Fabrication of Er:Y2O3 transparent ceramics for 2.7 μm mid-infrared solid-state lasers

Laser grade 7 at.% Er:Y₂O₃ transparent ceramics with submicron grain size were fabricated by using one-step vacuum sintering followed by hot isostatic pressing (HIPing) technique. Through studying the sintering trajectory of Er:Y₂O₃ ceramics, the sintering temperature zone where sufficient relative density (>96%), no pore-boundary separation, and sub-micron grain size (<1 μm) ceramic samples could be identified. The samples pre-sintered in this zone were readily densified by HIPing. To maximum the densification and achieve high transparency, it is critical to suppress the final-stage grain growth. After HIPing at 1520 °C, the Er:Y₂O₃ ceramics were fully densified without further grain growth, and exhibited in-line transmission of about 81.6% at 2000 nm. Continuous wave (CW) room temperature laser operation of the Er:Y₂O₃ transparent ceramic at 2.7 μm was demonstrated.     

Hot isostatic pressing of transparent Yb3+-doped Lu2O3 ceramics for laser applications

Yb³⁺-doped Lu₂O₃ nanoparticles produced by laser ablation were used to fabricate transparent ceramics by a combination of pressureless sintering in vacuum (PS) followed by a hot isostatic pressing (HIPing). The samples were subjected to various PS and HIPing conditions and the microstructure evolution and its correlation with the transmittance were investigated. Relative densities of over 97% were achieved after PS at the temperatures of 1250–1700 °C. Rapid grain growth occurred within PS and HIPing temperatures above 1500 °C leading to formation of intragranular porosity which is deleterious for optical quality. Higher transmittance (81.7% at λ = 1080 nm) and ultrafine microstructure with an average grain size of 0.35 μm were obtained by PS at 1250 °C followed by HIPing at 1400 °C for 5 h under 207 MPa. Output power of 2.02 W with a slope efficiency of 46.5% was obtained under a quasi-continuous wave end pumping at 929.4 nm.     

Preparation and characterization of porous alumina ceramics through starch consolidation casting technique

This work aims at studying the influence of thermal treatment on the microstructure, resistivity and technological properties of porous alumina ceramics prepared via starch consolidation casting (SCC) technique. Colloidal suspensions were prepared with three different contents of alumina solid loading (55, 60 and 65 mass%) and corn starch (3, 8 and 13 mass%). The sintered samples at 1400, 1500, 1600 and 1700 °C, show open porosity between 46 and 64%, depending on the starch content in the precursor suspensions and sintering temperature. The pore structures were analyzed by SEM. The effect of corn starch content on the apparent porosity, pore size distribution, linear shrinkage and electrical resistivity as well as cold crushing strength of the sintered porous alumina ceramics was also measured. These porous alumina ceramics are promising porous ceramic materials for using in a wide range of thermal, electrical and bioceramics applications as well as filters/membranes and gas burners, due to their excellent combination properties.     

Pump laser induced photodarkening in ZrO2-doped Yb:Y2O3 laser ceramics

5 at.% Yb:Y₂O₃ transparent ceramics were fabricated using vacuum sintering plus HIP. The ceramics doped with 1 at.% ZrO₂ as the sintering additive were densified at 1700 °C in vacuum followed by HIPing at 1775 °C, while those without sintering additives were densified at 1520 °C in vacuum followed by HIPing at 1450 °C. After sintering, both ceramics had relatively high in-line transmittance. However, during laser experiments, the ZrO₂-doped Yb:Y₂O₃ (Zr-YbY) ceramics were photodarkened when irradiated by 940 nm pump light. The discoloration might be attributed to the formation of Zr³⁺ color centers during lasing. In contrast, no photodarkening effect was detected in the pure Yb:Y₂O₃ ceramics without sintering additives (P-YbY). The P-YbY ceramics exhibited much higher lasing efficiency (17%) than the Zr-YbY ceramics (9%). To our best knowledge, it is the first time that the photodarkening effect was detected in rare-earth doped sesquioxide laser ceramics.     

添加剂对低温烧结95氧化铝陶瓷性能的影响

采用MnO2-TiO2-CaO-La2O3复相添加剂作为氧化铝陶瓷的烧结助剂,研究了MnO2添加量对95氧化铝陶瓷烧结性能、力学性能和微观结构的影响。结果表明:当配方中MnO2含量为3.0%时,在1550℃烧结温度下制得的氧化铝陶瓷的综合性能最佳,烧结试样的体积密度达到3.76g.cm-3,试样的抗弯强度和洛氏硬度(HRA)分别达到355.22MPa和84.3。     

Charge trapping characteristics of alumina based ceramics

The space charge dynamics is very important for electrical breakdown of alumina based ceramics. In this paper, the charge trapping/detrapping characteristics of alumina based ceramics were studied by means of isothermal surface potential decay (ISPD) method. For alumina and zirconia toughened alumina (ZTA) ceramic samples, the ISPD curves charged by corona discharge as well as microstructure characterization were carried out. For the first time, crossover phenomenon and hollow shaped potential profile were observed and reported in alumina based ceramics, indicating a surface potential decay process dominated by charge injection and volume conduction affected by the trap states in materials. In addition, the comparative trapping characteristics were evaluated based on a charge detrapping controlled decay model. The correlation between trap distribution and microstructure of alumina based ceramics was investigated. It was proposed that different charge trapping characteristics of alumina based ceramic samples was caused by varied shallow trap density of grain boundary.     

Recycling bagasse and rice hulls ash as a pore-forming agent in the fabrication of cordierite–spinel porous ceramics

Bagasse and rice hulls ash are both waste materials. In recent years, in order to meet environmental protection, these materials have been recycled in the production of porous ceramics. A solid-state reaction mechanism of calcined alumina and talc was used to prepare cordierite–spinel porous ceramics. Talc was added from 30 to 60 wt.% at the expense of alumina and sintered at 1400°C for 2 h. The effect of bagasse and rice hulls ash (as a pore forming agent) on the densification parameters, cold crushing strength (CCS), and pore size distribution was also studied. The phase composition (X-ray diffraction) and microstructure (scanning electron microscopy) of sintered samples were investigated. The results showed that the main phases present in the samples are cordierite, corundum, spinel, and sapphirine. In the sample with a higher amount of talc additions (60 wt.%), only the formation of the cordierite and spinel phases was observed. The bulk density of the samples and the apparent porosity ranged from 1.77 to 2.26 g/cm³ and from 28.6% to 48.21%, respectively. The CCS of the samples ranges from 13.9 to 36.3 MPa. The microstructures of the sintered samples were observed for the formation of cordierite phase, alumina phase, and spinel phase in an excellent crystallization and phase arrangement.     

浸渍及热处理对常压烧结AlN/h-BN复相陶瓷微观结构及性能的影响

采用常压烧结方法在1 700℃保温2 h制备出AlN/20%(体积分数)h-BN复相陶瓷,对烧结后的样品分别采用A10铝溶胶和硅溶胶/酚醛树脂进行浸渍处理,随后在1 450℃氮气气氛下热处理2 h。对比研究了浸渍及热处理前后复相陶瓷的致密度、抗弯强度、Vickers硬度、微观结构和物相组成,并分析了复相陶瓷的强化机理。结果表明:A10铝溶胶浸渍处理后样品的抗弯强度和Vickers硬度略有提高;经过硅溶胶/酚醛树脂处理的样品抗弯强度和Vickers硬度大幅提高,抗弯强度和Vickers硬度分别从81.5MPa和1.99 GPa提高到130.1 MPa和3.58GPa;硅溶胶/酚醛树脂处理后的样品在孔隙界面处生成的碳化硅及氮氧化铝是样品抗弯强度和Vickers硬度显著提高的主要原因。     

Tailoring of functionally graded zirconia–mullite/alumina ceramics

A new tailored zirconia–mullite/(0–100 vol%) alumina as functionally graded ceramics (FGCs) was designed and synthesized by reaction sintering of zircon and alumina. Zircon and alumina powder mixtures were mixed, stacked, compacted in a cylindrical die and sintered. The sintered samples made of 11 layers and varied gradually in composition by 10 vol% from one layer to the other layer (i.e. from zirconia–mullite layer to alumina layer) resulted in continuous functionally graded ceramics without sharp interfaces. Phase composition and densification behaviors of the samples were investigated. Microstructure, mechanical and thermal properties of FGC and its non-layered composites were studied. Results showed that the tailored FGZM/A gave continuous homogenous structure with highly improved physical, mechanical and thermal properties. The different properties of tailored FGZM/A recorded average values or rather better of its non-layered composites which gave a new way for material design.     

Transparent alumina ceramics: Effect of standard and plasma generated stabilizing approaches in colloidal processing

The study is focused on an optimization of the slip-casting process used for the fabrication of the transparent/translucent alumina ceramics; more precisely, on specifying the most appropriate way to stabilize the cast alumina suspensions. An innovative method of the particles’ stabilization by plasma treatment was compared with the classical electrostatic and the most frequently used electrosteric approach. Properties of green bodies (pore size distribution, density) and sintered samples (density, mean grain size, real in-line transmittance) were measured in term to evaluate the impact of the individual stabilization mechanism on the final properties of the transparent/translucent ceramics. The results showed that all tested approaches enable the preparation of the transparent/translucent alumina ceramics by Hot Isostatic Pressing. Ceramics prepared from the plasma treated as well as the electrostatically stabilized powders exhibited narrower pore size distribution, higher density, and lower mean grain size in comparison to ceramics fabricated from only electrosterically stabilized powders. Despite these promising properties the plasma-treated samples resulted in an unexpectedly low RIT of 36% caused by the presence of thin cracks. However, the electrostatically stabilized samples achieved the highest RIT value of 57%.     

Gas permeability and mechanical properties of porous alumina ceramics with unidirectionally aligned pores

Porous alumina ceramics having unidirectionally aligned cylindrical pores were prepared by extrusion method and compared with porous ceramics having randomly distributed pores prepared by conventional method, and their gas permeability and mechanical properties were investigated. SEM micrographs of the porous alumina ceramics prepared by the extrusion method using nylon fibers as the pore former showed excellent orientation of cylindrical pores. The bending strength and Weibull modulus of the extruded porous alumina ceramics with 39% porosity were 156 MPa and 17, respectively. These mechanical properties of extruded samples were higher than those of the conventional porous alumina ceramics. The strength decreased from 156 to 106 MPa with increasing pore size from 8.5 to 38 μm. The gas permeability of the extrusion samples is higher than that of the conventional samples and increased with increasing of porosity and pore size.     

Highly oriented α-Al2O3 transparent ceramics shaped by shear force

Alumina platelets were arranged horizontally in submicron alumina particles by shear force in the flow of slurries during casting. The obtained alumina green bodies with platelets were pressureless-sintered in vacuum, producing ceramics with thoroughly oriented grains and high transmittance. The effects of sintering parameters on the densification, microstructure evolution, and orientation degree of alumina ceramics were investigated and discussed. The results showed that the densification, grain size, orientation degree, and in-line transmittance were increased with increasing sintering temperature. The enhancement of orientation degree was mainly coherent with grain growth. The grain-oriented samples exhibited a much higher in-line transmittance (at 600 nm) of 61 % than that of the grain random sample (29 %). Moreover, the transmission remained a high level in the ultraviolet range (<300 nm).     

A novel colloidal processing route to alumina ceramics

The fabrication of complex-shaped alumina ceramics following a new near-net shape technique based on hydrolysis induced aqueous gelcasting (GCHAS) is reported in this paper. Aqueous suspension containing 50 vol.% solids loading was prepared by dispersing alumina in an aqueous solution of methacrylamide and methlynebisacrylamide (17 wt.% in 6:1) using polycarboxylic acid as dispersing agent. Consolidation was accomplished by adding a polymerization initiator, a catalyst and AlN powder (4 wt.%). For comparison purposes, alumina ceramics were also consolidated by aqueous gelcasting (GC) and by hydrolysis assisted solidification (HAS) from the same concentrated suspensions, and by conventional dry pressing (DP) from freeze dried granules prepared from the same suspensions by freeze granulation. Among the four shaping techniques used, GCHAS was found to be best for consolidating near-net shape alumina components like thin wall radomes with the highest green strength ever reported for alumina ceramics. Green samples were sintered for 2 h at 1600 °C and then characterized for microstructure and mechanical properties.     

Pressure-less joining of alumina ceramics by the reaction-bonded aluminum oxide (RBAO) method

The present work demonstrates a pressure-less and reliable joining technique for alumina ceramics through a reaction-bonded aluminum oxide (RBAO) method. Effective joining relies on the RBAO mechanism, in which Al particles are converted to alumina through oxidation and bond with alumina particles from the parts to be joined upon sintering. Alumina ceramics in a green state were successfully joined with the use of an Al/Al₂O₃ powder mixture as an interlayer. The oxidation behavior of the Al particles was confirmed by thermogravimetry and X-ray diffraction analyses. Joining was performed in ambient air at 1650 °C for 2 h without applying any external pressure. Microstructural observations at the joining interfaces indicated a compact joining. The joining strengths were assessed by determining the biaxial strengths at room temperature, and the joined samples exhibited no fractures at the joining interfaces. Moreover, the joints had a strength of almost 100 % when compared with those of the parent alumina ceramics.     

Advanced Alumina and Zirconia Ceramics Produced by the Electroconsolidation Method

The possibility to produce advanced alumina and zirconia ceramics by the electroconsolidation method is studied.The technological parameters of Al_2O_3 and ZrO_2( 3 mass% Y_2O_3) production were developed and optimized.Electroconsolidated alumina and zirconia ceramics have higher values of properties in comparison with ordinary sintered samples in air.Advanced properties of electroconsolidated ceramics are defined by homogeneous,ultradense and fine-crystalline structure that was formed due to the effect to consolidate the materials to high density for a shortest time.     

Hydraulic Alumina Binder for Extrusion of Alumina Ceramics

The present work describes the effect of hydraulic alumina (HA) as a new inorganic binder on extrusion of alumina ceramics. The addition of a small amount of HA imparted significant flowability and rigidity to the extruded alumina. Under optimum processing conditions such as amounts of HA and water, and curing period, alumina rods and tubes were successfully extruded using a single screw extruder. Sintered specimens have achieved a relative density of 97% after sintering at 1600°C for 2 h. Dense alumina samples prepared using organic and inorganic binders had similar mechanical properties, that is, Young's modulus >380 GPa and H _v>2000. These results suggest that HA is a new kind of inorganic binder applicable to the extrusion of ceramics.     

Hydrothermal Corrosion of Alumina Ceramics

The corrosion behavior and strength degradation of alumina ceramics with 99%, 99.9%, and 99.99% Al₂O₃ were studied in water at 300°C and 8.6 MPa for 1 to 10 d. The weight loss in alumina ceramics was mainly attributed to the dissolution of SiO₂ and Na₂O grain-boundary impurities. Intergranular corrosion proceeded in the alumina ceramics by preferential attack at the grain boundaries. The extent of the strength reduction for corroded alumina ceramics was related to the impurity level in the alumina ceramics.     

Assessment on mechanical properties controlling of alumina ceramics for harsh service conditions

Improvement of mechanical properties of alumina ceramics is investigated by addition of MgO and ZrO₂ starting from inorganic precursors. The doping of alumina ceramics is performed by infiltration of solutions of magnesium chloride and zirconium oxychloride into preformed samples. Two types of alumina powders were used. The impregnated samples were fired in 1550–1780 °C temperature range and characterised by bulk density, bending strengths and their microstructure are analysed. The dispersion and concentration of reinforcing particles is analysed by electron microprobe (energy dispersive analysis).After firing the infiltration method leads the dopants concentrate especially at the surface of samples. The surface enrichment in dopants determines increased strengths with about 17–27% as function of sintering temperature.     

Extrusion-based additive manufacturing of functionally graded ceramics

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.     

Chemically Bonded Phosphate Ceramics: II, Warm-Temperature Process for Alumina Ceramics

This is the second of three papers on a dissolution model that describes the formation of chemically bonded phosphate ceramics. In this paper, we discuss the kinetics of formation of aluminum phosphate ceramics between 100° and 150°C. Using basic thermodynamic formulations, we calculated the temperatures of maximum solubility of alumina and its hydrated phases and predicted the temperatures of formation of ceramics. Differential thermal and X-ray diffraction analyses on samples made in the laboratory confirm these temperatures. The resulting ceramics of alumina bonded with aluminum phosphate (berlinite) show a high compressive strength of 16 000 psi. We have concluded that rapid evaporation of excess water in the slurry generates porosity in the ceramics, and that better processing methods are needed. A consolidation model is presented that describes the microstructure of the ceramic. It predicts that a very small amount of alumina must be converted to form the bonding phase; hence, the product is mostly alumina with a thin coating of berlinite on the surface of alumina particles.