Residual porosity and optical properties of spark plasma sintered transparent polycrystalline cerium-doped YAG

Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.     

Spheroidization of SiC powders and their improvement on the properties of SiC porous ceramics

Spherical SiC powders were prepared at high temperature using commercial SiC powders (4.52 µm) with irregular morphology. The influence of spherical SiC powders on the properties of SiC porous ceramics was investigated. In comparison with the as-received powders, the spheroidized SiC powders exhibited a relatively narrow particle size distribution and better flowability. The spheroidization mechanism of irregular SiC powder is surface diffusion. SiC porous ceramics prepared from spheroidized SiC powders showed more uniform pore size distribution and higher bending strength than that from as-received SiC powders. The improvement in the performance of SiC porous ceramics from spheroidized powder was attributed to tighter stacking of spherical SiC particles. After sintering at 1800 °C, the open porosity, average pore diameter, and bending strength of SiC porous ceramics prepared from spheroidized SiC powder were 39%, 2803.4 nm, and 66.89 MPa, respectively. Hence, SiC porous ceramics prepared from spheroidized SiC powder could be used as membrane for micro-filtration or as support of membrane for ultra/nano-filtration.     

Dry Powder Deposition and Compaction for Functionally Graded Ceramics

Functionally graded ceramics, with property variations realized via local compositional changes or porosity, are fabricated by powder deposition and compaction. A removable fixture is used to control the location of dry powders to produce a macroscopic powder array with 2-D or 3-D compositional variations. A new method is presented that relies on traditional powder pressing techniques to create spatial variation in composition and density. The fixture is removed before conventional powder compaction. This dry powder deposition and compaction method is capable of producing large monolithic bodies with functional gradients. This method produces designs with 3 mm discrete regions or pixels that have a normalized positional tolerance of 0.6% for a 39 mm substrate size. The method is used to realize a UHF antenna substrate with spatially variable dielectric properties.     

Fabrication of Aluminum Oxynitride (γ‐AlON) Transparent Ceramics with Modified Gelcasting

Transparent aluminum oxynitride (AlON) ceramics have been prepared through aqueous gelcasting forming technique starting from the raw materials of single phase AlON powders. The powder was specially treated for anti‐hydrolysis in ethanol before the shaping technique. The surface‐treated AlON powders could then be dispersed in an aqueous‐organic solution to prepare stable slurries containing 35 vol% solids loading. The obtained stable slurries were subsequently casted, calcined, and pressureless sintered at 1950°C for 8 h in nitrogen atmosphere. High transparent AlON ceramics with an average grain size of 112 μm and the in‐line transmittance of 81% at wavelength 1100 nm have been obtained.     

Fabrication,microstructure, mechanical and luminescence properties of transparent Yb3+-doped Sr5(PO4)3F nanostructured ceramics

The Sr₅(PO₄)₃F (S-FAP) crystal material is regarded as one of the most ideal optical materials for diode pumping owing to its huge absorption and emission cross sections and long fluorescence lifespan. In this investigation, S-FAP powders with varying Yb concentrations (0.1–5%) were produced using the coprecipitation method. Then a variety of S-FAP transparent ceramics with varying Yb content were fabricated using hot-pressing sintering. The crystalline phase structure of hexagonal Sr₅(PO₄)₃F was verified by XRD analysis of the precursor powder and the final ceramics. According to the powder SEM, the average grain size and the long axial-radial ratio of powders are decreasing as the Yb³⁺ concentration increases. Thermal-etched surface SEM reveals nanostructured S-FAP transparent ceramics with an average grain size of less than 200 nm were synthesized. The highest transmittances of the 3% ceramics at 500 and 1100 nm wavelengths are 51% and 79.78%, respectively. The ceramic cross-sectional SEM demonstrated that porosity is the primary scattering source influencing the enhancement of optical characteristics. The absorption, emission, and fluorescence lifetimes of S-FAP transparent ceramics with varying Yb concentrations were tested and discussed, and the absorption and emission cross sections corresponding to the major peak were reported. Some physical parameters of this set of ceramic samples were shown, including thermal diffusivity, specific heat capacity, and thermal diffusivity data, as well as micro-hardness.     

Effect of Excess PbO on the Densification of PLZT Ceramics

The densification behavior and electrical properties of PLZT ceramics as a function of excess PbO in the starting powder have been investigated. The hot-pressed PLZT ceramics, prepared from powders containing 0% to 4% excess PbO, were transparent, and their electrical properties were similar in general. PLZT with PbO deficiency up to 1.4 mol% from stoichiometry can be regarded as a single phase-PLZT.     

Fabrication and Evaluation of Porous Piezoelectric Ceramics and Porosity–Graded Piezoelectric Actuators

Porous ceramics of lead zirconate titanate (PZT) were prepared by sintering powder compacts consisting of PZT and stearic acid powders in an air atmosphere; stearic acid was added as a pore-forming agent (PFA). The dielectric, elastic and piezoelectric properties of uniformly porous PZT ceramics were investigated as a function of the porosity volume fraction. Furthermore, a beam-shaped PZT actuator sample with a graded porosity content across its thickness was fabricated by sintering PFA-graded powder compacts. The electric-field-induced bending displacement characteristics of the actuator samples were measured by using strain gauges and were found to be in good agreement with the theoretical predication based on a classical lamination theory.     

New insights in structural characterization of transparent ZnS ceramics hot-pressed from nanocrystalline powders synthesized by combustion method

Zinc sulfide transparent ceramics have been fabricated by hot pressing (HP) powders prepared by a newly developed combustion method. Chemical, structural and microstructural properties of powders and ceramics were characterized using different experimental techniques (XRD, SEM-EDS, laser granulometry, TEM, BET, FT-IR spectroscopy). ZnS powders were densified to full density by HP under vacuum atmosphere. The ceramics exhibit highly dense microstructure with mean grain size of 1 μm. TEM characterization identified, both in powders and ceramics, twins and simple stacking faults due to the aperiodic distribution of hexagonal domains. With optical transmission of the theoretical level (～75%), without absorption band (at 6 μm) and with negligible optical loss, in the 4–12 μm region, the ceramics exhibit better optical performances than standard grade CVD ZnS, and unprecedented performances for hot-pressed ZnS.     

Effect of Powder Surface Modifications on the Properties of Silicon Nitride Ceramics

The effect of surface oxygen concentration of silicon nitride powders on the properties of resulting ceramics was studied. A high-purity silicon nitride powder was treated physically and chemically to modify its surface oxygen content. The resulting powders were hot-pressed into dense ceramics using 6 wt% yttria as a sintering aid. Strength and oxidation resistance of these ceramics were measured and correlated with the powder and ceramic compositions as well as the resulting intergranular phases. Results show that the phases developed in yttria-containing silicon nitride ceramics vary with slight changes in the initial powder oxygen content, as predicted, and that strength can be correlated to initial oxygen concentration. The mechanical strength vs oxygen content curve has a definite maximum; i.e., there is a small oxygen concentration range at which optimum ceramic strength is realized. Best results are obtained when the oxygen content is increased by thermal oxidation; other techniques such as chemical oxidation or addition of silica are not as effective, particularly in attaining high strength at elevated temperatures.     

Influence of powder physicochemical characteristics on microstructural and optical aspects of YAG and Er:YAG ceramics obtained by SPS

The present work aims to establish a correlation between the characteristics of YAG and Er:YAG commercial powders produced by two different synthesis routes and sintered ceramic microstructures and their optical aspect by taking into account the influence of pressure applied during the Spark Plasma Sintering (SPS) process. Physical and chemical characteristics of the powders were compared using various techniques such as SEM, XRD, laser diffraction and chemical analyses. Their behaviours were evaluated through a rheological study, compressibility tests and dilatometry cycles using SPS. This paper pinpoints the most important powder features which influence the optical quality of YAG and Er:YAG ceramics. The optical quality is mainly affected by the porosity, related to powder characteristics that affect particle rearrangement, densification and grain growth. The applied pressure induces microstructural heterogeneities depending on the starting material used and resulting in core-shell aspects of sintered ceramics.     

Dielectric properties of barium titanate ceramics with different materials powder size

The dielectric properties of barium titanium ceramics fabricated with nano-size fine powders (about 40 nm) are compared with that fabricated with micro-size coarse powders (about 2 μm). Three kinds of ceramics were fabricated; one using pure nano-size fine powders, the other using pure micro-size coarse powders, and the third using the combination of both. The sintering temperature of the ceramics with pure nano-size fine powders is 150 °C lower than that with pure micro-size coarse powders. For the same sintering conditions, the relative density of the ceramics is increases with the amount of nano-size fine powders. The grain size of the ceramics body with pure micro-size coarse powder is about 5 μm, but that of pure nano-size fine powder is about 1 μm. The room temperature dielectric constant of the ceramics increases with the increasing of the amount of nano-size fine powder. For pure nano-size fine powders, the room temperature dielectric constant is about 5000, and that of micro-size coarse powders is about 2200.     

Co-precipitation synthesis and two-step sintering of YAG powders for transparent ceramics

Yttrium aluminum garnet (Y₃Al₅O₁₂, YAG) precursor was synthesized by the co-precipitation method with ammonium hydrogen carbonate as the precipitant. The influence of aging and calcination temperature on the precursor composition and transformation temperature of the YAG phase was investigated. On that basis, a two-step sintering (TSS) method (heating the sample up to 1800 °C followed by holding it at 1600 °C for 8 h) was used to fabricate bulk transparent YAG ceramics in vacuum (10⁻³ Pa) in this communication. A variety of techniques, such as X-ray powder diffraction, infrared spectra, scanning electron microscopy and UV–vis–NIR spectrophotometry were adopted to characterize the resulting YAG powders and ceramics. The results showed that aging had a dramatic effect on the precursor composition, which in turn influenced the transformation temperature of the YAG phase. Loosely agglomerated YAG powders with a mean particle size of 50 nm were obtained by calcinating the precursor without aging at 1000 °C. Finally, a transparent YAG ceramic specimen, achieving the in-line transmittance of 41% in the visible wavelength region and a nearly pore-free microstructure with uniform grains of about 4 μm, was produced via the TSS technique.     

Effect of freeze-drying treatment on the optical properties of SPS-sintered alumina

This study looks at the influence of alumina powder processing on the preparation of transparent alumina by Spark Plasma Sintering (SPS). Zeta potential measurements were carried out on alumina suspensions in order to determine the best dispersion conditions. Stable slurries were submitted to a spray freeze drying process and their sintering behavior was compared with the corresponding non spray freeze dried powders. Transparent alumina samples were successfully prepared from alumina powders by Spark Plasma Sintering. An optical model considering pore and grain size distributions has been developed to obtain information about porosity in dense materials. It was found that the final density and, accordingly, the optical properties were improved when spray freeze dried starting powder was used.     

Effects of Surface Modification of Calcined PLZT Powder on Optical Transparency

The effects of surface modification of lead lanthanum Zirconate titanate (PLZT) powder on sintering and transparency were investigated by sintering hydrophobic powders modified with various alcohols. When alcohols with longer chain lengths were used for surface treatment, aggregation forces among the particles were reduced, resulting in dense, highly transparent ceramics.     

Properties of calcium phosphates ceramic composites derived from natural materials

In this study, ceramics containing mixed phases of hydroxyapatite/beta-tricalcium phosphate (HA/β-TCP) were fabricated by a solid-state reaction technique. The HA powder was synthesized from cockle shells while the β-TCP powder was synthesized from egg shells. Pure HA and β-TCP fine powders were successfully obtained. The HA and β-TCP were mixed and subjected to a thermal treatment up to 1100 °C. To form the mixed phase ceramics, the resulting powders were sintered at 1350 °C. Effects of HA concentration on the properties of the studied ceramic were investigated. X-ray diffraction analysis revealed that all samples presented multiphase of calcium phosphate compounds. Average grain size of the ceramics decreased with the HA additive content. The 75 wt% HA ceramic showed the maximum hardness value (5.5 GPa) which is high when compared with many calcium phosphate ceramics. In vitro bioactivity test indicated that apatite forming increased with the HA additive content. To increase antibacterial activity, selected ceramics were coated with AgNO₃. Antibacterial test suggested that an Ag compound coating on the ceramics could improve the antibacterial ability of the studied ceramics. In addition, the antibacterial ability for the Ag coated ceramics depended on the porosity of the ceramics.     

非化学配比对Mg_((1-3χ))Al_((2-2χ))O_4 尖晶石透明陶瓷显微结构的影响

本研究通过真空烧结和热等静压两步烧结制备了一系列的镁铝尖晶石透明陶瓷.我们利用XRD对粉体和陶瓷的物相成分进行了实验分析,并发现配比为0≤χ≤0.20 的Mg_((1-3χ))Al_((2-2χ))O_4 粉体具有单一的尖晶石成分.SEM 观察发现陶瓷主晶相边界上有利于增韧陶瓷的氧化铝晶粒.通过氧化铝颗粒的透过率和增韧效果的综合考虑,在较小的光损失条件下,采用配比χ为1.4的Mg_((1-3χ))Al_((2-2χ))O_4 粉体制备的陶瓷的显微结构将会最有利于陶瓷的强韧化.     

钇铝石榴石(YAG)激光透明陶瓷研究进展

钇铝石榴石(YAG)激光透明陶瓷由于具有单晶、玻璃激光材料无可比拟的优势而成为研究热点,并得到迅速发展,高性能的稀土元素掺杂YAG透明激光陶瓷被相继报导.本文综述了近年来国内外关于YAG激光透明陶瓷的最新研究成果.主要包括YAG微细粉体合成、烧结添加刺及多晶YAG透明陶瓷的致密化烧结技术,并对比了YAG透明陶瓷相对于Y...     

Y_2O_3透明陶瓷的研究进展

Y2O3透明陶瓷具有优异的光学和热学性能,是一种有较高应用价值的功能材料,现已成为单晶的可替代材料。介绍了Y2O3透明陶瓷的研究进展,阐述了Y2O3透明陶瓷的粉体合成、坯体成型、高温烧结和机械加工的制备工艺,并分析了成型时坯体开裂的原因。     

Fabrication and characterization of nanoporous SiO2 ceramics via pyrolysis of silicone resin filled with nanometer SiO2 powders

Mesoporous SiO₂ ceramics are fabricated by pyrolysis of silicone resin filled with nanometer SiO₂ powders in air at 1273 K. With the increase of shaping pressure, open porosity and average pore size decrease, while bulk density and fracture strength are improved. The fracture surface of porous SiO₂ ceramics was observed.     

Oxidation bonding of porous silicon carbide ceramics with synergistic performance

Porous silicon carbide (SiC) ceramics were fabricated by an oxidation-bonding process, in which the powder compacts are heated in air so that SiC particles are bonded to each other by oxidation-derived SiO₂ glass. It has been shown that a high porosity can be obtained by adding a large amount of graphite into the SiC powder compacts and that the pore diameter can be controlled by the size of graphite particles and/or SiC powders. When a 0.3-μm SiC powder was used, a high strength up to 133 MPa was achieved at a porosity of 31.5%. Moreover, oxidation-bonded SiC (OBSC) ceramics were observed to exhibit an excellent resistance to oxidation and thermal shock.