Reaction During Sintering of Barium Titanate with Lithium Fluoride

The reactions occurring during sintering of stoichiometric BaTiO₃ with small additions of LiF were studied at temperatures between 700° and 900°C. BaTiO₃ reacts with LiF to form a cubic solid solution and Li₂TiO₃ During sintering, the cubic solid solution coexists with Li₂Ti0₃ and forms a liquid phase at 740°± 5°C. The occurrence of a liquid phase at this temperature results in an enhancement of the sintering process and leads to the development of a highly dense microstructure.     

Shape Changes by {100} Lithium Fluoride Ridge-Channel Arrays and of Lithium Fluoride Particles at Sintering Temperatures

Parallel channels of rectangular cross section were etched into an LiF {100} single–crystal surface, by a photolithography technique, to produce ridge–channel arrays of controlled size and shape. The shape changes undergone by these arrays and by LiF isolated particles and powder beds at sintering temperatures were studied by SEM. The ridge–channel arrays and particles developed either faceted or rounded shapes, depending on temperature and atmosphere. Adsorption of an impurity, presumably H₂O or O₂, although it causes faceting,¹ increases the rate of mass transport from the ridges to channels. The influence of array geometry on mass transport rates is compared to models to show that the rate-limiting mass transfer step is probably a surface step when the surfaces remain faceted and may be a surface step when the surfaces are rounded.     

Degradation of Magnesium Aluminum Spinel by Lithium Fluoride Sintering Aid

The effect of LiF sintering aid on the degradation of transparent magnesium aluminate spinel during hot-pressing was studied. LiF is used to etch spinel particles during the hot pressing process. The LiF was found to react with the aluminum in the spinel structure, thereby leaving Mg-rich regions behind that do not sinter well and result in opaque white regions in the otherwise transparent matrix.     

Microstructural Development during the Microwave Sintering of Yttria—Zirconia Ceramics

The microstructural development of 3 and 8 mol% yttria—zirconia ceramics during microwave sintering was studied and compared to conventionally heated samples. Microwave heating enhanced the densification processes occurring during constant-rate heating for both materials. No change was found in the grain size—density relationship for the 8 mol% yttria—zirconia. However, a small, but statistically significant shift favoring densification was found for the 3 mol% yttriazirconia. Differences in the responses of the two materials are not completely understood but may be due to the differences in the activation energy for grain growth and grain-boundary mobility.     

Microstructural Evolution during the Sintering of Nanostructured Ceramic Oxides

Using small-angle scattering, together with density measurements, X-ray diffraction, nitrogen adsorption, and both transmission and scanning electron microscopy, the microstructural evolution during sintering in nanostructured ceramic oxides has been studied. The availability of samples with sufficient size and uniformity has permitted absolute calibration of the scattering data and made possible determinations of microstructural parameters and surface areas that are statistically representative of the undisturbed nanophase morphologies. A fully quantitative microstructural model has been developed, and the effects of varying processing parameters during powder synthesis and sintering have been assessed. Predictions are made regarding the achievable sintered full-density microstructures.     

利用主烧结曲线预测TiO2陶瓷无压烧结显微组织演变

纳米金红石TiO2粉末在高温热膨胀仪中进行恒速无压烧结,升温速率为2℃/min和5℃/min,热膨胀仪自动记录烧结收缩量,根据全期烧结模型,建立TiO2主烧结曲线;利用阿基米德法测量烧结体的相对密度,利用扫描电镜观察烧结体在不同温度的显微组织演变.结果表明:利用主烧结曲线得到的相对密度和Archimedes法实测的密度吻合,证明了主烧结曲线对烧结路径不敏感,烧结体的相对密度仅是时间和温度的函数.烧结体的显微组织是烧结温度和时间的函数,烧结体的相对密度和显微组织有关联,主烧结曲线理论可以用来预测和控制烧结体的相对密度、显微组织和陶瓷性能.     

Microstructural Development During Gas-Pressure Sintering of α-Silicon Nitride

Gas-pressure sintering of α-Si₃N₄ was carried out at 1850 ° to 2000°C in 980-kPa N₂. The diameters and aspect ratios of hexagonal grains in the sintered materials were measured on polished and etched surfaces. The materials have a bimodal distribution of grain diameters. The average aspect ratio in the materials from α-Si₃N₄ powder was similar to that in the materials from β-Si₃N₄ powder. The aspect ratio of large and elongated grains was larger than that of the average for all grains. The development of elongated grains was related to the formation of large nuclei during the α-to-β phase transformation. The fracture toughness of gaspressure-sintered materials was not related to the α content in the starting powder or the aspect ratio of the grains, but to the diameter of the large grains. Crack bridging was the main toughening mechanism in gas-pressure-sintered Si₃N₄ ceramics.     

Microstructural Evolution during the Sintering of TiC–Mo–Ni Cermets

TiC-Ni-Mo cermet specimens were prepared by using a mixture of fine (1.5 μm) and coarse (30 μm) TiC powders. When the fraction of fine TiC particles was 80%, a (Ti,Mo,Ni)C complex carbide phase was observed deposited on the coarse TiC particles and resulted in a typical cored structure. As the fraction of fine TiC particles decreased, the coarse TiC particles exhibited a unique microstructural evolution with the development of a concave interface. This microstructural change of the coarse TiC grains can be explained in terms of the coherency strain energy.     

Densification and Microstructural Development of the Reaction Sintering of Strontium Barium Niobate

SrNb₂O₆ and BaNb₂O₆ were selected as the precursor powders for the reaction sintering of strontium barium niobate of the composition Sr _x Ba_{1− x} Nb₂O₆ ( x = 0.5 and 0.6) (SBN). In comparing the densification behavior of the reaction sintering and the normal sintering of SBN, it was observed that the densification rate of the former was slower during reaction but became faster after reaction. It was also found that the chemical reaction occurred prior to densification, but a high-density and uniform microstructure could be obtained for the reaction-sintered sample. The densification rate of the reaction-sintered sample was pronouncedly enhanced when the reaction was almost completed. We also proposed the reason for the enhancement of the densification rate of the reaction-sintered sample when the reaction was almost completed. A high-density (>98% of the theoretical density), uniform, and fine-grained (3-4 µm) SBN microstructure could be achieved by using reaction sintering.     

Anisotropic Microstructural Development During the Constrained Sintering of Dip-Coated Alumina Thin Films

Alumina thin films were manufactured from aqueous slurries by dip coating. Film thickness as function of substrate withdrawal velocity could be correctly modeled by Landau and Levich's theory. Samples were sintered on a rigid substrate at 1350°C for different isothermal times to achieve relative densities from 84% to 97%. Microstructural analysis of polished cross sections revealed a continuous development of pore alignment, as expected by theoretical considerations. With increasing density pores become more anisometric and orientate along the thickness direction. A further preferential orientation of pores was found in the normal plane, apparently due to the coating process. Constraining conditions had less influence on the size and shape of the grains; they tend to become more equiaxed in the constrained plane, presumably due to the biaxial tensile stress state.     

Pore-Grain Boundary Configurations in Lithium Fluoride

A high-angle tilt boundary of LiF with attached pores was made to migrate under the influence of temperature and an imposed boundary curvature. The boundary was fractured along the boundary plane to observe its leading and trailing faces by stereo scanning electron microscopy. A detailed analysis of the attached pore shape and pore drag mechanism has been made. The quasi-equilibrium pore shape developed by pores detached from the boundary was used to determine the orientation dependence of surface energy in LiF.     

Microstructural Evolution during Sintering of Aluminum Nitride Ceramics Doped with Alumina and Yttria

The microstructural evolution of AlN sintered at >1950°C was studied in a specimen doped with 10 wt% Al₂O₃ and 5 wt% Y₂O₃. The constituent phases of the specimen were AlN, YAG, γ-AlON, and AlON polytypoids (compositional polytypes). Transmission and scanning electron microscopy revealed the microstructural characters: platelike 7AlN·Al₂O₃ first crystallized with concurrent formation of a residual liquid, then spherical AlN crystals formed. The liquid itself changed composition with the progress of the crystallization and reached the eutectic composition in the pseudobinary system AlN–YAG, and crystallized to an aggregate of AlN and YAG during cooling. As a product of the reaction of 7AlN·Al₂O₃, γ-AlON was formed.     

Microstructural Coarsening During Sintering of Boron Carbide

The sintering behavior of boron carbide was investigated with particular attention given to microstructure development at various stages in the sintering process. Hot-pressing and pressureless sintering techniques were employed and the effects of heating rate, firing atmosphere, and composition were used to characterize the sintering behavior. Pressureless sintering at temperatures up to 2300°C produces only limited densification. Microstructural coarsening is responsible for this since it leads to conditions where densification is slow. Hot-pressing and carbon additions suppressed coarsening and permitted densification to >95% of theoretical density.     

Intensified Sintering of Lithium Aluminosilicate Ceramics

Experimental data are presented showing that the sintering of lithium aluminosilicate-based glass ceramics, produced by ceramic technology, can be accelerated by adding 0.1 – 0.7% Cr₂O₃, without detriment to the product quality.     

Deformation and Fracture of Polycrystalline Lithium Fluoride

Techniques for the fabrication of polycrystalline LiF test specimens were developed and evaluated using single-crystal LiF as a control. An etch was developed which revealed dislocations on all crystallographic faces of LiF. Large-grained polycrystalline specimens tested in four-point loading underwent 0.076 to 0.798% plastic strain before fracture. In most cases their yield stress was similar to that for single crystals favorably oriented for flow on {110}〈110〉 slip systems. Deformation was inhomogeneous among the grains because of differences in orientation with respect to the applied stress and within individual grains because of interactions at grain boundaries. Grain boundaries were barriers to slip, but stresses resulting from slip in one grain were transmitted to neighboring grains and often caused local deformation near the boundary. In one case, local boundary slip occurred on an (010) plane. Three-grain junctions were areas of high residual stresses, and fractures originated at boundaries at or near three-grain junctions. Fractures were mixed transgranular and intergranular.     

Dimensionless Parameters for Microstructural Pathways in Sintering

Dimensionless parameters have been developed to study microstructural pathways for the sintering of powders. These parameters are designed to facilitate the comparison of microstructural paths for any sintering experiments, independent of the characteristic length scales in the microstructures. Microstructural pathways constructed with the dimensionless parameters are found to be similar for three different alumina ceramics. The systematic differences between the experimental results and the predictions of models based on simplifying geometric assumptions are explained in terms of the packing disruptions in the green microstructures.     

Barium Titanate Perovskite Sintered with Lithium Fluoride

The sintering of barium titanate with, respectively, 1 and 2 wt% LiF for two stoichiometries, Ti/Ba=1 and 0.975, was studied using two calcined powders. One was pure barium titanate; the other contained BaTi0₃ plus BaC0₃ and TiO₂ that did not react when burning. The sintering chronology—intermediate phases, appearance, and disappearance of a liquid phase that has been pointed out for the first time—is directly dependent on the used calcined powder, on the LiF amount, and on the firing schedule. In the same way, the obtained perovskite symmetry varies during sintering from tetragonal to cubic and then to a second tetragonal form, whereas most of the Li and F disappear from the ceramic with two different kinetics.     

Fabrication of Dense Lithium Fluoride for Neutron Shielding

Polycrystalline LiF suitable for use as a thermal neutron shielding material was fabricated by both hot-pressing and pressureless sintering to greater than 99.5% theoretical density. To achieve this, the poorly sinterable as-received LiF powder had to be (a) ball-milled to reduce particle size, (b) calcined to substantially remove porosity from the cubic particles, and (c) ball-milled again to break up agglomerates formed during calcination.