Obtaining polycrystalline alumina fibers

Refractories and Industrial Ceramics -     

Blunt scratch strength of polycrystalline alumina

A model for the fracture strength of brittle materials controlled by blunt (spherical) scratches is developed and compared with measurements on a polycrystalline alumina. The model is based on a residual stress‐intensity factor for median cracks at scratches that include a localized plastic deformation zone formed by dragged spherical contacts. The stress‐intensity factor depends nonlinearly on the normal contact load P, resulting in a predicted strength variation of P^?3/4. The strength result validates previous claims and extends the overall indentation‐strength framework. However, the result has only limited effectiveness in describing experimental measurements, pertaining only to ideal blunt scratches formed over a limited load domain.     

Controlled wet erosive wear of polycrystalline alumina

Controlled wet erosive wear tests were performed on polycrystalline alumina specimens of mean grain size G = 1.2, 3.8 and 14.1 μm and on sapphire specimens. The tests were performed by using an apparatus consisting of a jet body that rotates immersed in the slurry medium (SiC grits). Impacts are normal to the target surface. The construction and calibration of the apparatus are described. The impacting velocity used was 2.7 m s⁻¹. The weight loss of polycrystalline alumina and sapphire specimens increased linearly with impacting time. The wear rate of polycrystalline alumina specimens increased with grain size. Wear rates of 1.83, 8.36 and 11.3 nm s⁻¹ correspond to G = 1.2, 3.8 and 14.1 μm, respectively. For sapphire specimens the wear rate was 9.56 nm s⁻¹. Worn surfaces of both polycrystalline alumina and sapphire specimens were analysed by scanning electron microscopy.     

Laser shock processing of polycrystalline alumina ceramics

Laser shock processing (LSP) has been applied to polycrystalline α‐Al₂O₃ ceramics. X‐ray characterizations have revealed that LSP results in significant compressive residual stresses which can extend to a depth of more than 1.2 mm from the surface. The presence of compressive residual stresses improves the resistance of α‐Al₂O₃ ceramics to indentation cracking. Microstructural characterization suggests that the majority of α‐Al₂O₃ grains on the surface remains intact after LSP. However, damaged regions are occasionally present, which shows intergranular fractures and a limited plastic deformation in the vicinity of grain boundaries.     

Cr3+ microspectroscopy measurements and modelling of local variations in surface grinding stresses in polycrystalline alumina

Surface residual stresses caused by grinding and polishing of alumina are thought to influence materials properties but have previously been measured only by low spatial resolution techniques which sample average stresses. In this work confocal Cr³⁺ fluorescence microscopy has been used to investigate the spatial distribution of the residual stresses. A model for the residual stresses, accounting for both surface plastic deformation and “pullout” of material from the surface by brittle fracture, was developed to help in analysing the results. After coarse diamond grinding, the results showed that the residual stresses fluctuate greatly with position. Large tensile stresses (up to ～600 MPa) were found below the plastically deformed surface layer in regions between the “pullouts”. These tensile stresses are expected to aid crack propagation and further surface pullout. They arise because pullout removes parts of the plastically deformed surface layer. The stresses beneath the pullout sites themselves were compressive, but the largest compressive stresses (≈?1.5 GPa) were within the plastically deformed surface regions and extended to a depth of 1.3 μm. The plastically deformed surface layer was much shallower following polishing with 3 μm diamond paste but the compressive stress within it was of similar magnitude to that in the plastically deformed surface layer caused by grinding.     

Optimized etching of porcelain and polycrystalline alumina with a glass phase

Chemical and thermal etching techniques are commonly used in ceramography to enhance microstructural features. While thermal etching works well for high purity ceramic systems, this technique may not be appropriate where glass is present. Porcelain and industrial alumina both contain a glass phase, typically 40?60 vol% and 4?30 vol%, respectively, and these glass chemistries are proposed to be similar. Chemical etching of porcelain is common, but the images published in the literature are frequently over-etched. When glass is present in the grain boundaries of alumina, thermal etching can cause the glass to disappear or to recrystallize, obscuring the microstructure. Because of this, it is proposed that both chemical and thermal etching are necessary to prepare an industrial alumina microstructure for grain size measurements. In addition, it was observed that chemical etching is sensitive to the residual stress in the glass phase, becoming more aggressive when there is residual tension in the glass.     

Controllable synthesis of mesoporous alumina with large surface area for high and fast fluoride removal

Gamma phase of mesoporous alumina (MA) with large surface area was successfully synthesized by a facile hydrothermal method followed by thermal treatment for fluoride removal. The as-synthesized MA nanoparticles with average size of 20 nm–150 nm have ordered wormhole-like mesoporous structure. The pore size is 5 nm with a narrow distribution, and the specific surface area reaches 357 m² g⁻¹ while the bulk density is 0.45 cm³ g⁻¹. Glucose as a small-molecule template plays an important role on the morphology, surface area and pore diameter of the MA. As an ionic adsorbent for fluoride removal, the maximum adsorption capacity of MA is 8.25 mg g⁻¹, and the remove efficiency reaches 90% in several minutes at pH of 3. The Langmuir equilibrium model is found to be suitable for describing the fluoride sorption on MA and the adsorption behavior follows the pseudo-second-order equation well with a correlation coefficient larger than 0.99. The larger surface area and relatively narrow pore size of MA are believed to be responsible for improving the adsorption efficiency for fluoride in aqueous solution.     

Light scattering models for describing the transmittance of transparent and translucent alumina and zirconia ceramics

The original Apetz-van-Bruggen model for predicting the transmittance of dense polycrystalline ceramics with randomly oriented birefringent crystallites is shown to be incorrect. In a correct Apetz-van-Bruggen equivalent composite model (ECM) the volume fraction must be 1/3 instead of 1/2 and the factor 2/3 in front of the maximum birefringence must be abandoned. This ECM is in good agreement with the simplified version of Pecharromán’s dense polycrystalline model (DPM) and can be improved by replacing the Jobst approximation (i.e. the large-size limit of the Rayleigh-Gans approximation) by the van-de-Hulst approximation. Similarly, a new DPM is proposed that combines the van-de-Hulst approximation with Pecharromán’s texture function. However, all these models fail for small grains. Therefore two new models (infimum-supremum-based ECM and DPM) are proposed, which combine the full Rayleigh-Gans approximation (for small grains) with the van-de-Hulst approximation (for large grains) and provide (for all grain sizes) predictions almost identical to Mie theory.     

稳定多晶氧化铝纤维质量之研究

通过改变胶体添加剂和热处理参数,提高纤维化器的转速,达到了稳定胶体粘度、改善成纤性能、降低纤维直径、稳定并提高氧化铝纤维质量、降低生产成本之目的。     

溶胶--凝胶法制备多晶氧化铝纤维的研究

本文探讨了溶胶－凝胶法制备多晶氧化铝过程中影响体性能的因素,成纤方法与工艺参数,热处理工艺制度及晶相转移关系等一系列问题。实验得到了以莫来石为主晶相长期使用温度超过１６００℃的多晶氧化铝纤维。     

Fracture mechanics of sharp scratch strength of polycrystalline alumina

The strength of a polycrystalline alumina containing controlled scratches introduced by translated sharp contacts is investigated and described by a multiscale fracture mechanics model. Inert strength measurements of samples containing quasi‐static and translated Vickers indentation contacts showed that scratches degraded the strength at normal contact loads an order of magnitude less than those for quasi‐static indentation. The fracture mechanics model developed to describe strength degradation by scratches over the full range of contact loads included toughening effects by crack‐wake bridging at the microscale and lateral crack‐based residual stress relaxation effects at the mesoscale. A critical element of the model is the nonlinear scaling of the residual stress field of a scratch with the normal contact load acting during scratch formation. The similarities and differences in the scratch model in comparison with prior indentation‐strength fracture mechanics models are highlighted by parallel development of both. Central to the scratch model is the use of easily controlled normal contact load as the scratch‐strength measurement variable. Scratch length and orientation are shown to have significant effects on strength. The distributions of scratch widths controlling the intrinsic strengths of as‐received samples are determined and agreement with the observed scratch dimensions is demonstrated.     

In situ quantitative three-dimensional characterisation of sub-indentation cracking in polycrystalline alumina

The three-dimensional deformation beneath a Vickers indentation in polycrystalline alumina has been measured, in situ, by digital volume correlation of high resolution synchrotron X-ray computed tomographs. The displacement fields at the peak indentation load and after unloading are used to study the shape and orientation of sub-surface cracks induced by the indentation; lateral cracking due to residual stresses, bounded by a system of radial cracks, is revealed. For the first time, it is shown that radial cracks have mixed mode opening displacements, which are affected by the relaxation of residual stresses via lateral cracking. This novel technique may find applications in the study of surface damage by abrasive wear in brittle materials.     

大孔氧化铝载体制备条件的研究

本文以拟薄水铝石为原料,槽法炭黑为扩孔剂制备了大孔氧化铝载体,并对制备条件进行了研究,找到了制备条件对载体孔结构及强度的影响规律。     

Light scattering from polyethylene solutions

The technique of measurement of light scattering from dilute polyethylene (PE) solutions is reviewed. Conditions are discussed under which various PE samples can be characterized. The choice of the solvent affects the accuracy of molecular weight determination. It appears that measurement with thermodynamically poorer solvents yields results subjected to a smaller error. The interpretation of light scattering data is often disturbed by the presence of supermolecular structures in PE solutions. The effect of reprecipitation on the behavior of PE samples is described. The measurements were mostly performed on PE standards and on the Czechoslovak sample Liten Macro.     

Transparent polycrystalline alumina obtained by SPS: Green bodies processing effect

Starting from a commercial slurry of high purity α-Al₂O₃, freeze-dried powders, cast, filter-pressed or cold isostatically pressed samples were produced. Resulting powders or green bodies showing different particles packing were densified by spark plasma sintering (SPS) to obtain transparent polycrystalline α-Al₂O₃. Microstructure and real in-line transmittance (RIT) after SPS were dependent on the particles packing quality. Avoiding large agglomerates, narrowing the pore size distribution, reducing the most-frequent pore size (D_mode) and avoiding macroscopic heterogeneities within the green bodies enabled high RIT values to be achieved in the visible and near-infrared spectrum. However, a limit was achieved in the preparation of green bodies for which reducing the D_mode had no more influence on the optical behaviour of samples sintered by SPS. Finally, pure α-Al₂O₃ samples presenting a high RIT_640 nm value of 53% were produced from all the green bodies obtained by the following techniques: filter-pressing, slip casting and cold isostatic pressing.     

Light scattering by lead-borate melts

Specific features of changes in the intensity of light scattered by single-phase lead-borate melts containing from 0.6 to 12 mol % PbO are investigated. It is established that, under these conditions, the total intensity of light scattered by the investigated objects is mainly contributed from the concentration component, and the anisotropic scattering contribution is relatively low. Based on the obtained data with the use of the critical phenomena theory, spinodal temperatures T _s are determined, corresponding to the absolute instability limit for the investigated melts. It is found that an increase in the content of lead oxide in the melt results in a gradual development of specific features in the behavior of the light scattering intensity, which are revealed as its instability, dependence on the position of a scattering volume in the melt, and widened and textured glares from the primary light beam passed through the melt. The found effects cause irreproducible experimental results for melts containing more than 12 mol % PbO. Possible reasons for the discovered “anomalies” may be related to both lead oxide precipitation from the melt and interactions of lead-borate melts with the walls of a quartz cuvette.     

Light scattering of acetylated lignin

Organosolv lignin was fractionated on a Sephadex G 75 column with 0.1M aqueous NaOH resulting in 14 fractions. These fractions were acetylated and a high-molecular-weight fraction (F3) was investigated by means of combined static and dynamic light scattering (LS) in solvents acetone, tetrahydrofuran (THF), and trifluoroethanol (TFE). The measurements were found to be reproducible, and recycling of lignin by freeze drying caused slight but unessential changes in solution properties. Depending on the solvent used, weight average molecular weights M_w between two and ten million were found. By contrast, M_n of the fractions, measured by vapor pressure osmometry (VPO), was in the range of a few thousands only. Analysis of the angular dependence in static LS by means of a Casassa–Holtzer plot and the fractal dimensions showed the presence of chain stiffness, most distinct in TFE. Also, the dynamic light scattering results in TFE are typical for semiflexible chains, while in THF, and to some extent in acetone, the dynamic measurements including viscosity suggest the presence of spherical structures. These findings are being explained by large lignin clusters that consist of stiff subunits.