Grain Boundary Nanohardness of Coarse Grain Alumina

Here, we present the results of nanoindentation experiments conducted at ultralow peak loads of, for example, 1000, 5000, and 10,000 μN on a dense (~99.5%), coarse grain (~10 μm) alumina ceramic. The results showed that the nanohardness values of alumina were sensitive to the applied loads as well as to the locations of measurements, for example, the center of the grain, away from the center of the grain and at the vicinity of the grain boundary. The location sensitivity of nanohardness was linked to compositional variations of the different locations and to the presence of an amorphous phase.     

Relating Grain Boundary Complexion to Grain Boundary Kinetics II: Silica-Doped Alumina

This second paper in a series describes the relationship between grain growth kinetics and grain boundary complexions in silica-doped alumina. Dense high-purity silica-doped alumina samples were annealed for various times in the temperature range of 1300° and 1900°C and their grain growth behavior was quantified. Four different grain boundary complexions were observed in silica-doped alumina, all of which enhanced the kinetics relative to the intrinsic undoped alumina. These complexions included a thick crystallized film that was likely amorphous at high temperatures, a thin intergranular film, multilayer adsorption, and a type of boundary that showed no observable film by high-resolution transmission electron microscopy. A generational change in the population of grains occurred at 1500°C where all of the abnormal grains impinged and reestablished a new normal distribution. At higher temperatures a new set of abnormal grains containing different complexions formed in the microstructure. The activation energy of the normal and abnormal grains was approximately the same. The effects of varying dopant concentration were analyzed. The results for silica-doped alumina are compared with previous results for calcia-doped alumina in order to draw some generalized conclusions about the effect of complexions on grain growth.     

Creep Due to a Non-Newtonian Grain Boundary Phase

A model has been developed for creep due to viscous flow in a material containing a non-Newtonian, amorphous grain boundary phase. The model describes the process whereby glass is squeezed out from between grain boundary layers in compression, and flows toward grain boundary layers under tension. A pseudo steady state, in which the creep rate is constant, is seen at small strains. However, beyond a critical strain, the strain rate starts to decrease rapidly, and no creep is possible once adjacent grains come into contact. The maximum possible strain is twice as large in tension as in compression. This leads to a form of anisotropy which may explain the shift in neutral axis which occurs during flexural creep.     

Grain Boundary Evolution in Hot-Pressed ABC-SiC

Silicon carbide with aluminum, boron, and carbon additions (ABC-SiC) was hot-pressed to full density. The samples were examined by transmission electron microscopy (TEM), with an emphasis on high-resolution electron microscopy (HREM). Amorphous grain boundary interlayers, typically less than 2 nm wide, were formed between SiC grains. Heat-treating the ABC-SiC at temperatures as low as 1100°C in Ar crystallized the grain boundary interlayers completely without significantly changing the dominant chemical constituents. Chemical microanalyses demonstrated Al and O enrichment for all examined grain boundaries in both as-prepared and annealed samples. Quantitative EDS analyses revealed Al₂OC- and Al₂O₃-related species (with Si, C, B, or S substitutions) as two of the most likely grain boundary interlayer materials, both before and after heat treatment. Al₂O₃, and (Al_{1− x} Si _x )₂OC with a 2H-type wurtzite structure, were identified as grain boundary films by HREM images. The structural evolution in the grain boundary phases during the hot pressing and postannealing is discussed.     

Intrinsic Grain Boundary Mobility in Alumina

Ultra-high purity alumina was sintered in hydrogen at temperatures ranging from 1325° to 2020°C. Normal grain growth was maintained in all samples and grain boundary mobilities were measured. The data represent the most complete measure of intrinsic grain boundary mobility for alumina grain growth currently available. There are two orders of magnitude of discrepancy between the calculated and measured intrinsic grain boundary mobility. It is proposed that this discrepancy results from the poor assumption of the Burke–Turnbull model, which suggests that grain boundaries are ideally disordered and atomically rough. Implications of these results on the mechanism of solid-state single-crystal conversion in alumina are also discussed.     

Grain Boundary Films in Rare-Earth-Glass-Based Silicon Nitride

The thickness of the intergranular films in Si₃N₄ densified with lanthanide oxides has been systematically investigated using high-resolution transmission electron microscopy. Four lanthanide oxides—La₂O₃, Nd₂O₃, Gd₂O₃, and Yb2O3—as well as Y₂O₃ are chosen so that the results will reflect the overall trend in the effect of the lanthanide utilized. The film thicknesses increase with increasing ionic radius of the lanthanide. In addition, Si₃N₄ particles flocculated into isolated clusters in the lanthanide-based glasses are also characteristically separated by an amorphous film whose thickness is similar to that in the comparable polycrystalline ceramics, demonstrating that the film thickness is dictated entirely by the composition and not by the amount of the glass phase present.     

Grain Boundary Diffusion of Magnesium in Zirconia

This paper reports the transport kinetics of Mg in cubic yttria-stabilized zirconia (containing 10% mol of Y₂O₃ (10YSZ)) involving the bulk and the grain boundary diffusion coefficients. The diffusion-controlled concentration profiles of Mg were determined using secondary ion mass spectrometry (SIMS) in the range 1073–1273 K. The determined bulk diffusion coefficient and the grain boundary diffusion product may be expressed as the following functions of temperature, respectively: D = 5.7 exp[(−390 kJ/mol)/ RT ] cm²·s⁻¹ and D 'αδ= 3.2 × 10⁻¹⁵ exp[(−121 kJ/mol)/ RT ] cm³·s⁻¹, where α is the segregation enrichment factor and δ is the boundary layer thickness. The grain boundary enhancement factor decreases with temperature from 10⁵ at 1073 K to 10³ at 1273 K.     

Effect of Grain Boundary Structure on Diffusion-Induced Grain Boundary Migration in BaTiO3

The effect of grain boundary structure, either rough or faceted, on diffusion-induced grain boundary migration (DIGM) has been investigated in BaTiO₃. SrTiO₃ particles were scattered on the polished surfaces of two kinds of BaTiO₃ samples with faceted and rough boundaries and annealed in air for the samples with faceted boundaries and in H₂ for those with rough boundaries. In the BaTiO₃ samples with rough boundaries, an appreciable grain boundary migration occurred. In contrast, grain-boundary migration hardly occurred in the BaTiO₃ samples with faceted boundaries. The migration suppression observed in the sample with faceted boundaries was attributed to a low boundary mobility. The present experimental results show that DIGM is strongly affected by the boundary structure and can be suppressed by structural transition of boundaries from rough to faceted.     

天然磁铁矿处理铬废水

利用重金属离子在水合金属氧化物和氢氧化物矿物表面产生吸附作用的原理,进行了天然磁铁矿处理含铬离子废水的实验研究,通过本实验证明:当实验温度为25℃、粒径为220目、试样用量为0.5g、p H值为6.4、吸附平衡时间为60 min、离子强度为零时,初始浓度为20 mg/L的铬离子在天然磁铁矿上的吸附率可以达到99%,这样处理可以使废水中的铬离子浓度降低以达到国家的排放标准。     

边界层陶瓷电容器的研究进展

介绍了陶瓷电容器的优点、性能要求以及边界型陶瓷电容器的产生机理和特点，同时还论述了边界层型陶瓷电容器的物理性能、发展现状和在21世纪的研究和应用前景。     

Grain Boundary Oxidation in PTCR Barium Titanate Thermistors

The phenomenon of grain boundary oxidation in PTCR BaTiO₃ thermistors is discussed. In particular, the energy spectra of the surface states were calculated for different samples, and these were related to the nominal composition, the impurity content of the base BaTiO₃ powder used, and the prevalent atmospheric conditions during cooling and/or annealing. It is proposed that the interaction of manganese with oxygen creates deep-lying traps, and, in general, some proof is offered that the majority of the surface states are due to different oxidizing chemisorbed gases. It is believed that the ability of a particular sample to adsorb such gases in adequate amounts, and thus exhibit an appreciable PTCR effect, is related to the presence of acceptor-type dopants perferentially segregated onto the grain surfaces. Notably, the role of 3 d transition metal cations in this process is discussed in some detail.     

Devitrification of the Grain Boundary Glassy Phase in a High-Alumina Ceramic Substrate

Devitrification of the (Na, Mg, Ca) aluminosilicate glass phase in a high-alumina (>99%) ceramic substrate has been studied by TEM. MgAl₂O₄ (spinel) and a Ca-rich plagio-clase (a solid solution between CaAl₂Si₂0₈ (anorthite) and NaAlSi₃O₈ (albite)) are the dominant crystalline phases. Significant spatial variability exists both in the extent of crystallization and in the glass composition.     

Sintering Temperature Dependence of Grain Boundary Resistivity in a Rare-Earth-Doped ZnO Varistor

We present a rare-earth-doped ZnO ceramic with nonohmic electrical properties. Analysis of the microstructure and composition indicates that the ceramic is composed of the main phase of ZnO and the second phase of rare-earth oxides (e.g., Dy₂O₃, Pr₆O₁₁, Pr₂O₃). The average grain size is markedly increased from 3 to 18 μm, with an increase in the sintering temperature from 1150° to 1350°C. The corresponding varistor voltage and nonlinear coefficient decrease from 1014 to 578 V/mm, and from 15.8 to 6.8, respectively. The resistivity of grain and grain boundary evaluated by the complex impedance spectrum indicates that the resistivity of the grain is approximately constant (∼10³Ω), and the resistivity of the grain boundary decreases. The relative dielectric permittivity of the sintered ceramic samples is much larger than that of pure ZnO ceramic, which should be ascribed to the internal boundary layer capacitance effect.     

Grain Boundary Diffusion of Oxygen in Alumina Ceramics

The volume and grain boundary diffusion coefficients (D_v and δD_gb) of oxygen at 1673 K in pure and Mg-doped alumina were studied by using a resistive anode encoder of SIMS. The effect of Mg-doping on D_v, was observed clearly. The differences on δD_gb in pure and Mg-doped alumina were about 3 orders and 1 order of magnitude, respectively. The Mg-doping in alumina is useful for homogenizing grain boundary diffusion.     

Characterization of the Grain Boundary in Shock-Compacted Diamond

Microscopic observations of shocked diamond compacts with an initial grain size of 2 to 4 μm have revealed that the grain boundaries consist of three phases: (1) extremely small, recrystallized diamond particles with a uniform size of 50 nm, (2) a mixture of the recrystallized andlor fragmented diamond and low-density phase of carbon, and (3) low-density phases of carbon only. These differences are caused by the pore-size distribution of the green compact.     

天然磁铁矿包膜TiO2磁性光催化剂的制备及其性能研究

易回收、可重复使用的光催化剂一直是光催化领域的研究热点。本文以天然磁铁矿为原料,通过沉淀法在磁铁矿表面包覆一层TiO₂,制备了可磁力分离回收的天然磁铁矿包膜TiO₂高效光催化材料。使用XRD、SEM和TEM等手段对制得的磁性光催化剂的结构进行了表征。磁性光催化剂对酸性红G的降解率可以达到99%,经过五次循环后仍可达到93%。     

Grain Boundary Sliding Measurements during Tensile Creep of a Single-Phase Alumina

The grain boundary sliding (GBS) behavior of a single-phase (relatively coarse-grained) alumina material was studied after tensile creep experiments were performed at 1500°C at stress levels of 20 and 35 MPa. Specimens tested at 35 MPa exhibited a number of modes of GBS, including Mode II (shear) displacements, Mode I (opening) displacements, out-of-plane sliding displacements, and in-plane grain rotation. Strains in the grain boundaries due to Mode II GBS ranged from 940% to 4400%. Average Mode II GBS displacements ranged from 0.08 to 0.29 µm in samples tested for 120 and 480 min, respectively, at 35 MPa. The GBS displacements were shown to fit a Weibull distribution. Tensile creep under a 35 MPa stress yielded a GBS rate of 9.5 10^-6µm/s, while the 20 MPa stress resulted in a GBS rate of 2.2 10^-6µm/s. The average Mode II GBS displacements increased linearly with specimen strain, suggesting that GBS may play an important role in creep cavitation during tensile creep. The data also revealed that compatibility and constraint rules appear to govern GBS behavior during tensile creep. GBS behavior during compressive creep will be compared to the tensile creep GBS measurements presented.     

SiC边界层陶瓷电容器的制备和性能特点

陶瓷电容器,尤其是边界层陶瓷电容器由于具有许多优良的性能,而越来越受到人们的重视.通常用于陶瓷电容器的材料是具有弛豫铁电性能的钛酸盐类材料.本文则介绍了将SiC材料用于制备陶瓷电容器,结果发现,电容器的介电常数高达2 910 000,远远高于传统材料;而同时,其介质损耗也非常高.在此,对降低损耗的工艺过程进行了探讨.     

Grain Boundary Film Thicknesses in Superplastically Deformed Silicon Nitride

The thickness of intergranular films in polycrystalline β-Si₃N₄ ceramics, both before and after superplastic deformation, has been systematically investigated using high-resolution transmission electron microscopy. In characterizing the film thickness, care was taken to correlate the grain boundary orientation with the direction of the compressive stress applied during the hot-pressing and the superplastic deformation. The film thickness shows a dependence on the intersection angle between the grain boundary and the applied force direction, typically ranging from around a characteristic value for most of the boundaries to zero for a boundary which has an overall short length and is perpendicular to the applied force direction. The film thicknesses in the deformed material, as compared with those before deformation, are marked by a wider distribution and an increased fraction of boundaries free of films, unequivocally demonstrating that during the superplastic deformation the liquid phase is redistributed within short ranges, a process governed by the local stress level as well as kinetic factors. Possible consequences of the liquid-phase redistribution on the deformation behavior are also discussed.