Phase formation,microstructure development and thermoelectric properties of (ZnO)kIn2O3 ceramics

The (ZnO)_kIn₂O₃ system is interesting for applications in the fields of thermoelectrics and opto-electronics. In this study we resolve the complex homologous phase evolution with increasing temperature in polycrystalline ceramics for k = 5, 11 and 18 and its influence on the microstructural development and thermoelectric properties. The phase formation at temperatures above 1000 °C is influenced by the local ZnO-to-In₂O₃ ratio in the starting-powder mixture. While the Zn₅In₂O₈ equilibrium phase for k = 5 is formed directly after sintering at 1200 °C, the formation of the k = 11 and k = 18 equilibrium phases proceeds at higher temperatures by diffusion between the initially formed phases, the lower k Zn₅In₂O₈/Zn₇In₂O₁₀ and the higher k Zn_kIn₂O_k+3 (9 < k < ∞). Such phase formation affects the sintering and grain growth, and consequently, with the degree of structural and compositional homogeneity, also the thermoelectric characteristics of the (ZnO)_kIn₂O₃ ceramics.     

Preparation and microwave dielectric properties of B2O3 bulk

The transparent B₂O₃ bulks were prepared by quenching B₂O₃ melt or cooling it down to room temperature with the rate of 0.1-1°C/min. The quenched sample exhibited the dense microstructure, while the pores were observed for low cooling rates, and this is consistent with the slight decrease in density and permittivity (ε_r) with decreasing the cooling rate. Both the amorphous and cubic phases of B₂O₃ were indicated by XRD, and the content of the cubic phase increased with the decrease in cooling rate, which was responsible for the decreasing Qf value with the lower cooling rate. The temperature coefficient of resonant frequency (τ_f) was insensitive to the cooling rate, and the microwave dielectric properties with ε_r = 3.86-3.97, Qf = 3,200-5,300 GHz and τ_f = −48.9 to −42.8 ppm/°C were obtained in the B₂O₃ bulks. The reported microwave dielectric properties of B₂O₃ bulk are helpful to estimate the effects of the residual B₂O₃ on the properties of the microwave dielectric ceramics with B₂O₃ and H₃BO₃ as the sintering aid.     

Thermoelectric properties of Graphene/Mn0.7Zn0.3Fe2O4 composites

The Graphene/Mn_0.7Zn_0.3Fe₂O₄ composites were synthesized by coprecipitation and sintered by a spark-plasma-sintering (SPS) method. The thermoelectric properties of the sintered composites were evaluated in the temperature range of 343–973 K. The effect of graphene on the thermoelectrical properties of Mn_0.7Zn_0.3Fe₂O₄ was investigated. The dispersion of 2 wt% graphene in Mn_0.7Zn_0.3Fe₂O₄ effectively enhanced the electrical conductivity and the absolute value of Seebeck coefficient, while thermal conductivity was decreased. The results showed that the maximum ZT value of 0.035 at 973 K was obtained in the composite with 2 wt% graphene.     

Citrate gel process and thermoelectric properties of Ge-doped In2O3 bulk ceramics

Indium oxide ceramics doped with different doping levels of germanium (from 0 to 10 at.%) were prepared with nano-sized powders obtained by citrate gel process. Ge for In substitution in the In₂O₃ bixbyite structure below the solubility limit (about 0.5-1 atom%) leads to a large decrease in the electrical resistivity and the absolute value of the Seebeck coefficient. X-ray diffraction and scanning electron microscopy show the presence after sintering of well dispersed secondary phases In₂Ge₂O₇ when the Ge solubility in the In₂O₃ matrix is passed. The thermal properties measurements confirm that the decrease in the lattice thermal conductivity observed in these composite materials can be attributed to the presence of insulating In₂Ge₂O₇ phases. Furthermore, preliminary microwave sintering experiments have been tested in order to keep the nanoscale after sintering. Particular nano-microstructures were obtained due to minimal grain growth induced by this rapid sintering process.     

Low Temperature Growth of In2O3 and InN Nanocrystals on Si(111) via Chemical Vapour Deposition Based on the Sublimation of NH4Cl in In

Indium oxide (In₂O₃) nanocrystals (NCs) have been obtained via atmospheric pressure, chemical vapour deposition (APCVD) on Si(111) via the direct oxidation of In with Ar:10% O₂ at 1000 °C but also at temperatures as low as 500 °C by the sublimation of ammonium chloride (NH₄Cl) which is incorporated into the In under a gas flow of nitrogen (N₂). Similarly InN NCs have also been obtained using sublimation of NH₄Cl in a gas flow of NH₃. During oxidation of In under a flow of O₂ the transfer of In into the gas stream is inhibited by the formation of In₂O₃ around the In powder which breaks up only at high temperatures, i.e. T > 900 °C, thereby releasing In into the gas stream which can then react with O₂ leading to a high yield formation of isolated 500 nm In₂O₃ octahedrons but also chains of these nanostructures. No such NCs were obtained by direct oxidation for T _G < 900 °C. The incorporation of NH₄Cl in the In leads to the sublimation of NH₄Cl into NH₃ and HCl at around 338 °C which in turn produces an efficient dispersion and transfer of the whole In into the gas stream of N₂ where it reacts with HCl forming primarily InCl. The latter adsorbs onto the Si(111) where it reacts with H₂O and O₂ leading to the formation of In₂O₃ nanopyramids on Si(111). The rest of the InCl is carried downstream, where it solidifies at lower temperatures, and rapidly breaks down into metallic In upon exposure to H₂O in the air. Upon carrying out the reaction of In with NH₄Cl at 600 °C under NH₃ as opposed to N₂, we obtain InN nanoparticles on Si(111) with an average diameter of 300 nm.     

In2O3/CdS复合物的制备及光催化性能

以氧化铟(In_2O_3)纳米球作为基体,采用水热法制备了氧化铟/硫化镉(In_2O_3/CdS)复合光催化剂,并利用XRD、SEM等对所制备复合光催化剂进行了表征。结果表明:复合光催化剂由立方相的In_2O_3纳米球和六方相CdS棒状结构组成,且In_2O_3纳米球附着于CdS棒状结构表面上。光学性能测试和光降解实验发现:所得复合光催化剂与纯In_2O_3和纯CdS相比,不仅光响应范围增加,而且光催化亚甲基蓝(MB)的活性也得到显著改善。当In_2O_3/CdS中n(In_2O_3)∶n(CdS)=1∶4时,光催化效率改善尤为明显,当复合催化剂的质量为0.05 g时,MB转化率达到96.2%;这可能是由于CdS接受In_2O_3表面上的光生电子,减少了光生电子与空穴的复合机会,因而提高了光催化降解能力。     

Thermoelectric properties of Sb-doped tin oxide by a one-step solid-state reaction

Recently, oxide-based materials have proven to be potential thermoelectric materials at high temperatures. In this work, the thermoelectric properties of one-step solid-state sintered Sn_1-xSb_xO₂ (x = 0, 0.005, 0.01, 0.02, 0.03, 0.04) ceramic pellets were investigated in detail. It was confirmed that the addition of Sb significantly alters the thermoelectric properties of SnO₂ due to the increase in the carrier concentration, which increases the electrical conductivity. The Seebeck coefficient values of all the solid solutions were negative, which indicates that these samples have n-type conduction. The thermoelectric performance of the material was evaluated by determining the zT value and the best composition was Sn_0·98Sb_0·02O₂ with zT ～0.06 at 1073 K.     

Thermoelectric properties of La3+ and Ce3+ co-doped CaMnO3 prepared by tape casting

The structural and thermoelectric transport properties of Ca_0.9La_0.1–xCe_xMnO_3–δ (x = 0 – 0.1) prepared through tape casting process were investigated. The thermoelectric transport properties of CaMnO₃ were optimized by the La³⁺ and Ce³⁺ co-doping and tape casting process. The Ce³⁺ substitution substantially enhanced the dimensionless figure-of-merit (ZT) due to the increases in electrical conductivity and absolute Seebeck coefficient. Of the studied samples, Ca_0.9La_0.025Ce_0.075MnO_3–δ had the maximum ZT (0.17) at 800 °C, and this value was 42% and 70% larger than those of singly doped Ca_0.9La_0.1MnO_3–δ (0.12) and Ca_0.9Ce_0.1MnO_3–δ (0.10), respectively. We demonstrate that both the La³⁺ and Ce³⁺ co-doping and tape casting process are highly promising approaches in enhancing the high-temperature thermoelectric transport properties of CaMnO₃.     

Thermoelectric properties of Cu2Sex prepared by solution phase methods and spark plasma sintering

Cu-Se compounds have attracted more attentions as a low-cost, low toxicity thermoelectric materials. In this work, p-type Cu₂Se_x compound powders were prepared by solution syntheses, including the wet chemistry method and hydrothermal synthesis method. The reaction mechanisms were investigated, showing that the NaOH contents affected the phase structure of the final product. Polycrystalline Cu₂Se_x bulk materials were obtained by densifying the powders using spark plasma sintering (SPS) and obtained the maximum ZT value of 0.75 at 623 K, which is currently the highest reported value for the Cu-Se system at this temperature. We also found that the introduced oxides could suppress the long-range migration of Cu ions in Cu-Se system to avoid the decomposition of the materials.     

Thermoelectric properties of plasma sprayed of calcium cobaltite (Ca2Co2O5)

The thermoelectric properties of calcium cobaltite deposits produced by the plasma spray process are investigated from room-temperature to 873 K. Synthesis of Ca₃Co₂O₆ and Ca₂Co₂O₅ powders were prepared by the solid-state reaction from CaO and CoO_x starting powders. During their subsequent plasma spray Ca particles experience preferential evaporation within the plasma, resulting in a complex interplay among process conditions, stoichiometry, and resultant phases. The as-sprayed material predominantly contains amorphous and secondary phases. Upon annealing, the deposits show sensitivity to phase evolution and therefore thermoelectric properties. Through screening studies, optimal annealing conditions were identified which show a p-type Seebeck coefficient value of 180 μV K^?1, electrical conductivity of 1.09 × 10⁴ S m^?1, thermal conductivity of 1.16 W m^-1 K^-1 at 873 K. The resultant figure of merit value reached 0.266 for this combination of processing and thermal treatment and is in line with data reported from other techniques for this system.     

Preparation of In0.5Sc1.5Mo3O12 nanofibers and its negative thermal expansion property

Orthorhombic In_0.5Sc_1.5Mo₃O₁₂ nanofibers were prepared by electrospinning followed by a heat treatment. The effects of post-annealing temperatures on the phase composition, microstructure and morphology were investigated by XRD, SEM, HRTEM and XPS. Negative thermal expansion (NTE) behaviors of the In_0.5Sc_1.5Mo₃O₁₂ nanofibers were analyzed by high-temperature XRD. Results indicate that the as-prepared In_0.5Sc_1.5Mo₃O₁₂ nanofibers show an amorphous structure with smooth and homogeneous shape. The average diameter of the as-prepared In_0.5Sc_1.5Mo₃O₁₂ nanofibers is around 515 nm. Well crystallized orthorhombic In_0.5Sc_1.5Mo₃O₁₂ nanofibers could be prepared after post-annealing at 550 °C for 2 h with an average diameter of about 192 nm. The crystallinity of In_0.5Sc_1.5Mo₃O₁₂ nanofibers gradually improved with the increase of annealing temperature. However, too high post-annealing temperature leads to a damage of sample's fiber structure. The high-temperature XRD results reveal that In_0.5Sc_1.5Mo₃O₁₂ nanofibers show an anisotropic NTE, and the coefficients of thermal expansion (CTEs) along a-axis and c-axis were −5.95 × 10⁻⁶ °C⁻¹ and -3.54 × 10⁻⁶ °C⁻¹, while the one along b-axis is 5.61 × 10⁻⁶ °C⁻¹. The volumetric CTE of In_0.5Sc_1.5Mo₃O₁₂ nanofibers is −3.90 × 10⁻⁶ °C⁻¹ and the linear one is 1.3 × 10⁻⁶ °C⁻¹ in 25–700 °C.     

Thermoelectric properties of Al and Mn double substituted ZnO

The effects of Al and Mn single and double substitution on structure, composition, and thermoelectric properties of ZnO have been investigated in three series of compounds; Zn_1−xAl_xO, Zn_1−xMn_xO (x=0,0.02,0.04,0.06,0.08) and Zn_1−2xAl_xMn_xO (x=0,0.01,0.02,0.03,0.04) prepared by thermal decomposition method. While the lattice structure is not affected by the substitutions, properties of the material are. Al and Mn have opposite effects on electrical conductivity and Seebeck coefficient of ZnO. Al substitution leads to an increase in electrical conductivity while Mn substitution increases absolute value of Seebeck coefficient. Double substituted samples seem to exhibit the effects from both ions though the increase in absolute value of Seebeck coefficient is less significant comparing to that observed in Mn single substituted samples. Nevertheless, the change in electrical conductivity is more pronounced and dominant in the power factor calculation. Thus the most conductive sample in this work, Zn_0.98Al_0.02O, shows the highest power factor of 1.03×10⁻⁴ WK⁻² m⁻¹at 800 K. The best double substituted sample is Zn_0.98Mn_0.01Al_0.01O which gives a power factor of 4.79×10⁻⁵ WK⁻² m⁻¹ at the same temperature.     

Ni0.5Zn0.5Fe2O4-dispersed In2O3-spotted ZnO nanoparticles: Ammonia-source and surface and photocatalytic properties

Ammonia-source, used to attain the desired pH during synthesis, is conceived to influence the physical characteristics of ZnO-based nanomaterials, and the catalytic activity is susceptible to surface characteristics of semiconductor–photocatalyst. In this context, Ni_0.5Zn_0.5Fe₂O₄-dispersed In₂O₃-spotted ZnO nanoparticles have been obtained by using either tetramethyl ammonium hydroxide or ammonium carbonate as ammonia-source at identical pH (9) using identical quantities of the precursors following identical synthetic procedure. The nanoparticles have been characterized using energy dispersive X-ray spectroscopy, elemental mapping, selected area electron and X-ray diffractometries, transmission electron microscopy, etc. The nanoparticles obtained using ammonium carbonate possess larger (1) pore width, (2) pore volume, and (3) surface area compared with nanoparticles prepared employing tetramethyl ammonium hydroxide. Although the electrical properties of both the samples do not differ remarkably, the violet light-absorption of the sample prepared using the carbonate is slightly larger than that of the other sample. Further, the In₂O₃-spotting is slightly larger on using ammonium carbonate than using tetramethyl ammonium hydroxide. To degrade dye under visible light, the sample obtained using ammonium carbonate shows larger catalytic activity compared with nanoparticles prepared using tetramethyl ammonium hydroxide. The observed photocatalytic activities are explained based on the surface characteristics.     

Thermoelectric properties and thermal stability of metal-doped cuprous sulfide thermoelectrics

Mn doping and S-evaporation are strategies used to improve the thermoelectric properties and thermal stability of cuprous sulfide thermoelectric materials. Cu_1.8S and Mn-alloyed Cu_1.8S powders were prepared via ball milling, and different samples were obtained via current-assisted sintering at different times. It was found that Mn and S-evaporation optimized the carrier concentration and thus improved the figure of merit (ZT) of the samples. The introduction of pore defects induced by S-evaporation also improved the ZT. The maximum ZT of the optimized sample reached 0.89 at 500 °C. Mn in the samples reacted with oxygen to form an oxide film on the surface of the block, which inhibited the kinetic process of Cu_1.8S decomposition and improved the thermal stability of the samples. However, the reaction between Mn and oxygen led to a continuous loss of metal cations in the material, resulting in changes in the thermoelectric properties.     

Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl₂O₄) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl₂O₄ spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl₂O₄ spinel ceramics have been successfully fabricated by co-doping CeO₂ and La₂O₃ via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO₂ and La₂O₃ on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl₂O₄ spinel ceramics were systematically investigated. The results show that by co-doping CeO₂ and La₂O₃ can increase the defect concentration due to the lattice distortion. This could promote the movement of Al³⁺ and Mg²⁺ at high temperature, which is beneficial to the formation of more secondary MgAl₂O₄ spinel. t-ZrO₂ with more Ce⁴⁺ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO₃ that was mainly distributed along the grain boundary of the MgAl₂O₄ phase, both of which were favorable for the formation of dense microstructure of MgAl₂O₄ spinel materials. At the same time, the formation of more secondary MgAl₂O₄ spinel and sintering densification also improve the mechanical properties of spinel ceramics. La³⁺ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO₂ and 3 wt% La₂O₃ co-doping, the bulk density of the sample increased from 3.02 g∙cm⁻³ to 3.55 g∙cm⁻³; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.     

Realizing enhanced thermoelectric properties in Cu2GeSe3 via a synergistic effect of In and Ag dual-doping

In this work, we propose a modulation doping strategy for simultaneous achievement of low lattice thermal conductivity and high Seebeck coefficient in the Cu₂GeSe₃ compound. The Ag and In dual-doping can optimize the hole carrier concentration to balance electrical conductivity and Seebeck coefficient, achieving a high power factor of ～6.4 μW cm^?1 K^?2 for the Cu₂GeSe₃ compound. The Ag point defect makes a great contribution to blocking the propagation of phonons besides the phonon-phonon Umklapp process, yielding a minimum lattice thermal conductivity of ～0.38 W m^–1 K^–1. Remarkably, a maximum ZT value of ～0.97 at 723 K is achieved for Cu_1.8Ag_0.2Ge_0.95In_0.05Se₃ compound, which is the highest value for the Cu₂GeSe₃-based systems in the temperature range of 323–723 K.     

Luminescent properties and LED Application of Broadband Near-Infrared Emitting NaInGe2O6: Cr3+ phosphors

A NIR-emitting Cr³⁺-activated phosphors (NaInGe₂O₆: Cr³⁺) covering whole NIR-I region (700–1200 nm) were successfully designed and prepared via solid-state reaction. XRD and Rietveld refinement verified that the octahedral In³⁺ site is the preferred site of Cr³⁺ substitution in NaInGe₂O₆ structure. The synthesized NaInGe₂O₆: Cr³⁺ phosphors exhibit two strong absorption bands at 480 and 700 nm, and show a mountain-like single-band emission at 900 nm with FWHM = 175 nm. The crystal field parameters are calculated using steady-state spectral data, in which a low Dq/B value of 1.89 is obtained and results in this broadband NIR emission. NaInGe₂O₆: Cr³⁺ exhibits good emission thermal stability, i.e. 55 % of room temperature intensity at 373 K. Besides, an efficient NIR pc-LED is fabricated and shows NIR output of 25.2 mW@120 mA. This broadband NaInGe₂O₆: Cr³⁺ NIR phosphor could be merged into pc-LED package for hand-held spectrometers, security cameras and vivo biomarkers.     

蠕虫孔状介孔Cr_2O_3的制备与表征

以硝酸铬为金属源,活性炭为硬模板,用真空辅助浸渍法制备了蠕虫孔状介孔Cr_2O_3,采用XRD、BET、TEM、TPR和UV-Vis对样品的物化性能进行了表征。结果表明,制备的介孔Cr_2O_3是具有六方晶型和比表面积高达112m^2·g^(-1)的晶体,平均孔径约6.5nm,与体相材料相比,介孔Cr_2O_3表现出良好的低温还原性能和吸光性能,这与其表面形貌和晶体结构有关。     

Combustion synthesis of SiC-MoSi2-Al2O3 composites

Combustion synthesis of SiC-MoSi₂-Al₂O₃ composites by using highly exothermic reactive system of MoO₃-Al-Si-SiO₂ was investigated. It has been shown that, by using the combustion synthesis method, it is feasible to prepare, at 1500 °C, an electrical heating element with room temperature resistivity and element watt loading of 12 Ω cm and 20 W/cm² respectively. The colloid containing nano-sized material 30 wt% SiO₂, was used as the plasticizer medium as well as combustion source. The β-SiC functioned as a filler material to control the combustion reaction and also constituted part of the final product. The amount of SiC additive varied in the range from 45% to 65% wt. and was optimized with respect to porosity level, reaction conversion and net-shape quality of the heating elements. The element containing about 60% wt. SiC reveals low porosity level, with full conversion for the reaction and superior net-shape quality. However, despite the relatively high level of porosity present in the final synthesized product, it can be used as an electrical heating element in accordance with the investigation at the element temperatures up to 1500 °C.