工程陶瓷

早期的陶瓷是粘土质耐火材料。最普通的粘土砖广泛用于高炉、化铁炉、热处理炉等冶金工业,组分为20～45%的Al_2O_3和50～80%的SiO_2与常用的几种粘土、以及几种能减小烧成收缩和增加使用稳定性的添加剂。烧成温度为1500C。典型的烧结材料含有50%玻璃、35%莫来石和15%方英石。耐火粘土的承载能力直接关系到玻璃相的数量及其粘度。Al_2O_3含量较高的耐火粘土能在艰难条件下更好地使用。软化温度通常为1750     

美国高技术陶瓷持续增长

高技术陶瓷的销售额从1989年的35亿美元增加到1999年的81亿美元,平均年增长率为8.7％。据美国商业资讯公司的调查,在高技术陶瓷市场中,电子陶瓷不仅将继续占有最大市场份额,而且是未来增长的领头羊,结构陶瓷紧随其后。1999年,美国高技术陶瓷部件的总产值大约为81亿美元,预     

结构陶瓷—一个需要耐心的行业

众所周知,要制备高质量的先进陶瓷零部件、元器件,粉料的质量是个关键环节。近15年来,通过国际陶瓷界(当然还有国内)对制备方法和工艺技术的精心研究,粉料的纯度、细度、颗粒分布以及性能的重现性均有极大的进展,导致高性能结构陶瓷的性能有了显著的提高(当然还包括配方、工艺的改进,这里只是强调了粉料)。例如:①力学性能由20MPa提高到2GPa;②K_(IC)由1提高到     

陶瓷中氧化锆的研究及应用现状

综述了陶瓷中氧化锆的研究动态;介绍了氧化锆的几个主要应用领域及常用氧化锆陶瓷系列。     

对我国工程陶瓷的科研和发展的调查

几位教授介绍了我国南方几所大学有关工程陶瓷的科研情况,以及在京专家介绍了部分工程陶瓷的生产情况。     

可加工陶瓷研究现状

工程陶瓷因为具有极高的硬度、良好的耐磨耐蚀性和很高的脆性,使其成为难加工材料,现存的陶瓷材料加工技术均存在成本高、效率低和对材料损伤性大等问题。通过陶瓷自身显微结构设计来增强陶瓷材料的可加工性是解决陶瓷难加工问题的关键。综述了国内外对可加工陶瓷研究的现状。     

晶内型Al2O3—SiC纳米复合陶瓷的制备

研究了沉淀法制备Ａｌ２Ｏ３－ＳｉＣ纳米复合陶瓷的工艺过程，利用Ａｌ２Ｏ３从γ相到α相的蠕虫状生长过程，使大部分纳米ＳｉＣ颗粒位于Ａｌ２Ｏ３晶粒内，用沉淀法制得的、含有５ｖｏｌ％ＳｉＣ的Ａｌ２Ｏ３－ＳｉＣ纳米复合陶瓷，其强度为４６７ＭＰａ，韧性为４．７ＭＰａ．ｍ＾１／２，与一般的Ａｌ２Ｏ３陶瓷相比有较大的提高，显示了沉淀法制备Ａｌ２Ｏ３－ＳｉＣ纳米复合陶瓷的优点。     

Positron Lifetime Study of the Single-Phase B2 Ni100-xAlx and Ni50-xCoxAl50 Intermetallics

To whom correspondence should be addressedE-mail: jtguo@imr.ac.cn1. IntroductionIt is now well established that the mechanicalproperties of NIAI are strong functions of composition and often of thermal history. These dependencieshave been correlated in part to the presence of pointdefects. The observation of off-stoichiometric hardening in NiAI, for instance, has been attributed tothe constitutional point defects[1'2]. Previous investigators have attempted to relate this hardening tothe co…     

Positron lifetime calculations on vacancy clusters and dislocations in Ni and Fe

Positron lifetime calculations have been performed on vacancy clusters (stacking fault tetrahedra (SFT), vacancy loops), such clusters on dislocation line, interstitial clusters, such clusters on a dislocation line and dislocation line itself in order to investigate the so-called intermediate lifetimes observed in the experiments, namely, positron lifetimes between that at a matrix and that at a single vacancy. Before lifetime calculations, various defects were constructed in the model lattices and were relaxed completely to obtain the stable atomic structure by using N-body potentials. Then positron lifetime calculation was carried out for each defect. It was shown that positron lifetime for a SFT in Ni dependes on its size and becomes smaller with increasing the size. The positron wave function is mainly localized at the corner of a SFT, which gives rather lifetime, e.g., 130 ps for V₂₈, but when the cluster size is small, e.g., less than 10 vacancies, it gives a rather longer lifetime, e.g., 177 ps for V₆ because of the wave function localized at the inner space of a cluster. These behaviours are consistent with the experimental results. It was also found that the positron lifetime on a dislocation line and that at a jog are short (113 and 119 ps, respectively for Ni, 117 and 117 ps, respectively for Fe), close to the lifetime at matrix (110 ps for both Ni and Fe) and in these cases trapping potentials for a positron are shallow both for Ni and Fe.     

Investigation of Er-doped Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> transparent ceramics by positron annihilation spectroscopy

0.25 at.% Er-doped Sc₂O₃ transparent ceramics fabricated using the two-step sintering method with different combinations of sintering temperatures were investigated by positron annihilation spectroscopy. Analysis of the broadening of the annihilation photopeak revealed the presence of the same type of defect in all samples. The lack of long lifetimes (τ ≥ 2 ns) suggested no positronium formation or the lack of trapping sites large enough to trap positronium for long enough time for the annihilation to be observed. Analysis of positron annihilation lifetime revealed the presence of a single lifetime that ranged from 208 to 219 ps, depending on the sintering conditions. These results also suggest the absence of a significant presence of vacancy clusters and other larger open-volume defects, and that the dominant open-volume defect corresponds to monovacancies and/or complex defects associated with monovacancies. The bulk lifetime of Er-doped scandia is estimated to be equal or lower than 208 ps.     

Local Electronic Structure, O–T Phase Transition and Superconductivity in the Ba-site Nd-substituted YBCO System

A series of YBa_2-xNd_xCu₃O_y (x = 0–0.4) samples have been systematically studied by means of X-ray diffraction, transport property measurements and positron annihilation technology. The positron lifetime parameters show strong Nd substitution dependence. There is an obvious change of positron lifetime parameters around the O–T phase transition. The local electron density n_e and vacancy concentration C_v as a function of x were calculated from the positron lifetime results. The correlations between local electronic structure, O–T phase transition and superconductivity are discussed. The results confirmed that n_e mainly has an effect on high-T_c superconductivity by affecting the charge transfer between CuO₂ planes and Cu–O chains region or Ba–O layer. The vacancy properties in the orthorhombic phase and tetragonal phase are two intrinsic different types. Positron lifetime is very sensitive to the O–T phase transition in the YBCO systems that can be used as a useful technique to determine the O–T phase transition in these systems.     

掺杂TiO2的ZnO压敏电阻正电子寿命谱研究

利用正电子湮灭技术(PAS)和扫描电子显微镜(SEM),分析了掺杂TiO2的ZnO压敏电阻的晶界缺陷,以及不同降温速率对晶界特性的影响.实验结果表明,向样品中掺杂TiO2或者快速冷却样品,都能使得样品晶界处Zn空位团尺寸变大,浓度减小.     

Defect depth profiling after sphere indentation and blasting in aluminum and aluminum alloy detected by positron annihilation

The paper presents the experimental results of positron annihilation measurements of the subsurface zones in aluminum and aluminum alloy exposed to the indentation of the small steel ball or blasting by the silicon carbide particles. The measurements of the Doppler broadening of the annihilation line and positron lifetime (PL) enabled the depth profile of open volume defects induced by indentation and blasting processes to be obtained. The coincidence of the von Mises criterion for yield with the onset of the increase of the defect concentration at certain depth was detected. It was established that the defects created in the pure aluminum and the aluminum alloy detected by positrons are different. However, the kinds of induced defects do not change with depth. The PL measurements indicate the presence of vacancy clusters in samples exposed to blasting. The vacancy clusters created in the aluminum alloy are larger than in the pure aluminum.     

Defects in FZ-silicon after neutron irradiation—A positron annihilation and photoluminescence study

Float-zone (FZ) Si irradiated with 1 MeV neutrons was investigated by means of positron annihilation lifetime spectroscopy and photoluminescence. Three types of defects were observed: di-vacancies, small vacancy clusters and an unknown defect with the defect-related lifetime of (285 ± 5) ps that contribute to positron trapping only at low temperatures. Two annealing stages were observed: one at 350°C and another at 500°C. While the former is due to the annealing of divacancies, the latter is caused by the annealing of vacancy clusters and the unknown defect.     

BaTiO3基PTCR陶瓷中Bi2O3蒸汽掺杂和Mn的协同作用

BaTiO3基陶瓷材料的PTCR效应与施受主掺杂密切相关。通过BaTiO3、Sb2O3等蒸汽掺杂，材料的PTCR效应可以得到提高。然而，蒸汽掺杂之后，PTCR效应提高的幅度在含受主Mn的材料中比纯施主掺杂的材料中要大得多。这与BaTiO3蒸汽掺杂和Mn协同作用密切相关。这种协同作用可能进一步形成三阶Mn或中性钡缺位相关的更为稳定的复合缺陷，从而增大了电子捕获中心的浓度，使材料PTCR效应大幅提高。     

Cold-worked and hydrogen chargedNbHf as studied by TDPAC and positron lifetime measurements

Time differential perturbed angular correlation (TDPAC) and positron life-time measurements on cold-worked and hydrogen chargedNbHf indicate a strong binding of hydrogen decorated vacancy clusters with Hf impurities. Both TDPAC and positron lifetime results are mutually consistent on this aspect. Transformation of H-vacancy clusters into H-bubbles is indicated by the positron lifetime behaviour at annealing temperatures beyond 900 K. Impurity effects like oxygen pickup and coating of voids cannot be ruled out at elevated temperatures.     

Divacancies at room temperature in germanium

We have studied the annealing of vacancy defects in neutron and proton irradiated germanium. After neutron irradiation the Sb-doped samples were annealed at 473, 673 and 773 K for 30 min. The positron lifetime was measured as a function of temperature (30 - 295 K). A lifetime component of 330 ps with no temperature dependence is observed in as irradiated samples, identified as the positron lifetime in a neutral divacancy. The average positron lifetime in the samples annealed at 473 K has a definite temperature dependence, suggesting that the divacancies become negative as the crystal recovers and the Fermi level moves upward in the band gap. Proton irradiation of germanium at 37 K with subsequent room temperature annealing also resulted in a similar lifetime component 315 ps, in good agreement with the neutron irradiation experiment.