锌硼玻璃掺杂低压ZnO压敏电阻电性能及晶粒生长动力学研究

研究了锌硼玻璃掺杂量对低压ZnO压敏电阻微观结构和电性能的影响.结果表明,当掺杂量x=0.1wt%时,可以得到较好综合性能的ZnO压敏电阻:E1mA=36.7V/mm,α=30.4,IL=0.1μA.并应用晶粒生长动力学唯象理论研究了锌硼玻璃掺杂低压ZnO压敏电阻的晶粒生长规律,探讨了锌硼玻璃掺杂对低压ZnO压敏陶瓷晶粒生长的作用机理.当烧结温度T≤1000℃时,其晶粒生长动力学指数n≈4.54,激活能Q≈316.5kJ/mol,这是由于未熔融的锌硼玻璃通过颗粒阻滞机理阻碍了ZnO压敏陶瓷晶粒的生长;而当T1000℃时,其晶粒生长动力学指数n≈2.92,激活能Q=187kJ/mol,这是由于熔融的锌硼玻璃通过液相烧结机理促进了晶粒的生长.     

液相烧结大晶粒UO2及其烧结动力学分析

大晶粒二氧化铀(UO₂)芯块因具有低辐照肿胀、低裂变气体释放及优异的燃料包壳相互作用效应(PCI),而成为未来先进反应堆关注的候选燃料。本文采用液相烧结工艺制备大晶粒UO₂芯块，研究了液相烧结对UO₂芯块烧结性能、显微结构和烧结动力学的作用机制和影响规律。结果表明：通过添加金属铀粉形成的液相烧结可明显促进UO₂芯块的致密化速度，液相烧结UO₂芯块的烧结特征指数为2.937,烧结机理主要为晶界扩散，烧结激活能为103.00 kJ/mol,低于普通UO₂芯块的烧结激活能(110.65 kJ/mol);液相烧结UO₂芯块晶粒生长指数为2.831,晶粒生长活化能为463.97 kJ/mol,低于普通UO₂芯块晶粒生长激活能(510 kJ/mol),加入的金属铀粉形成的液相烧结可促使晶粒长大；添加金属铀粉液相烧结工艺具有不引入非铀杂质元素、加快芯块致密化速度和增大芯块晶粒尺寸的多重作用。     

多元纳米复合氧化锌压敏陶瓷的晶粒生长

研究了多元纳米复合ZnO电压敏粉体在高温下的烧结行为.应用晶粒生长的动力学方程Gn-G0=K0texp(-Q/RT),确定了晶粒生长的动力学指数n和激活能Q.实验结果表明,随着烧结温度的提高和保温时间的延长,ZnO压敏陶瓷的晶粒不断长大,其动力学指数n=3.2,激活能Q=(185±28)kJ/mol.     

氧化锌压敏电阻微波烧结行为的研究

采用微波和传统烧结工艺制备了ZnO压敏电阻,比较了微波和传统烧结ZnO压敏电阻的相组成、表面微观结构和电性能,探讨了烧结温度和保温时间对微波烧结样品的致密化和电性能的影响.与传统工艺相比,微波烧结工艺明显改善了ZnO压敏电阻的致密化行为,缩短了烧结周期,改善了电性能.优化的微波烧结样品的压敏电压U1mA为521.8V,非线性系数α是61.4,漏电流IL为1.25×10-6A,残压比Kr为1.45,通流量Im达11600A,均达到或超过了传统工艺水平.微波烧结样品的通流量Im更是比传统烧结样品高约50%.     

纳米氧化锌半导体块材晶粒生长的研究

以平均粒径20nm的ZnO超微粉为原料,研究了纳米ZnO块材烧结过程中的晶粒生长行为,由实验结果得出在700～900℃温度范围内,纳米ZnO烧结的晶粒生长动力学指数n为6,晶粒生长的表观活化能Q为64kJ/mol,导出了纳米ZnO的晶粒生长动力学方程.与粗晶ZnO的晶粒生长进行了对比,初步分析了纳米ZnO的烧结机制.     

ZnO压敏陶瓷的晶粒生长和电学性能

研究了不同价态Co、Mn添加物对低压ZnO压敏陶瓷晶粒生长和电学性能的影响,分析了由于不同价态Mn和Co掺杂所产生的缺陷类型,应用晶粒生长的动力学方程：Gⁿ=Dtexp(-E/RT)确定了晶粒生长的动力学指数和激活能．实验结果表明：对于低压ZnO压敏陶瓷,其晶粒生长的动力学指数n＝6,激活能E＝224±17kJ／mol,随着Mn、Co价态的增加,ZnO压敏陶瓷的平均晶粒大小增加,提高烧结温度,ZnO压敏陶瓷的压敏场强E1mA降低,漏电流IL增加,非线性系数α降低．在低压ZnO压敏陶瓷的制备过程中,烧结温度以不超过1250℃为宜．     

Al2O3掺杂ZnO压敏陶瓷的晶粒生长研究

研究了Al2O3掺杂对ZnO压敏陶瓷晶粒生长规律的影响,应用晶粒生长动力学方程确定了晶粒生长的动力学指数和激活能.实验结果表明,对于Al2O3掺杂ZnO压敏陶瓷,其晶粒生长的动力学指数n等于4,激活能Q等于(400±26)kJ/mol.Al2O3掺杂ZnO压敏瓷的晶粒生长机理是ZnAl2O3尖晶石颗粒在ZnO压敏瓷晶粒边界钉扎过程中Al3+和O2-通过ZnAl2O4尖晶石的扩散.     

稀土氧化物Pr2O3掺杂对高压ZnO压敏电阻性能的影响

采用低温固相化学反应法制备了Pr₂O₃掺杂的ZnO纳米复合粉体, 并用此粉体在不同烧结温度下制备了高压ZnO压敏电阻。采用X射线衍射、 比表面测试、 透射电镜、 扫描电镜等手段对制备的ZnO纳米复合粉体及高压ZnO压敏电阻进行了表征, 并与未掺杂ZnO压敏电阻进行了对比研究, 探讨了稀土氧化物Pr₂O₃掺杂对高压ZnO压敏电阻电性能的影响机制。结果表明: 较低的烧结温度(1030～1130 ℃)时, 掺杂的稀土氧化物Pr₂O₃偏析于ZnO晶界中, 有活化晶界、 促使晶粒生长的作用; 同时, Pr₂O₃掺杂导致1080 ℃烧结的ZnO压敏陶瓷体中晶体相互交织形成晶界织构, 比未掺杂的更均匀和致密, 这有助于高压ZnO压敏电阻晶界性能的改善, 从而提高其综合电性能。当烧结温度为1080 ℃时, Pr₂O₃掺杂的高压ZnO压敏电阻的综合电性能最佳: 电位梯度为864.39 V/mm, 非线性系数为28.75, 漏电流为35 μA。     

铋硼玻璃掺杂对ZnO-Bi_2O_3-TiO_2系压敏电阻性能影响

研究了铋硼玻璃掺杂对ZnO-Bi_2O_3-TiO_2系压敏电阻微观结构和非线性特性的影响.由SEM结果可知:在900℃烧结时,低熔点铋硼玻璃通过液相烧结机制能够促进氧化锌晶粒生长和提高晶粒分布的均匀性;然而,未熔融的锌硼玻璃通过颗粒阻滞机理阻碍了氧化锌压敏陶瓷晶粒的生长.当铋硼玻璃掺杂量为2wt%时,可以得到最佳非线性特性:电位梯度E_(1mA)=124.9V/mm,非线性系数a=46.2,漏电流密度J_L=0.2μA/cm~2.对铋硼玻璃掺杂压敏电阻来说,其晶粒生长动力学指数和激活能Q要远小于锌硼玻璃掺杂压敏电阻,仅为n≈2.15和146.2 kJ/mol.以上分析表明:与锌硼玻璃相比,铋硼玻璃能够更有效的促进氧化锌晶粒生长,改善ZnO-Bi_2O_3-TiO_2系压敏电阻的微观结构,提高非线性特性.     

稀土氧化物Pr2O3掺杂对高压ZnO压敏电阻性能的影响

采用低温固相化学反应法制备了Pr2O3掺杂的ZnO纳米复合粉体,并用此粉体在不同烧结温度下制备了高压ZnO压敏电阻.采用X射线衍射、比表面测试、透射电镜、扫描电镜等手段对制备的ZnO纳米复合粉体及高压ZnO压敏电阻进行了表征,并与未掺杂ZnO压敏电阻进行了对比研究,探讨了稀土氧化物Pr2O3掺杂对高压ZnO压敏电阻电性能的影响机制.结果表明:较低的烧结温度(1030～1130℃)时,掺杂的稀土氧化物Pr2O3偏析于ZnO晶界中,有活化晶界、促使晶粒生长的作用;同时,Pr2O3掺杂导致1080℃烧结的ZnO压敏陶瓷体中晶体相互交织形成晶界织构,比未掺杂的更均匀和致密,这有助于高压ZnO压敏电阻晶界性能的改善,从而提高其综合电性能.当烧结温度为1080℃时,Pr2O3掺杂的高压ZnO压敏电阻的综合电性能最佳:电位梯度为864.39 V/mm,非线性系数为28.75,漏电流为35 μA.     

Effect of microwave sintering on the microstructure and electrical properties of low-voltage ZnO varistors

Coarse-grained ZnO varistors for low-voltage applications were prepared by microwave sintering technique under different soaking times of 5–150?min. For comparison, a low-voltage ZnO varistor was also prepared through a conventional sintering process. Microwave sintering remarkably enhanced the grain growth rate of ZnO varistors. Average grain size of the sample prepared by microwave sintering in 15?min was about 20?µm, which is similar to the grain size of sample prepared conventionally in 150?min time. In addition to grain growth, an increase in microwave sintering time led to precipitation of zinc titanate (Zn₂TiO₄) on the top surface of samples which sintered for long dwell times. X-ray diffraction and scanning electron microscopy results from different points of the samples declared that precipitation of Zn₂TiO₄ phase is due to the high rate of bismuth evaporation of Bi-rich liquid from top surface and the reaction between remaining titanium ions on the surface with ZnO. The results showed that increasing sintering time from 5 to 150?min increased the grain size from 14 to 33?µm, consequently, the breakdown field decreased from 90 to 27?V/mm, respectively. These changes led to a switch in the varistor application, from low to very low voltage.     

Microwave sintering of β-SiAlON–ZrO2 composites

Densification, phase transformation, microstructure evolution and hardness of microwave sintered β-SiAlON–ZrO₂ composites were investigated and compared with conventionally sintered samples. Sintering trials were performed by a high vacuum capable 2.45 GHz microwave furnace without decomposition. Microwave sintered samples showed better densification behavior than conventional sintered samples. The higher density observed in the case of microwave sintered samples was attributed to volumetric fast heating. X-ray diffraction results of conventionally sintered samples showed β-SiAlON, tetragonal ZrO₂ and ZrN phases, while, ZrO₂ reacted with nitrogen and completely transformed to ZrN in the case of microwave sintered samples. The aspect ratios of microwave sintered β-SiAlON grains were higher than conventional sintered samples whereas, hardness remained lower.     

Mass transport via grain boundary in Pr-based ZnO varistors and related electrical effects

Mass transport and the grain boundary formation behaviors in Pr, Co-doped ZnO ceramics are strongly affected by firing atmosphere and temperature. Drastic change of grain boundary morphology and mass transport behavior were observed in the specimens sintered between 1350 and 1500°C which is ascribed to the increase of the vaporization contents of zinc oxide and praseodymium oxide with the formation of liquid phase. The penetration of liquid (ZnO–PrO_x) into the grain boundaries of sintered, Co-doped ZnO pellets resulted in varistors with breakdown voltages per grain boundary in the 1–3 V range and nonlinearity coefficients of 15–58.     

Mechanical properties of hydroxyapatite–zirconia compacts sintered by two different sintering methods

Microwave sintering is traditionally employed to reduce the sintering temperature required to densify powder compacts. The effect of microwave heating on hydroxyapatite (HA)–zirconia (ZrO₂) green bodies has been investigated in order to understand how microwave energy may affect the physical and mechanical properties of the resultant densified composites. Laboratory synthesised nano-sized HA and a commercial nano-sized ZrO₂ powder have been ball milled to create mixtures containing 0–5 wt% ZrO₂ loadings. Compacts were microwave sintered at either 700, 1000 or 1200°C with a 1 h hold time. Comparative firings were also performed in a resistive element furnace using the same heating profile in order to assess the differences between conventional and microwave heating on the physical, mechanical and microstructural properties of the composites. Samples sintered at 700°C show little sign of densification with open porosities of approximately 50%. Composites conventionally sintered at 1000°C were between 65 and 75% dense, whereas the samples microwave sintered at this temperature were between 55 and 65% dense. Samples sintered at 1200°C showed the greatest degree of densification (>80%) with a corresponding reduction in open porosities. TCP generation occurred as a consequence of sintering at 1200°C, even with 0 wt% ZrO₂, and increased degradation of the HA phase to form significant amounts of TCP occurred with increasing additions of ZrO₂, along with increasing open porosity. Nanosized ZrO₂ prevents the densification of the HA matrix by effectively pinning grain boundaries and this effect is more pronounced in the MS materials. Similar strengths are achieved between the microwave and conventionally sintered samples. Greater amount of open porosity and pore interconnectivity are seen in the MS samples, which are considered to be useful for biomedical applications as they can promote osteo-integration.     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

Effect of two-step sintering on rare earth (RE = Y2O3, Pr6O11) doped ZnO–Bi2O3 varistors processed from ‘nano-precursor’ powders

Nano size ZnO–Bi₂O₃ varistor precursor powders containing Y₂O₃ and Pr₆O₁₁ rare earth dopants were prepared by low temperature refluxing at 80 °C. Effect of rare earth dopants, densification by two-step sintering, evolution of microstructures and their influence on varistor properties were investigated. Chemically synthesized nano- precursor varistor powders produced controlled grain size in two-step sintering in which the average sintered ZnO grain size was reduced to at least half compared to the conventionally processed ZnO–Bi₂O₃ varistors. The study revealed that such grain size reduction is highly beneficial to attain enhanced varistor properties.     

Microstructure development in SnO2 with and without additives

The microstructure development in pure and ZnO-, Nb₂O₅- or (ZnO + Nb₂O₅)-containing SnO₂ has been studied. Sintering of pure SnO₂ proceeds by an evaporation-condensation mechanism and grain and pore sizes increase without densification. The pores are open with no closed pores developed. Grain and pore structures become homogeneous with sintering time. Sintered pure SnO₂ is porous and has large grains. Nb₂O₅ suppresses particle coarsening and pore growth but does not promote densification, leading to porous sintered bodies with small grains. ZnO and (ZnO + Nb₂O₅) promote densification but do not suppress particle and grain coarsening. The sintered body is composed of large grains with pores located on grain boundaries.     

Comparison of sintering behavior and piezoelectric properties of (K,Na)NbO3-based ceramics sintered in conventional and microwave furnace

In this study (1 − x) K_0.48Na_0.48Li_0.04Nb_0.96Ta_0.04O₃ − xSrTiO₃ (0.0 ≤ x ≤ 0.10) ceramics were fabricated by sintering in microwave furnace for first time as well as in conventional furnace (either via single step or two-step procedures). Sintering behavior and piezoelectric properties of sintered samples were studied and compared. It was found that two-step sintering decreases sintering temperature effectively and enhances densification compared to single step sintering. Microstructure analysis revealed that, two-step sintering suppresses grain growth and promotes densification. On the other hand, microwave sintering enhanced densification more effectively and reduced sintering time and temperature. The maximum piezoelectric constants of ceramics were measured for those sintered in microwave furnace. Piezoelectric constant of the sample containing 1 mol% SrTiO₃ which was sintered in microwave furnace was measured 310 pC N⁻¹ while by sintering in conventional furnace via single and two-step sintering it was obtained 208 and 278 pC N⁻¹, respectively.     

ZnO压敏陶瓷的微波烧结

对用纳米粉体制备的ZnO压敏生坯进行了微波烧结,通过XRD、SEM分析和电性能测试,与普通烧结比较,微波烧结可使ZnO压敏材料快速成瓷,显著缩短烧结时间;在相同晶粒尺寸下,微波烧结温度更低,瓷体更致密;并能获得较好电性能．微波烧结为ZnO压敏陶瓷材料制备提供了一条新的、高效节能的途径．     

Electronic paramagnetic resonance (EPR) study of the structure of ZnO varistors prepared by various chemical methods

The structure of ZnO varistors prepared by different chemical methods was studied by the electronic paramagnetic resonance (EPR) method. The presence of Mn²⁺ ions in both ZnO lattice and electroconductive phase was used as a sensitive probe for the analyses of structural changes which occur during sintering of ZnO varistors. Potential mechanisms which can contribute to the formation of resonant lines were considered. The concentration of paramagnetic centres was quantitatively analysed. The variation of EPR signals of Mn²⁺ ions in ZnO phase was registered as a function of the chemical methods used for the preparation of powders only when samples were sintered at lower temperatures and non-linear characteristics of varistor ceramics had not yet been reached. At higher sintering temperatures EPR signals of Mn²⁺ ions in electroconductive phase differed only in the case of powders obtained by NaOH coprecipitation.