Co2O3的理化性能对ZnO压敏陶瓷电性能的影响

本文详细研究了氧化钴的理化性能对ＺｎＯ压敏陶瓷性能的影响。论述了Ｃｏ２Ｏ３的热特性对压敏陶瓷烧成工艺产生的影响及控制措施，从理论上探讨了氧化 中所含杂质Ｆｅ、Ｃｕ，Ｎａ等对ＺｎＯ压敏陶瓷电性能的不利作用。提出了控制的上限，并试制出一种用于氧化锌压敏陶瓷的性能优良的Ｃｏ２Ｏ３粉体。     

Influence of processing parameters on the densification and the microstructure of pure zinc oxide ceramics prepared by spark plasma sintering

Due to the sensitivity of nanopowders and the challenges in controlling the grain size and the density during the sintering of ceramics, a systematic study was proposed to evaluate the densification and the microstructure of ZnO ceramics using spark plasma sintering technique. Commercially available ZnO powder was dried and sintered at various parameters (temperature (400–900?°C), pressure (250–850?MPa), atmosphere (Air/Vacuum) etc.). High pressure sintering is desirable for maintaining the nanostructure, though it brings a difficulty in obtaining a fully dense ceramic. Whereas, increasing the temperature from 600 to 900?°C results in fully densified ceramics of about 99% which shows to have big impact on the grain size. However, a high relative density of 92% is obtained at a temperature as low as 400?°C under a pressure of 850?MPa. The application of pressure during the holding time seems to lower the grain size as compared to ceramics pressed during initial stage (room temperature).     

Microwave-sintering preparation and densification behavior of sodium zirconium phosphate ceramics with ZnO additive

The dense NaZr₂(PO₄)₃ (NZP) ceramics were successfully prepared by a microwave sintering process with x wt.% ZnO as a sintering aid (where x = 0, 0.5, 1.0, 2.0 and 3.0). The effects of ZnO additive on the crystal structure, microstructure and sintering behavior of as-prepared NZP ceramics were systematically investigated. The single NZP phase can be achieved in the wide temperature range of 900–1200 °C holding for 2 h by microwave-sintering technology. The addition of ZnO sintering aid would not noticeably change the crystal structure of NZP. Importantly, ZnO additive significantly promoted the densification and 1.0 wt% ZnO-added sample possessed the maximum relative density of 96.5% after sintering at 1100 °C, considerably higher than that of pure NZP sample. Besides, the Vickers hardness of the above sample could attain near 800 MPa, which is about four times as hard as the pure NZP ceramics without ZnO additive. It was suggested that the combination of microwave sintering with appropriate addition of ZnO sintering aid would provide a convenient and efficient method for rapid fabrication of dense NZP ceramics.     

Cold sintering ZnO based varistor ceramics with controlled grain growth to realize superior breakdown electric field

Controlling the grain growth and grain boundary morphology is of great importance in the manipulation of electrical properties of electro-ceramics. However, it has been a challenge to achieve dense varistor ceramics with grain sizes in submicrons and nanometers using conventional thermal sintering at high temperatures. Here we present a strategy to fabricate dense ZnO based ceramics with controlled grain growth and thin grain boundaries using cold sintering process (CSP). With CSP, the sintering temperature of ZnO based ceramics dramatically drops from 1100 °C to 300 °C. The Bi₂O₃, Mn₂O₃, and CoO dopants suppress the grain growth of ZnO under CSP conditions, and Bi-rich intergranular films (2?5 nm) can be observed along grain boundaries. The cold sintered ZnO-Bi₂O₃-Mn₂O₃-CoO ceramic shows a non-linear coefficient of 33.5, and a superior breakdown electric field of 3550 V/mm. This work thus demonstrates that CSP is a promising technique for designing new submicron-/nano-ceramics with superior performances.     

Cold sintering process of ZnO ceramics: Effect of the nanoparticle/microparticle ratio

We show the efficiency in the preparation of >95 % dense ZnO ceramics by cold sintering process through the incorporation of ZnO nanoparticles in the 1−10 wt% range at temperatures of 170 °C, pressures of 750 MPa and a pellet height/diameter ratio of 0.38. Morphological, structural and physical properties are dependent on the amount of ZnO nanoparticles incorporated into the system. After the densification by cold sintering process, ZnO ceramics show a reduction of the average valence indicating the deficiency of oxygen, similar to ceramics sintered by the conventional route. Besides, the generation of structural disorder and modifications into the ZnO lattice are identified in sintered ceramics, inducing intrinsic defects related to the loss of oxygen ions, the diffusion of zinc and zinc vacancies, which depend on the sintering process and the starting powders. These characteristics influence the final functional properties of the sintered ZnO ceramics, such as the visible photoluminescence signal.     

Domain structure and evolution in ZnO-modified Pb(Mg1/3Nb2/3)O3–0.32PbTiO3 ceramics

The doping of ZnO is efficient to improve the piezoelectric property and thermal stability of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ (PMN–PT) based ceramics. However, the underlying physics, especially the local domain structures of the ZnO modified PMN–PT ceramics, which is strongly associated with the electric properties, is not clear yet. In this paper, we investigated the local domain structures and their evolution as a function of x in PMN–0.32PT:xZnO ceramics. It was found that, the domain evolution is mainly caused by the growth of grain size induced by the sintering aiding effect of ZnO at x < 0.04, and the domain evolution can be attributed to the phase transition induced by the partial replacement of Mg²⁺ by Zn²⁺ in the B-site of PMN–PT lattice at x > 0.06. Furthermore, we also investigated the domain structure evolution as functions of temperature and local external electric field in PMN–0.32PT:0.06ZnO ceramics, which exhibited superior piezoelectric property relative to other compositions. We found that the irregular nanodomains are more stable at high-temperature range, and the regular non-180° domains exhibited more complex rotation behavior under local electric field, which probably leads to the thermal stability and piezoelectric property enhancement in the ZnO-modified PMN–0.32PT ceramics.     

ZnO和Na2O对CaO-B2O3-SiO2介电陶瓷结构与性能的影响

研究了烧结助剂ZnO和Na2 O对CaO -B2 O3 -SiO2 (CBS)系微波介质陶瓷介电性能、相组成及结构特性的影响。烧结助剂ZnO在烧结过程中与B2 O3 及SiO2 生成低熔点玻璃相 ,有效地降低了材料的致密化温度 ,烧结机理为液相烧结。碱金属氧化物Na2 O虽然能够有效降低材料的烧结温度 ,但会破坏硅灰石晶体结构 ,引起材料微波性能显著降低。通过实验 ,制备出了具有优良微波介电性能的陶瓷材料 ,适用于LTCC基板及滤波器等高频微波器件的生产     

Near-net-size preparation of porous mullite matrix ceramics with in situ formed 3D mullite whisker network

Porous mullite matrix ceramics have excellent thermal and mechanical properties suitable for applications such as in thermal insulation. However, their applications are limited by processing defects from nonuniform sintering shrinkage and the trade-off between high porosity (preferred for low thermal conductivity) and high mechanical strength. Herein, we seek to minimize the sintering shrinkage by near-net-size preparation and improve the strength by in situ formed whisker network structure. Gelcasting forming technology and pressureless sintering were used to prepare porous mullite matrix ceramics using kyanite and α-Al₂O₃ powders as the starting materials and using MoO₃ to promote the growth of mullite whiskers. The results showed that the sintering shrinkage could be compensated by the volume expansion from solid-state reaction during reaction sintering. The in situ formed three-dimensional (3D) whisker network further reduced sintering shrinkage and effectively improved the strength of the ceramics. An ultralow sintering shrinkage of .78% was achieved. The near-net-shape porous mullite matrix ceramics strengthened by 3D whisker network had a high porosity of 63.9%, a high compressive strength of 83.8 MPa and a high flexural strength of 53.5 MPa.     

Layered ceramics based on InGaO3(ZnO)2: Preparation and experimental investigation of high-temperature heat capacity and thermal conductivity

This paper reports on a method for producing ceramics from a high-purity, submicron InGaO₃(ZnO)₂ powder synthesised using a PVA-assisted gel combustion method, as well as an experimental study of the thermophysical properties of the ceramic materials obtained. The platelet-like crystallites of the InGaO₃(ZnO)₂ obtained were several microns long and up to several hundred nanometres thick. Layered ceramics obtained by sintering compacted InGaO₃(ZnO)₂ powders at temperatures of 1373–1773 K had a bulk density that was 68–96 % of the theoretical density. The temperature dependence of heat capacity in the range 306–1346 K was studied experimentally for InGaO₃(ZnO)₂ using the DSC method. It was found that, in the range 323–1173 K, layered InGaO₃(ZnO)₂ ceramics had a low thermal conductivity, which decreased from 2.0–1.3 W/(m K. The results obtained make it possible to consider this material as a promising thermal barrier coating.     

Varistor Performance of ZnO-Based Ceramics Related to Their Densification and Structural Development

Electrical potential barriers are often observed in ZnO-based ceramics. Earlier studies on ZnO photoconduction have shown that the narrow regions, where the sintered grains have grown together, control the resistance of the entire sample. In those regions, the surface/volume ratio is sufficiently high for the acceptor concentration (which occurs because of adsorbed oxygen) to exceed the donor concentration inside the ZnO grains. More recent works have shown that Schottky barriers result from interface states because of the chemisorbed oxygen ion at the ZnO-ceramic grain boundaries. The work reported in this paper involves the relationship between the densification of the microstructure and the varistor performance of ZnO ceramics. The emphasis of densification percentage as an indicator of the degree of sintering shows the desirability of continuity across ZnO grain boundaries, without the presence of voids or films of second phases, in optimizing varistor behavior. The effect of oxygen partial pressure on the development of varistor microstructure and electrical properties, as well the kinetics of grain growth, during the sintering process have been determined and are discussed.     

A1N陶瓷低温烧结过程中氧化控制与性能研究

采用纳米级的A1N粉并以Y2O3-CaF2作烧结助剂于1600℃下制备A1N陶瓷,对AlN陶瓷物相组成、相对密度、微观结构和热性能进行了表征,针对A1N陶瓷烧结过程中易氧化的问题,分析了氮化铝陶瓷在烧结过程中氧化的机理,提出了防止A1N陶瓷制备过程中氧化的措施。研究表明：将A1N坯体置于含有一定量碳粉的A1N埋粉中于N2气氛下烧结,生成还原性气体CO,有效避免了A1N烧结过程中的氧化问题。其中添加3wt％Y2O3-2wt％CaF2作烧结助剂,1600℃常压条件下制备了高热导率的致密A1N陶瓷。     

Structure and enhanced thermoelectric properties of InGaO3(ZnO)m (m=1, 2, 3, 4, and 5) ceramics

InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics are a series of n-type oxide thermoelectric materials with layered structures and low thermal conductivities. Herein, InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics were fabricated by spark plasma sintering (SPS). Two different trends in the thermoelectric properties of the InGaO₃(ZnO)_m (m = 1, 2, 3, 4, and 5) ceramics were observed depending on the odevity of the m value. The InGaO₃(ZnO) sample exhibited a relatively high electrical conductivity and was therefore selected for vacuum annealing to further improve the electrical transport performance. Oxygen vacancy defects were introduced to the matrix during the annealing procedure, which improved the thermoelectric performance. A maximum ZT of 0.45 was obtained at 973 K for the InGaO₃(ZnO) sample with a 96 h vacuum annealing treatment, which is 30 times higher than that of the pristine sample.     

Al-doped ZnO varistors prepared by a two-step doping process

ABSTRACT

It is difficult to dope Al into main grains of ZnO varistor ceramics, especially for small doping amount. Generally, all raw materials including Al dopant are directly mixed together and sintered into ceramics. However, in this direct doping process, Al is apt to stay in grain boundaries, and almost does not enter grains. This does harm to the electrical properties of ZnO varistors. In this paper, we proposed a two-step doping process. Al₂O₃ powder was first mixed only with a part of the ZnO powder and pre-sintered. The pre-sintered powder was mixed with other additives such as Bi₂O₃ and the rest ZnO. Then ZnO varistor ceramics were prepared via solid state sintering processes. Results showed that two-step doped ZnO varistors exhibited improved electrical properties with a significant increased nonlinear coefficient and a great decreased leakage current compared to directly doped ones because more Al was incorporated into ZnO grains.     

Synergistically optimizing thermoelectric performance of ZnO ceramics by interfacial band alignment and self-doping defects

ZnO is a promising thermoelectric ceramic material due to non-toxicity and abundance in resources. However, its thermoelectric performance is limited by the intrinsic low carrier concentration and high thermal conductivity. In this work, we synthesized the (1 ? x)ZnO/xZnS (x = 0–0.05) powders by a two-step solution method followed by microwave sintering in an oxygen-deficient environment at 1000 ℃, and then produced the self-doped ZnO ceramics with ZnO/ZnS interfaces. The electrical and thermal properties was investigated from room temperature to 900 K. The ZnO/ZnS interface and self-doping significantly increased the electrical properties of ZnO ceramics, the electrical conductivity (σ) and Seebeck coefficient (α) increased simultaneously with temperature for (1 ? x)ZnO/xZnS (x > 0), and the highest power factor (PF, 3675 µW·m^?1·K^?2) was obtained from 0.98ZnO/0.02ZnS at 900 K. At the same time, the ZnO/ZnS interfaces and self-doped defects greatly reduced the lattice thermal conductivity. Finally, the highest ZT value of 0.94 has been reached in 0.95ZnO/0.05ZnS at 900 K.     

低压ZnO压敏陶瓷晶粒边界电子陷阱态的研究

应用４１９２Ａ低频阻抗分析仪测量了低压ＺｎＯ压敏陶瓷的复电容曲线,研究了烧结温度对复电容曲线压低角的影响规律,探讨了晶粒边界耗尽层中电子陷阱的种类和起因并对电子陷阱的特征参数进行了表征．实验发现：在低压ＺｎＯ压敏陶瓷中存在两种非理想的Ｄｅｂｙｅ弛豫现象,对于每一种弛豫,其复电容曲线存在压低现象,且其压低角随着烧结温度的升高而迅速减小．在低压ＺｎＯ压敏陶瓷的耗尽层区域内存在两种电子陷阱,它们分别位于导带底０．２０９和０．３４２ｅＶ处,其中０．２０９ｅＶ处的陷阱能级对应于本征施主Ｚｎ×ｉ的二次电离,而０．３４２ｅＶ处的陷阱能级对应于氧空位ＶＯ的一次电离．     

Microwave sintering of nano-sized ZnO synthesized by a liquid route

Zinc oxide is a widely used material in various applications in electronic, optic, and spintronic fields, in particular. The control of the final properties of ZnO requires the mastering of the final microstructure. To achieve this goal, the grain growth of ZnO has been examined as a function of the sintering conditions, in particular in using a specific microwave sintering method. In order to get nano-sized ZnO powder as a starting material, a liquid route was implemented. The latter is based on the direct precipitation of a zinc oxalate solution. After thermal treatment, pure ZnO powder was obtained with a very narrow grain size distribution, centered at around 20 nm. The sintering of this powder was then carried out in conventional and microwave furnaces. While an important grain growth occurs during the conventional sintering, it is shown that microwave sintering allows us to maintain the grain size at the nano-metric scale.     

Preparation of high density ZnO ceramics by the Cold Sintering Process

High-density ZnO ceramics were prepared by a Two-Step Cold Sintering Process (TS-CSP) at an ultralow-temperature. The densification process of ZnO ceramics was demonstrated by TS-CSP. And the density, microstructure and electrical properties of the ZnO ceramics were investigated. The results indicate that most of the crystallization of ZnO ceramic can be completed in the first step of sintered at 150 °C/200 MPa for 30 min, with a relative density approaching 97.36%. This relative density was further improved to 99.43% after the second sintering step was deployed at 200 °C/200 MPa for 30 min, which displayed an average grains size of 1.5 μm and resistivity of 0.125 Ω cm. This work demonstrated an effective method to reduce CSP temperature and pressure in the production of high-performance ZnO ceramics.     

Comparing hydrothermal sintering and cold sintering process: Mechanisms,microstructure, kinetics and chemistry

This study reports the sintering of zinc oxide (ZnO) through the comparison between the hydrothermal sintering (HS) and the cold sintering process (CSP) operating in closed and open conditions, respectively. Sintering was performed at 155 ± 5 °C applying a pressure of 320 MPa, and during different holding times (0 min, 20 min, 40 min and 80 min). Whatever the low sintering process used, ceramics characteristics are almost similar in terms of relative densities and ZnO structure. However, several differences such as the nature of stabilized phases, grain sizes and quantities of residual molecules in the densified pellets, were characterized and explained. The formation of zinc acetate “bridges” was observed ex situ in hydrothermally sintered samples. A detailed ReaxFF molecular dynamics simulation was performed to help understand the formation mechanisms of zinc acetate “bridges” and compare the chemical activities between HS and CSP.     

碳化硅陶瓷固相烧结的烧结机理及研究进展简

碳化硅陶瓷材料具有高硬度、高强度、抗氧化、耐高温、高热导率、低线胀系数等优良性能,同时具有优良的化学稳定性且能够耐大多数种类的酸碱溶液腐蚀,在石油、化工、建材、航空、机械等诸多领域得到了广泛应用。本文主要阐述了碳化硅陶瓷固相烧结的烧结机理,并对目前国内外关于碳化硅陶瓷固相烧结的研究进展进行了阐述。     

Pressure-assisted flash sintering of ZnO ceramics

Implementing pressure-assisted flash sintering of ZnO powder without pretreatment by a new experimental configuration is presented. Rapid and energy-concentrated heating of electrode-sample-electrode area by induction heating allows preheating and flash sintering of loose-pack powder in the die with pressure assistance. Using an insulated die enables the current to flow through the sample during flash sintering. ZnO ceramics with a relative density of 95.1% can be achieved in less than 3 min. The whole process includes 104 s of preheating by a low-power induction heating device and 30 s of flash sintering assisted by a pressure of 26 MPa using the pulsed direct current (DC). The process characteristics of pressure-assisted flash sintering using the pulsed DC are discussed. The effect of pressure on densification and grain size is analyzed in detail, and some potential mechanisms are provided.