal conductivity severely hinder its application. Carbon nanotubes (CNTs) are often used as a reinforcing phase in composites, but it is difficult to achieve uniform dispersion of CNTs due to van der Waals forces. Herein, we developed an effective in-situ growth strategy of homogeneous CNTs on ZnO nanoparticles by exploiting the chemical vapor deposition (CVD) technology, in order to improve their electrical conductivity and mechanical properties, as well as reducing the thermal conductivity. Meanwhile, magnetic nickel (Ni) nanoparticles are introduced as catalysts for promoting the formation of CNTs, which can also enhance the electrical and thermal transportation of ZnO matrices. Notably, the −1 electrical conductivity of ZnO is significantly boosted from 26 to 79 S·cm due to the formation of dense and uniform conductive CNT networks. The lattice thermal conductivity (κ ) is obviously declined by the intensification of phonon scattering, resulting from the abundant grain boundaries and interfaces in ZnO–CNT composites. Importantly, the maximum dimensionless figure of merit (zT) of 0.04 at 800 K is obtained in 2.0% Ni–CNTs/ZnO, which is three times larger than that of CNTs/ZnO prepared by traditional ultrasonic method. In addition, the mechanical properties of composites including Vickers hardness (HV) and fracture toughness () K are also reinforced. This work IC provides a valuable reference for dispersing nano-phases in TE materials to enhance both TE and mechanical properties. Keywords: ZnO; carbon nanotubes (CNTs); in-situ grown; thermoelectric (TE); mechanical properties * Corresponding authors. E-mail: L. Wang, wanglj@dhu.edu.cn; H. Yan, h.x.yan@qmul.ac.uk www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1933 ZnO-based materials [19]. References [20–23] show 1 Introduction that the zT value of ZnO can be also significantly improved by doping Mn, Sn, Bi, and nickel (Ni). Thermoelectric (TE) materials can directly convert heat Incorporation of a nanosized second phase to the TE to electricity, holding an attractive prospect for alleviating matrix is another promising approach for improving the global energy crisis. The conversion efficiency of the zT of TE materials by increasing the phonon TE materials is evaluated by the dimensionless figure scattering. In particular, nanosized second phases can of merit (zT), which can be expressed by zT  S T/ , be introduced into the matrix by special methods, such where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the as hot deformation, melting spinning, and hydrothermal synthesis [24–26]. Another popular method for forming absolute temperature [1]. In the past decades, great advances have been made in the classical TE materials, a nanosized second phase within the TE matrix is the direct addition of nanomaterials, which has proven to such as Bi Te -based, PbTe-based, and GeTe-based 2 3 materials [2–4]. However, these TE materials exhibit be a reliable and controllable procedure. In the family poor durability when fabricated into devices and usually of nanomaterials, two-dimensional materials, such as contain toxic or rare elements on earth. In contrast, graphene and MXene, have ultra-high electrical oxide-based compounds have outstanding chemical and conductivity and are regarded as the ideal reinforcements thermal stability at elevated temperatures and an oxidizing [27,28]. For example, Chen et al. [29] reported that the atmosphere, and are abundant in raw materials, which  was remarkably reduced by the phonon boundary endow them greater practical application potential [5–7]. scattering at the interface between the ZnO matrix and To date, single-crystalline Na CoO and BiCuSeO rGO. Guo et al. [30] prepared the MXene/ZnO composite x 2−δ have achieved competitive zT of over 1.0 through material by using a cold firing process. The electrical strategies, such as doping, alloying, and nano-structuring conductivity was improved by 1–2 orders of magnitude [8–11]. However, it is worth noting that the development due to the addition of MXene. Apart from this, both the of n-type oxide TE materials lags far behind those of hardness and elastic modulus were increased by the p-type counterparts [12]. On the other hand, most 40%–50%. high-performance oxide TE materials have complex Carbon nanotubes (CNTs) have attracted much compositions or require harsh preparation conditions. attention serving as a one-dimensional nanosized second For example, SiTiO -based compounds need to be phase since their discovery due to their rather low annealed under a reduction atmosphere and high density, excellent electrical conductivity, and outstanding temperatures for a long time, which restricts their mechanical properties. They have been thus widely reproducibility and large-scale application [13]. used as a reinforcement in many fields [31–33]. For ZnO is a promising n-type candidate in high- example, Dreßler et al. [34] mixed AZO powders and temperature TE field due to its advantages of simple multi-walled CNTs by ultrasonic dispersion and preparation process, low cost, high thermal stability, mechanical milling, and then sintered the mixture by and non-toxicity [14]. Unfortunately, the zT value of the spark plasma sintering (SPS). The results indicated ZnO is currently much inferior to those of other oxide that the thermal conductivity decreased from 31.2 to −1 −1 materials due to its low electrical conductivity and 26 W·m ·K after the incorporation of multi-walled high thermal conductivity, which severely limits its CNTs. At the same time, the electrical conductivity applications [15,16]. Doping is considered as an effective was significantly increased. The similar results were route to boost the TE performance by adjusting the also observed in Ref. [35], in which single-walled electrical transport properties [17]. Ohtaki et al. [18] CNTs were used as a reinforcement. CNTs have shown prepared Zn Al O (AZO) samples by solid-state considerable promise for enhanced TE performance. 0.98 0.02 reaction followed by pressed into a pellet, and then However, it is extremely difficult to achieve a uniform sintered at 1400 ℃ in air. The results showed that a dispersion of CNTs in the TE matrix owing to the −4 −1 −2 power factor of (10–15)×10 W·m ·K was obtained, strong van der Waals force, which has been a and the zT value reached 0.24 at 1273 K. After that, Ga bottleneck, limiting their broad development in TEs. In was added to AZO as a second doping element, and the addition, traditional methods, such as long-time mechanical zT value increased further to 0.65 at 1247 K, which ball milling, are easy to destroy the perfect structure of produced the highest zT value ever reported for CNTs, as well as severely deteriorate the TE performance www.springer.com/journal/40145 1934 J Adv Ceram 2022, 11(12): 1932–1943 2. 2 Synthesis of catalyst precursors of the materials, thus severely degrading their strengthening effect [36–38]. The catalyst precursors were synthesized as follows: In this work, we design a new approach to synthesize First, appropriate amounts of Ni(NO ) ·6H O and ZnO 3 2 2 CNTs/ZnO TE composites, where CNTs are in-situ powders were dissolved in 50 mL deionized water by grown on the pristine ZnO powders by using a chemical vigorous stirring to yield a mass ratio of Ni:ZnO = vapor deposition (CVD) method catalyzed by Ni. The 1.5:98.5. At the same time, appropriate amount of SPS is employed to prepare bulk samples. The NaOH was dissolved in 50 mL deionized water morphologies and distributions of CNTs were regulated 2+ − according to the molar ratio of Ni /OH = 1:2 and by controlling the catalyst contents, and the effect of quickly poured into the previous mixture, as shown in CNTs on the properties of ZnO is fully investigated. Fig. 1. Subsequently, the precipitate was collected by Benefitting from the uniform distribution of CNTs, the vacuum filtration, and then vacuum dried at 70 ℃ for electrical conductivity of ZnO is greatly improved, 24 h. To adjust the content of CNTs, a series of while the thermal conductivity is decreased, contributing precursors with different mass ratios of Ni:ZnO = a high zT value of 0.04 at 800 K. An outstanding 1.75:98.25 and 2:98 were prepared. mechanical property is also obtained owing to the intact structure of CNTs. 2. 3 In-situ growth of CNTs The growth of CNTs was carried out under atmospheric 2 Materials and methods pressure in a CVD system (CVD-12III-3Z, Tianjin Zhonghuan Furnace Co., China). After loading 3 g 2. 1 Materials precursor powders into a corundum crucible and then into a quartz tube, the tube furnace was firstly heated The ZnO matrix was prepared by using commercial −1 to 450 ℃ at 10 ℃·min and held in argon (Ar; powders (> 99.99%, Shanghai Aladdin Biochemical −1 150 mL·min , > 99.99%, Shanghai Haoqi Co., Ltd., Technology Co., Ltd., China) without any other China) for 2 h to covert nickel hydroxide to nickel purification. Sodium hydroxide (NaOH, AR, Sinopharm oxide. Ar was then switched off, and hydrogen (H ; Chemical Reagent Co., Ltd., China) and nickel nitrate 2 −1 150 mL·min , > 99.99%, Shanghai Haoqi Co., Ltd., hexahydrate (Ni(NO ) ·6H O, AR, Sinopharm Chemical 3 2 2 China) was introduced to reduce nickel oxide to Ni at Reagent Co., Ltd., China) were used to synthesize 450 ℃ for 1 h. Subsequently, the furnace was heated the catalyst precursors. Commercial multi-walled CNTs −1 up to 550 ℃ at 10 ℃·min with the flowing of H to (95 wt%, Timesnano, China) were purchased for grow CNTs. Once the growth temperature was reached, comparison. Fig. 1 Schematic illustration of the synthesis process. www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1935 a mixture of Ar/CH (> 99.99%, Shanghai Haoqi Co., concentrations and mobility were measured by the Hall Ltd., China) was introduced to replace H at a flow rate measurement system (Lakeshore 8400 Series, HMS, −1 of 150/50 mL·min , and the reaction was maintained at USA) according to the van der Pauw method, and the the growth temperature for 3 h. Finally, CH was test temperature of Hall measurement is from 300 to switched off, and the furnace was cooled down to room 800 K. The measurement errors for Seebeck coefficient, temperature in an Ar atmosphere. For simplicity, the , and Hall measurement electrical conductivity, D, c composite powders synthesized with different contents were about 7%, 3%, 5%, 5%, and 7%, respectively. of catalysts (1.5:98.5, 1.75:98.25, and 2:98) were The Poisson’s ratio (γ) and Young modulus (E) were denoted as 1.5% Ni–CNTs/ZnO, 1.75% Ni–CNTs/ZnO, measured by the advanced ultrasonic material and 2.0% Ni–CNTs/ZnO, respectively. For comparison, characterization system (UMS-100, TECLAB, France). commercially available multi-walled CNTs were dispersed The Vickers hardness (HV) was determined by the micro-hardness testing machine (HV-1000, Shenzhen into ZnO with a mass ratio of 2.0% by ultrasonic dispersion, which was marked as 2.0% CNTs/ZnO. Lunjie Technology Co., Ltd., China) with a load (P) of 4.9 N for 10 s. The fracture toughness was To fabricate bulk samples, 1 g of the as-prepared () K IC powders was loaded into a graphite die with a diameter calculated according to the Anstis equation, as presented by Eq. (1): of 12 mm and sintered by the SPS instrument (Dr Sinter:2000, Japan) at 1100 K for 5 min under an axis 0.5  1.5 pressure of 80 MPa and an Ar atmosphere. KP  0.016 C (1) IC  HV  2. 4 Characterization where C is the half crack length. The modified small punch (MSP) method was adopted to calculate the flexural The crystal structures of all samples were characterized strength ( ) of CNTs/ZnO composite by Eq. (2) [39]: by the X-ray diffractometer (DX-2700, Dandong 3P Haoyuan, Instrument Co., Ltd., China) with a Cu Kα (λ =   (2)     1 ba 1.5418 Å) radiation source. The range of 2θ was from 2πt1( 1)ln       5° to 90°. The field emission scanning electron 4 b       microscope (FE-SEM; MAIA3, TESCAN, Czech where t is the thickness of the sample, and a and b are Republic) equipped with an energy-dispersive X-ray the radius of the hole of the mold and the radius of the detector (X-MAX 65T, Oxford, UK) was employed to tip of cylindrical punch, respectively. characterize the distributions of CNTs and morphologies of the composites. The transmission electron microscope (TEM; JEM-2100F, JEOL, Japan) was performed to 3 Results and discussion study the growth mechanism of CNTs and the interfaces between different phases. In addition, the contents of Figure 2(a) shows the X-ray diffraction (XRD) patterns in-situ grown CNTs were studied by the thermogravimetric of different ZnO composite powders. These samples analyzer (STA 449 F5, Netzsch, Germany). include three categories: (1) CNTs/ZnO composite powers The Seebeck coefficient and electrical conductivity synthesized by using commercial CNTs combined with were measured synchronously by using power conversion ultrasonic dispersion (named as 2.0% CNTs/ZnO); (2) efficiency measuring instrument (ZEM-3, Ulvac-Riko, ZnO powders with 2.0 wt% Ni catalysts grown on the Japan) from 300 to 800 K under the protection of surface (named as 2.0% Ni/ZnO); and (3) ZnO powders helium (> 99.99%, Shanghai Haoqi Co., Ltd., China). with in-situ grown CNTs using different loading contents () The thermal conductivity was calculated according of Ni catalysts. It is shown that the main patterns of all to   Dc , where D is the thermal diffusivity tested samples can be indexed to the wurtzite structure of by the laser flash analyzer (LFA457, Netzsch, ZnO (PDF#36-1451) [40]. Sharp diffraction peaks of Germany) from 300 to 800 K, c is the specific heat ZnO can be obviously seen in all samples, indicating capacity tested by the differential scanning calorimeter good crystallinity. However, characteristic peaks of (DSC; 204F1, Netzsch, Germany) from 300 to 800 K, CNTs cannot be detected, neither in the sample with and ρ is the density tested according to the Archimedes the addition of commercial CNTs nor the samples with method. The temperature dependence of carrier in-situ grown CNTs, which is probably ascribed to the www.springer.com/journal/40145 1936 J Adv Ceram 2022, 11(12): 1932–1943 relatively small amount of CNTs [41]. Furthermore, an additional small diffraction peak located at around 43° can be detected in the samples with in-situ grown CNTs or merely Ni catalysts. This peak is corresponding to the reaction of Ni and ZnO. After sintering into bulk samples, as presented in Fig. 2(b), the diffraction peaks still match well with the standard pattern of ZnO. Compared with those of powder samples, an impurity diffraction peak corresponding to Ni–Zn–C phase is observed at around 50° for the samples with in-situ grown CNTs, which is due to the amorphous carbon [42]. The contents of the in-situ grown CNTs were studied by the thermogravimetric analysis (TGA). In Fig. 2(c), it can be observed that the TGA curves of the CNTs/ZnO hybrid powders fluctuate below 380 ℃, which is mainly due to the reaction of amorphous carbon and Ni with oxygen. Afterwards, there is an obviously weight loss varying from 1.25 to 1.85 wt% in 380–550 ℃, which is ascribed to the decomposition of in-situ grown CNTs under an oxidizing atmosphere. As expected, the amount of CNTs in the composites increases with the increasing catalyst, which is related to the number of Ni nanoparticles that serve as the growth sites of CNTs. Figures 3(a)–3(d) display the SEM images of pristine ZnO powders and the representative states of catalyst precursors. After deposition, calcination, and reduction, it can be clearly observed from Fig. 3(d) that the Ni nanoparticles marked by red circles are uniformly distributed on the surface of ZnO powders, which is crucial for achieving uniform distribution of in in-situ grown CNTs. Figures 3(e), 3(f), and 3(g) show the SEM images of ZnO hybrid powders with CNTs in-situ grown at different amounts of Ni catalysts (1.5, 1.75, and 2.0 wt%), respectively. CNTs can be obviously observed in all samples, demonstrating the feasibility of our Fig. 2 (a) XRD patterns of different ZnO composite proposed method. More importantly, the in-situ grown powders, (b) XRD patterns of bulk ZnO composite samples, and (c) TGA curves of CNTs/ZnO hybrid powders. CNTs are uniformly distributed in the ZnO matrix, which is mainly due to the uniform distribution of Ni and length of the obtained CNTs with the increasing catalyst particles, as presented in Fig. 3(d). This is a catalyst content. This is because they are mainly prerequisite to enhance the TE performance of the determined by the size of the catalyst, growth time, and composites utilizing CNTs [43]. Simultaneously, the proportion of gas used [44–46]. In contrast, the CNTs amount of in-situ grown CNTs increases as the Ni introduced by ultrasonic dispersion have poor concentration is increased from 1.5 to 2.0 wt%, which dispersion in the matrix, which can be clearly observed is consistent with the TGA results (Fig. 2(c)). This is in Fig. 3(h). To further explore the microstructures of due to the formation of more catalyst nanoparticles the as-grown CNTs, the sample named as 2.0% with the increasing catalyst precursors, which provide Ni–CNTs/ZnO is analyzed by the TEM, as shown in more nucleation and growth sites for CNTs [44]. Figs. 3(i)–3(k). It was found that almost every CNT However, there is no significant change in the diameter www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1937 Fig. 3 SEM images of precursors: (a) pure ZnO, (b) Ni(OH) /ZnO, (c) NiO/ZnO, and (d) Ni/ZnO. (e–g) SEM images of CNTs/ZnO composite powders with in-situ grown CNTs synthesized using different mass ratios of Ni catalysts: 1.5, 1.75, and 2.0 wt%; (h) SEM image of CNTs/ZnO composite powders mixed with commercial CNTs by ultrasonic dispersion; (i–k) TEM and HRTEM images of 2.0% Ni–CNTs/ZnO sample; (l) selected area electron diffraction pattern; and (m–p) EDS elemental mappings of 2.0% Ni–CNTs/ZnO sample. has a nanoparticle wrapped around its top, indicating 3(p)) of 2.0% Ni–CNTs/ZnO sample confirm that the that the in-situ grown CNTs on ZnO follows the Ni nanoparticles are uniformly distributed in the matrix, “tip-growth mode” [47]. The high-resolution TEM which ensures the uniformity of the as-grown CNTs. (HRTEM) images show that in-situ grown CNTs are To further investigate the distribution states of CNTs multi-walled CNTs, and the interlayer spacing of in the sintered samples, we characterized the graphite layers is about 0.36 nm. In addition, the microstructures of the CNTs/ZnO bulk composites. As graphitic layers are not parallel to the axis direction for the sample with commercial CNTs added by and are disordered, revealing that the in-situ grown ultrasonic dispersion, serious agglomeration of CNTs CNTs have many edges and defects (Fig. 3(j)) [48]. can be clearly seen in Fig. 4(a). In contrast, the in-situ Furthermore, the interlayer space of lattice fringes for grown CNTs are not easy to be observed in the the nanoparticles is ~0.21 nm, which corresponds to as-sintered samples, which may be due to the short the (111) plane of Ni, implying that metallic Ni acts as length of the CNTs [50]. Another obvious phenomenon a catalyst for the growth of CNTs [49]. The HRTEM is that the crystal grain sizes decrease with increasing image performed in Area 2 (Fig. 3(k)) also reveals that the content of the as-grown CNTs (Figs. 4(b)–4(d)). the in-situ grown CNTs are tightly anchored on the This is due to the pinning effect of CNTs that limits the ZnO matrix, ensuring good interface connectivity. The growth of grains during the sintering [51]. The increased selected area electron diffraction pattern (Fig. 3(l)) grain boundaries can enhance the scattering of phonon, identifies the wurtzite ZnO, which agrees with the leading to an improvement of the TE performance. XRD results. More importantly, the energy-dispersive However, it should be noted that this effect will be spectroscopy (EDS) elemental mappings (Figs. 3(m)– deeply weakened if the CNTs agglomerate in the www.springer.com/journal/40145 1938 J Adv Ceram 2022, 11(12): 1932–1943 Fig. 4 SEM images of fractured surfaces of ZnO bulk composites: (a) 2.0% CNTs/ZnO, (b) 1.5% Ni–CNTs/ZnO, (c) 1.75% Ni–CNTs/ZnO, (d) 2.0% Ni–CNTs/ZnO, (e) high-magnification image of 2.0% Ni–CNTs/ZnO, and (f) line scanning. matrix, which is the case in Fig. 4(a). In order to better mainly due to the agglomeration of CNTs, which observe the distributions of in-situ grown CNTs in the prevents the excellent conductivity of CNTs from as-sintered bulk, further characterization on the 2.0% being effectively utilized [34]. Figure 5(b) shows the Ni–CNTs/ZnO sample by the high-magnification SEM temperature dependence of Seebeck coefficient. The is presented in Fig. 4(e). The CNTs marked with red negative values of all samples indicate an n-type circles remain uniformly distributed in the bulk material electrical transport behavior. For the samples with and well connected with the matrix. In addition, the in-situ grown CNTs, the values of Seebeck coefficient CNTs are mainly located on the grain boundaries, with show little change over the entire measurement a small amount on the surface of the ZnO matrix. The temperature range. With increasing the amount of elemental line scanning plotted in Fig. 4(f) further in-situ grown CNTs, the absolute values of Seebeck proves that the marked area in Fig. 4(e) is the CNTs. coefficient increase firstly and then decrease. In general, Figure 5(a) shows the electrical conductivity as a the Seebeck coefficient is inversely proportional to the function of temperature for all bulk samples. It can be carrier concentration (n) according to Eq. (3) [54]: noticed that the electrical conductivity decreases with 2/3 8π k  π increasing the temperature, indicative of the metallic Sm  T (3)  3n 3eh  conducting behavior [52]. Obviously, the electrical where k is the Boltzmann constant, h is the Planck’s conductivity of the samples with in-situ grown CNTs is significantly enhanced compared with that of the constant, and m is the effective mass of carrier. The main contribution of the Seebeck coefficient for sample without CNTs. Furthermore, by controlling the amount of catalyst (from 1.5 to 2.0 wt%), the electrical ZnO-based composites lines to carrier concentration. However, the 1.75% Ni–CNTs/ZnO sample presents conductivity increases with the increasing content of CNTs. On the one hand, the enhanced electrical the highest Seebeck value and keeps a high carrier conductivity results from the inherent high concentration. This may be ascribed to the change of conductivity of CNTs [53]. On the other hand, a good m when the CNTs are introduced into ZnO, which conductive network is established in the matrix owing should be further investigated. To better understand the to the homogeneous dispersion of in-situ grown CNTs, impact of in-situ grown CNTs on the electrical transport which facilitates the transport of electrons. In contrast, of ZnO composites, the temperature dependence of for the sample with commercial CNTs by ultrasonic carrier concentration and carrier mobility (μ ) for all technology, the electrical conductivity is much lower, samples are measured and analyzed. www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1939 As shown in Fig. 5(c), the carrier concentration is is plotted in Fig. 6(a). The values of total thermal found to increase as the temperature rises from room conductivity decrease with the increasing temperature. temperature to 800 K, which can be ascribed to the It is clearly observed that the introduction of in-situ intrinsic excitation [55]. Moreover, there is an apparent grown CNTs has a significant effect on reducing the increase in carrier concentration as the content of total thermal conductivity at low temperatures. For −1 −1 in-situ grown CNTs in the ZnO matrix increases. A example, it is suppressed from 25 to 19 W·m ·K at 19 −3 maximum carrier concentration of 1.66×10 cm at 300 K. However, this effect is diminished if the CNTs 800 K is obtained for the sample catalyzed by 2.0 wt% are agglomerated in the matrix, as shown in the sample Ni, exhibiting an increase of ~168% compared with with the direct addition of commercial CNTs. This that of the sample catalyzed by 1.5 wt% Ni (0.62× demonstrates the advantage of introducing CNTs 19 −3 10 cm ). It is noteworthy that the sample with through in-situ growth technology. Both phonons and commercial CNTs also has a relatively high carrier electrons can act as carriers of heat, and their concentration. In contrast, μ (Fig. 5(d)) is negatively H contributions to the total thermal conductivity are referred correlated with temperature, which is caused by the to as  and the electronic thermal conductivity ( ), L e scattering of charge carriers. In addition, all samples respectively. Furthermore,  is calculated according with in-situ grown CNTs possess higher μ compared to the Widemann–Franz law (Eq. (4)): with the sample with mechanically mixed CNTs, which    LT (4) is due to the high μ of CNTs and the formation of where L is the Lorenz number.  is obtained by conducting channel between the ZnO and CNTs. Moreover, the tight bonding of ZnO and in-situ CNTs subtracting  from the total thermal conductivity. As shown in Fig. 6(b), decreases with increasing the may reduce the interface potential barrier for electron  temperature, showing the same changing trend as that transport, demonstrating the benefits of uniform dispersion of CNTs [56,57]. of the total thermal conductivity. As expected, the The temperature-dependent total thermal conductivity sample with 2.0% Ni–CNTs/ZnO has the lowest lattice Fig. 5 Temperature dependence of (a) σ, (b) S, (c) n, and (d) μ for ZnO-based composites. www.springer.com/journal/40145 1940 J Adv Ceram 2022, 11(12): 1932–1943 thermal conductivity over the entire temperature range. repetitions, indicating good reproducibility of this This can be attributed to three factors: Firstly, the work. Moreover, the zT values of the ZnO composites addition of CNTs limits the growth of grains during the prepared by our method are comparable to those sintering process, as evidenced by the SEM images in reported in Refs. [59–61] by doping metal elements, Fig. 4, thereby providing more grain boundaries to such as Al and Ni, or introducing second phase, as scatter phonons; secondly, the rich interface formed compared in Fig. 6(d). between the CNTs and the ZnO matrix enhances the Excellent mechanical properties are the basis for scattering of phonons; and thirdly, the magnetic Ni realizing practical applications from materials to nanoparticles formed during the in-situ growth of devices. Although many materials currently possess CNTs can also act as phonon scattering centers [58]. outstanding TE properties, they cannot be used in Figure 6(c) shows the temperature dependence of  . modules due to the poor mechanical properties [62]. As When the amount of catalyst is improved from 1.5 to shown in Fig. 7(a), HV is improved when the CNTs 2.0 wt%,  rises owing to the enhancement of electrical introduced through in-situ grown technology. A maximum conductivity. However, this increase can be compensated value of 4.12 GPa is achieved for the sample with by the decrease of  . 2.0% Ni–CNTs/ZnO, which is 11.6% higher than that As a consequence, the zT values of samples with of the sample denoted as 2.0% Ni/ZnO and 38.7% CNTs obtained by in-situ technology are enhanced, higher than that of the sample with commercial CNTs. benefitting from the increase in electrical conductivity This improvement may be ascribed to the change in and the decrease in thermal conductivity, as shown in grain size [63]. The MSP method is adopted to measure Fig. 6(d). The highest zT of 0.04 at 800 K was  , which is similar to the 4-point bending method [39]. achieved in 2.0% Ni–CNTs/ZnO sample, which is a Compared with that of the 2.0% Ni/ZnO sample, the for 2.0% Ni–CNTs/ZnO is increased by 18.5%, two-fold increase compared to that of the 2.0% Ni/ZnO  sample and a three-fold increase compared to that of from 111.06 to 131.66 MPa, as shown in Fig. 7(b). At commercial CNTs. Besides, comparable TE performance the same time, the K for 2.0% Ni–CNTs/ZnO can IC 1/2 reach 1.55 MPa·m , which is 55% higher than that of of 2.0% Ni/ZnO sample was achieved after three Fig. 6 Temperature dependence of (a) total thermal conductivity ( ), (b)  , (c)  , and (d) zT values for ZnO-based total L e composites. www.springer.com/journal/40145 J Adv Ceram 2022, 11(12): 1932–1943 1941 Fig. 7 Mechanical properties of ZnO-based composites: (a) HV and (b)  and K . IC the 2.0% CNTs/ZnO sample with mechanically mixed Declaration of competing interest commercial CNTs, suggesting that the evenly distributed CNTs in the matrix is vital to improve the K [64]. 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Commons Attribution 4.0 International License, which permits Thermoelectric properties of ZnO ceramics densified use, sharing, adaptation, distribution and reproduction in any through spark plasma sintering. Ceram Int 2020, 46: medium or format, as long as you give appropriate credit to the 5229–5238. original author(s) and the source, provide a link to the Creative [53] Zhang YC, Zhang QC, Chen GM. Carbon and carbon Commons licence, and indicate if changes were made. composites for thermoelectric applications. Carbon Energy The images or other third party material in this article are 2020, 2: 408–436. included in the article’s Creative Commons licence, unless [54] Nong J, Peng Y, Liu CY, et al. Ultra-low thermal indicated otherwise in a credit line to the material. If material is conductivity in B O composited SiGe bulk with enhanced not included in the article’s Creative Commons licence and your 2 3 thermoelectric performance at medium temperature region. intended use is not permitted by statutory regulation or exceeds J Mater Chem A 2022, 10: 4120–4130. the permitted use, you will need to obtain permission directly [55] Zhuang HL, Pei J, Cai BW, et al. Thermoelectric performance from the copyright holder. enhancement in BiSbTe alloy by microstructure modulation To view a copy of this licence, visit http://creativecommons.org/ via cyclic spark plasma sintering with liquid phase. Adv licenses/by/4.0/. www.springer.com/journal/40145

Journal

Journal of Advanced CeramicsSpringer Journals

Published: Dec 1, 2022

Keywords: ZnO; carbon nanotubes (CNTs); in-situ grown; thermoelectric (TE); mechanical properties

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