A study on the thermoelectric properties of ALD-grown aluminum-doped tin oxide with respect to nanostructure modulations

The thermoelectric properties of aluminum-doped tin oxide (ATO) thin films synthesized by thermal atomic layer deposition (ALD) were studied with respect to the aluminum concentration. The overall aluminum content in each layer was modulated by adjusting the relative number of tin oxide (SnO₂) and aluminum oxide (Al₂O₃) growth cycles, where a sequential process involving n cycles of SnO₂ growth followed by 1 cycle of Al₂O₃ deposition was performed (building up a super-cycle). The electrical conductivity (620 S/cm), free carrier concentration (1.23x10²¹ cm^-3), and power factor (0.49 mW/K²m) increase until their maximum values are reached when the Al content is approximately 1.50 at% of the cations, and decrease as more Al is added in. On the other hand, the Seebeck coefficient decreases monotonically as the Al content increases up to about 2.88 at%, and begins to increase with further Al doping. Here the thermoelectric efficiency is therefore determined primarily by the free carrier concentration, while the Seebeck coefficient appears to be influenced by the overall crystal structure.     

Densification,microstructure, and thermoelectric properties of AZO/SrTiO3 composites

Al-doped ZnO (AZO) has emerged as a potential high-temperature thermoelectric material with an appropriate Seebeck coefficient and high thermal stability, and hence is considered as a promising material for power generation applications. Herein, we report the fabrication of AZO/SrTiO₃ composites with improved thermoelectric performance. The densification, microstructure, and thermoelectric properties of the AZO/SrTiO₃ composites were investigated. The significant increase in the relative density of AZO from 89.1 to 98.0% after the addition of SrTiO₃ indicates that SrTiO₃ promoted the densification of the composites. Furthermore, the electrical conductivity of AZO increased after the addition of SrTiO₃, which can mainly be attributed to its enhanced relative density. The AZO/SrTiO₃ composite with 2.0 wt% SrTiO₃ showed the highest power factor at 1000 K because of its highest electrical conductivity. In addition, the composite showed the highest ZT value, which was 1.8 times higher than that of pure AZO.     

Synergistic effects of zirconium- and aluminum co-doping on the thermoelectric performance of zinc oxide

This work aims to explore zirconium as a possible dopant to promote thermoelectric performance in bulk ZnO-based materials, both within the single-doping concept and on simultaneous co-doping with aluminum. At 1100–1223 K mixed-doped samples demonstrated around ～2.3 times increase in ZT as compared to single-doped materials, reaching ～0.12. The simultaneous presence of aluminum and zirconium imposes a synergistic effect on electrical properties provided by their mutual effects on the solubility in ZnO crystal lattice, while also allowing a moderate decrease of the thermal conductivity due to phonon scattering effects. At 1173 K the power factor of mixed-doped Zn_0.994Al_0.003Zr_0.003O was 2.2–2.5 times higher than for single-doped materials. Stability tests of the prepared materials under prospective operation conditions indicated that the gradual increase in both resistivity and Seebeck coefficient in mixed-doped compositions with time may partially compensate each other to maintain a relatively high power factor.     

Enhanced thermoelectric properties of Cu2-xSe by coordinating carrier concentration to reduce thermal conductivity

Cu_2-xSe has been known as an ideal thermoelectric material for application in the middle-high temperature range due to the outstanding electric transmission conductivity and relatively low lattice thermal conductivity. However, its performance is significantly constrained by its high thermal conductivity and thermal stability issues, as well as its difficulty in element doping because of the influence of atomic size and atomic solid solubility. Here, we prominently reduced the thermal conductivity of Cu_2-xSe by addition different amounts of WS₂ to the Cu_2-xSe matrix, and successfully improved the power factor of the material because of the reduction of high Cu defects to coordinate the three mutually coupled parameters. The zT value of the WS₂-doping Cu_2-xSe sample was eventually enhanced by 56% compared with pristine Cu_2-xSe, and up to ca. 1 at 823 K. Further, we found that the symmetry of the Cu_2-xSe crystal had not been destroyed undergoing the doping of WS₂ into its crystal lattice, and the doped sample showed good thermal stability when cycle test was carried out twice between 315 K and 823 K.     

Enhanced optoelectrical performance of ultraviolet detectors based on self-assembled porous zinc oxide nanostructures

A templated self-assembly technique was utilized in the present study to grow porous zinc oxide nanostructures. The nanostructures were formed by the electrochemical deposition of ZnO through the interstitial spaces between polymer microsphere templates. After the deposition, polymer microspheres were removed by dissolving in chloroform solvent, leaving porous ZnO nanostructures. This technique is benefited from facile controllability of the pore morphology and size by varying the diameter of microspheres. X-ray diffraction analysis showed a dominant peak corresponding to the hexagonal ZnO structure. Moreover, no significant structural strain was observed after the removal of spheres unlike the other synthesis methods of porous materials. The improved photoluminescence (PL) properties revealed an enhancement in the light capturing capability of the systems due to the multiple scattering of light in the pore walls. The porous sample showed a PL blue-shift compared to the flat one, indicating a reduction in crystallite size of ZnO nanostructures. To assess the photonic applications of synthesized porous ZnO substrates, a metal-semiconductor-metal photodetector was developed via the metallization of ZnO nanostructures, and their optoelectrical properties were tested under UV radiation. The results showed an improvement in photosensitivity and quantum efficiency of devices based on porous ZnO substrates which can be assigned to the larger exposed area and elevated rates of electron-hole generation in this sample.     

Europium (III) doped bismuth telluride decorated on carbon-based materials for enhancing thermoelectric performance

Scalable synthetic approach for superior performance of thermoelectric (TE) materials is a crucial step for the TE technology progress. Herein, reduced graphene oxide (RGO), carbon nitride (g-C ₃N₄) and europium (Eu) are utilized as additives to bismuth telluride (Bi₂Te₃) matrix to prepare various novel nanocomposites (NCs): (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) with an enhanced TE performance. The novel NCs were synthesized via solvothermal method, physiochemically characterized and consolidated into pellets of 1 mm thickness to measure their TE properties. The new additives potentially affected the physicochemical and TE properties of Bi₂Te₃. Nanostructured hexagonal nanoplatelets with 12.5 nm thickness were observed by scanning and transmission electron microscopy (SEM and TEM) of the synthesized Bi₂Te₃. This thickness shrinked to 5.7 and 5.2 nm upon the formation of (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs, respectively. Energy dispersive X-ray Spectroscopy (EDS) of NCs proved the existence of Bi, Te, C, Eu and N atoms. Raman and Fourier-transform infrared (FT-IR) spectra confirmed the NC formation that led to narrowing the energy band gap of Bi₂Te₃ as displayed by UV–Vis spectra. Brunauer–Emmett–Teller showed specific surface area expansion of Bi₂Te₃ from 6.78 to 19.00 and 16.75 m²g^-1 of (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs, respectively. The electrical conductivity of Bi₂Te₃ rose by 56 and 69 times, whereas its thermal conductivity significantly dropped by 1.6 and 1.7 times upon (RGO@Bi_1.8Te₃Eu_0.2) and (RGO-g-C₃N₄@Bi_1.8Te₃Eu_0.2) NCs formation. Owing to extra channels of carrier transfer and phonon scattering induced by NCs heterointerfaces. Novel combination of carbon-based materials and Eu with Bi₂Te₃ matrix boosts its TE performance resulting in a worthy candidate for power generation applications at room-temperature.     

Synthesis and thermoelectric performance of titanium diboride and its composites with lead selenide and carbon

The exploration of new thermoelectric material is the current area of research in energy conversion and storage technologies, in that nanocomposite approach is a promising root to get desirable thermoelectric properties. The present study demonstrates a composite containing highly conductive titanium diboride (TiB₂), polyvinyl alcohol (PVA) as binder and lead selenide (PbSe) as semiconductor. The synthesis and transport physics are studied with an intention to increase the power factor and figure of merit (ZT) of TiB₂ by reducing thermal conductivity through creating inhomogeneity in microstructures. Sol gel method and carbothermal reduction reaction have been used to synthesize TiB₂. More than 95% of thermal conductivity is reduced due to the phonon scattering, which is desirable to achieve a high power factor and ZT. TiB₂/PVA composite possesses a very low Seebeck coefficient and exhibits three order of magnitude reduction in electrical conductivity, which hinders in achieving a good power factor and ZT. Power factor of 25.3?µW/mK², Seebeck coefficient of 36.3 μV/K at 550?K and electrical conductivity of 2.5?×?10⁴ S/m at ~300?K and ZT of 0.064 at 550?K are worth to report in this study. Finally, the synthesized TiB₂ is incorporated into PbSe to evaluate thermoelectric properties. Maximum ZT of 0.12 at 495?K, Seebeck coefficient of ?342?µV/K at 550?K, electrical conductivity of 2.8?×?10³ S/m at 400?K, thermal conductivity of 1.03?W/mK at 550?K and highest power factor of 280.2?µW/mK² at 495?K have been achieved in this composite.     

Improving optical and electrical performances of aluminum-doped zinc oxide thin films with laser-etched grating structures

A laser etching method was performed to achieve the dual purpose of fabricating grating structures and laser annealing on aluminum-doped zinc oxide (AZO) thin films, and thus improve the film photoelectric performances. Different line spacings and laser fluences were adopted to systematically explore the optimal laser etching condition. Too narrow line spacings or too high laser fluences led to light reflections at the grating external surface to cause more light dissipation, and too wide line spacings or too low laser fluences resulted in relatively small total grating lateral areas being detrimental to multiple internal light reflections. Moreover, too narrow line spacings brought about laser-caused lattice disorder and too high laser fluences produced laser-ablated spots or overburned traces. Therefore, using the medium line spacing and laser fluence, e.g. 40 μm and 0.6 J/cm² in the present work, was more suitable for synchronously realizing grating structure fabrication and laser annealing. The corresponding AZO film exhibited the maximum figure of merit of 2.89 × 10^?2 Ω^?1, which was 1.6 times that of the untreated AZO film. This study is expected to expand performance improvement methods of TCO films and promote the application of laser-etched grating structures.     

Preparation of biomorphic porous zinc oxide by wood template method

This paper presents an effective method to fabricate highly porous zinc oxide (ZnO) derived from different wood templates, where the microstructure features of wood template were well reproduced in the final product. Biomorphic ZnO was fabricated via the sol–gel method by the infiltration of the precursor sol into the wood template and the sintering of wood template at high temperature under air atmosphere. Many characterization methods such as x-ray diffraction (XRD), scanning electron microscopy (SEM) and pore size analyzer detection were used to investigate the crystalline phase and microstructure of the product as well as the pore size of biomorphic ZnO. In our work, highly porous ZnO derived from different wood templates had been prepared. The relevant results revealed that the specie of wood template and sintering temperature played vital roles in the pore size, specific surface area and pore volume of the product.     

Effects of microstructure evolution on transport properties of thermoelectric nickel-doped zinc oxide

We investigate the effects of microstructure evolution on transport properties of nickel-doped ZnO for thermoelectric waste heat recovery at high temperatures. A 3 at.% supersaturated Ni-alloyed ZnO solid solution was prepared by sintering at 1400 °C followed by controlled nucleation and growth of sub-micrometer size NiO-precipitates by aging at 750, 800, and 900 °C for different durations. Minimum thermal conductivity as low as 8.0 W m⁻¹ K⁻¹ at 700 °C is obtained for samples aged at 750 °C for 16 h due to precipitates with high number density of 1.3·10²⁰ m⁻³, which initiate phonon scattering. In turn, as-quenched samples exhibit the highest electrical conductivity, ca. 17.9 S cm⁻¹ at 700 °C. Further nucleation and growth of precipitates taking place for longer annealing durations reduce electrical conductivity and increase Seebeck coefficients, which is associated with dilution of the ZnO-matrix from Ni-atoms. This study provides us with guidelines for optimization of thermoelectric Ni-doped ZnO.     

氨配合氧化法由氧化锌烟灰制备活性氧化锌

本研究以氧化锌烟灰或粗氧化锌为原料，用氨水—碳酸氢铵—双氧水作浸取液。将氧化锌烟灰或粗氧化锌浸取、除杂净化、蒸氨沉锌、洗涤干燥、煅烧即可制得高品位的活性氧化锌。     

三聚磷酸钠辅助沉淀法制备纳米氧化锌及其抗菌性能研究

探讨了纳米氧化锌制备过程中的团聚问题及其抗菌性能。以三聚磷酸钠为表面活性剂,辅助沉淀法制备了纳米氧化锌粉体,用XRD,TEM,TG-DTA,BET等方法对产物进行了表征。结果表明:所得纳米氧化锌为高分散的球形颗粒,属于六方纤维矿的单晶结构,平均粒径为20 nm,比表面积可达70.478 1 m2/g。线性三聚磷酸钠与锌离子的络合作用,可以使前躯体均匀成核,并通过空间位阻抑制颗粒间的聚集,避免团聚的发生。采用杯碟法测试纳米氧化锌的抑菌活性。结果显示,纳米氧化锌对金黄色葡萄球菌、大肠埃希氏菌和沙门氏菌具有很好的抑菌活性,纳米氧化锌比表面积大小决定其抑菌活性。     

负载型纳米氧化锌在橡胶中的应用研究

氧化锌作为一种良好的硫化活性剂,在橡胶生产中已被广泛使用,但锌基材料对人体健康和生态环境存在不良影响,因此如何在橡胶配方中减少锌的含量已经成为目前橡胶行业重要的研究课题。纳米氧化锌因为具有纳米粒子特有的优势,因此在使用中可以达到锌减量的目的,但纳米粒子容易团聚,限制了纳米效应的发挥;广东韶关凯鸿纳米材料有限公司开发了一种负载型的纳米氧化锌,将纳米氧化锌粒子包覆在载体上的新型制品,提高反应活性,减少团聚,可以使纳米氧化锌充分发挥纳米粒子的优势,同时有效的减少锌在橡胶制品中的残余量。本工作分别采用普通氧化锌、纳米氧化锌、负载型纳米氧化锌(C型、A型、S型)作为活化剂,其中负载型纳米氧化锌依据载体不同进行分类,C型的载体为碳酸钙,A型的载体为氧化铝,S型载体为二氧化硅。通过各项性能测试实验,重点研究负载型纳米氧化锌对橡胶综合性能的影响。实验结果表明,相对于普通氧化锌,负载型纳米氧化锌作为活化剂,可以在锌的使用量明显下降的情况下,制备出达到甚至优于普通氧化锌制品性能的橡胶复合材料。其中,3份用量的A型负载型纳米氧化锌的活化作用最优。     

Cyclic voltammetric study of zinc and zinc oxide electrodes in 5.3 M KOH

The behaviour of zinc and zinc oxide in 5.3 M KOH in the presence of alkaline earth oxides, SnO, Ni(OH)₂ and Co(OH)₂ was examined by cyclic voltammetry. The influence of the alkaline earth oxides was compared with additives of established effects (Bi₂O₃, LiOH, Na₂CO₃ and CdO). The alkaline earth oxide each exhibits a distinct behaviour towards zincate. Whereas, a single process of interaction with zincate was shown by CaO; two modes of reaction were obtained with SrO and BaO. Solid solution formation was noticed with BeO and MgO. The other additives forming solid solution with ZnO were CdO, SnO. The ionic sizes of Ni(OH)₂ and Co(OH)₂ allow solid solution formation with Zn(OH)₂. Both Bi₂O₃ and Na₂CO₃ enter into complexation with zincate. LiOH forms two distinct zincates, of which one is an oxo zincate leaching the `hydroxyl' functionality. Cyclic voltammetry revealed the deposition of the oxide/hydroxide additives as metal prior to the onset of zinc deposition and the potential range for this additive metal deposition is almost the same for different additives (SnO, CdO, Ni(OH)₂). The beneficial action of these additives to zinc alkaline cells is associated with a substrate effect. The implication of this electrocatalytic deposition of metals on a zinc oxide electrode is also discussed.     

超声场中沉淀法纳米氧化锌的制备与表征

将超声辐射应用于以硫酸锌(ZnSO4·7H2O)和草酸(H2C2O4·2H2O)为原料的沉淀法制备纳米氧化锌粉体的工艺过程,制备了平均晶粒尺寸为26nm的氧化锌粉体。通过XRD,DTA—TG和SEM等技术研究了纳米氧化锌的合成过程及粉体性能。结果表明:超声辐射引入普通沉淀法,超声波的空化作用可使前驱体颗粒细化,抑制其团聚并延缓其向凝胶转变,从而可制备出氧化锌纳米粉体。这种方法所得纳米氧化锌粒子外貌为球形,粒度分布均匀,分散性好。     

锌灰制备碱式碳酸锌和超细氧化锌及表征

室温下采用氨浸出锌灰制得碱式碳酸锌,再经煅烧制得超细氧化锌。研究了在合成碱式碳酸锌过程中表面活性剂对碱式碳酸锌和氧化锌颗粒尺寸与形貌的影响。结果表明,聚乙二醇（PEG20000）和聚乙烯吡咯烷酮（PVP-K30）这两种表面活性剂对颗粒的分散效果最好,制得的碱式碳酸锌颗粒为无定形片状且分散均匀,平均粒径为1 μm,煅烧后的氧化锌颗粒为六方晶系纤锌矿结构,粒径约为0.7 μm。添加PVP-K30比添加PEG20000的碱式碳酸锌热分解温度高。添加PEG和PVP的碱式碳酸锌反应活化能分别为139.9 kJ/mol和146.8 kJ/mol。     

二氧化碳法制备活性氧化锌的过程研究

以次硫酸氢钠甲醛副产锌泥为原料，用高剪切乳化机乳化锌浆，采用双氧水作为氧化剂氧化锌浆中的金属锌使之转化为氢氧化锌，采用二氧化碳作为沉锌剂碳化锌浆后得到碱式碳酸锌沉淀，沉淀通过脱水、洗涤、干燥、煅烧后可得到活性氧化锌产品。该方法充分利用了锌资源，有效处理了大量闲置的锌泥，制得的活性氧化锌产品质量可达到HG2572-1994合格品标准。     

Enhanced electrochromic properties of ordered porous nickel oxide thin film prepared by self-assembled colloidal crystal template-assisted electrodeposition

Highly ordered porous NiO film is prepared by self-assembled monolayer polystyrene sphere template-assisted electrodeposition. The as-prepared NiO film shows an ordered hexagonal close-packed bowel-like array that is made up of macrobowls with about 500 nm in diameter. The electrochromic properties of NiO film are investigated in an aqueous alkaline electrolyte (0.1 M KOH) by means of transmittance, cyclic voltammetry and chronoamperometry measurements. The ordered porous NiO film prepared with PS sphere template exhibits a noticeable electrochromism with reversible color changes from transparent to dark brown, and presents quite good transmittance modulation with a variation of transmittance up to 76% at 550 nm. The ordered porous NiO film also shows high coloration efficiency (41 cm² C⁻¹), fast switching speed (3 s and 6 s) and good cycling performance, compared with the dense NiO film prepared without PS sphere template. The improvements of electrochromic performances are attributed to the highly porous morphology, which shortens the ion diffusion paths and provides bigger surface area.     

Fabrication of porous silica ceramics by gelation-freezing of diatomite slurry

Diatomite-derived porous silica ceramics with high porosities of up to 90% were fabricated using a gelation-freezing method, which resulted in unidirectional cellular or random microstructure with micrometer-sized cells. The ice crystals that were formed during freezing of a diatomite powder dispersed gel were removed by sublimation during vacuum drying, and the green bodies were sintered at 1150–1350 °C for 2 h in air. The thermal conductivity of the porous ceramics prepared with initial solid loadings of 5 and 10 vol% ranged from 0.09 to 0.16 W/(mK) at room temperature. The proposed method is therefore promising for the preparation of ceramic thermal insulators with very low thermal conductivity.     

Foam-gelcasting preparation,microstructure and thermal insulation performance of porous diatomite ceramics with hierarchical pore structures

Hierarchically pore-structured porous diatomite ceramics containing 82.9∼84.5% porosity were successfully prepared for the first time via foam-gelcasting using diatomite powder as the main raw material. Sizes of mesopores derived from the raw material and macropores formed mainly from foaming were 0.02∼0.1 μm and 109.7∼130.5 μm, respectively. The effect of sintering temperature, additive content and solid loading of slurry on pore size and distribution, and mechanical and thermal properties of as-prepared porous ceramics were investigated. Compressive strength of as-prepared porous ceramics increased with sintering temperature, and the one containing 82.9% porosity showed the highest compressive strength of 2.1 ± 0.14 MPa. In addition, the one containing 84.5% porosity and having compressive strength of 1.1 ± 0.07 MPa showed the lowest thermal conductivity of 0.097 ± 0.001 W/(m·K) at a test temperature of 200 ̊C, suggesting that as-prepared porous ceramics could be potentially used as good thermal insulation materials.