Lost Mold-Rapid Infiltration Forming of Mesoscale Ceramics: Part 2, Geometry and Strength Improvements

Iterative process improvements have been used to eliminate strength-limiting geometric flaws in mesoscale bend bars composed of yttria-tetragonal zirconia polycrystals (Y-TZP). These improvements led to large quantities of high bend strength material. The metrology of Y-TZP mesoscale bend bars produced using a novel lost mold-rapid infiltration-forming process (LM-RIF) is characterized over several process improvements. These improvements eliminate trapezoidal cross sections in the parts, reduce concave upper surfaces in cross section, and minimize warping along the long axis of 332 × 26 × 17 μm mesoscale bend bars. The trapezoidal cross sections of earlier, first-generation parts were due to the absorption of high-energy ultraviolet (UV) light during the photolithographic mold-forming process, which produced nonvertical mold walls that the parts mirrored. The concave upper surfaces in cross section were eliminated by implementing a RIF-buffing process. Warping during sintering was attributed to impurities in the substrate, which creates localized grain growth and warping as the tetragonal phase becomes destabilized. Precision in the part dimensions is demonstrated using optical profilometry on bend bars and a triangular test component. The bend bar dimensions have a 95% confidence interval of <±1 μm, and the tip radius of the triangular test component is 3 μm, consistent with the UV-photolithographic process used to form the mold cavities. The average bend strength of the mesoscale Y-TZP bend exceeds 2 GPa with a Weibull modulus equal to 6.3.     

先进陶瓷快速无模成型技术的研究与进展

系统综述了先进陶瓷快速无模成型技术及其最新研究进展,其中主要包括熔融沉积成型技术、喷墨打印成型技术、三维打印成型技术、分层实体成型技术、激光选区烧结成型以及立体光刻成型技术。最后论述了陶瓷快速无模成型技术所具有的独特优势和当前研究工作中面临的问题和挑战。     

多孔陶瓷的制备及应用

根据成孔原理，多孔陶瓷成型工艺可分为七大类，对各种工艺进行了简要论述结合作者的工作对有机泡沫体浸渍工艺中的关键技术进行了较详细的探讨，并简要描述了两次离心挂浆工艺的过程及实验结果。最后，展望了多孔陶瓷的应用前景及尚待解决的问题。     

Superplastic Forming of SiAlON Ceramics

Superplastic SiAlON's of the nominal composition Y _{m /3}-Si_{12-( m + n )}AL _{m + n} O _n N_{16– n} are reported in this study using a transient-phase-forming approach. They encompass the fields of single-phase α'-SiAlON, single-phase β'-SiAlON, and their two-phase mixtures. Excellent formability is obtained at 1550°C for the β'and α'+β'materials, and at 1600°C for the α'material. Typically in the nonequilibrium state before deformation, these fine-grained materials undergo dramatic phase and microstructure evolutions during superplastic forming. In particular, the stress-biased α-Si₃N₄→β'-SiAlON reaction is found to result in elongated and aligned grains with fiber-strengthening effect and excellent formability.     

New High-Permittivity AgNb1-xTaxO3 Microwave Ceramics: Part II, Dielectric Characteristics

Ceramic samples with the AgNb_{1− x} Ta _x O₃ (0.46 < x < 0.54) composition were synthesized and analyzed, with respect to their dielectric properties, in a microwave-frequency range and at radio frequencies. The permittivity over the compositional range 0.46 lessthan equal to x lessthan equal to 0.52 was >410, whereas the Q × f value, measured at 2 GHz, reaches values of 640–900 GHz. Dependence of the resonant frequency on temperature was not linear but did display a minimum at a particular temperature, which was dependent on the composition of the ceramic. Samples that were measured at radio frequencies displayed low dissipation factors ( tg δ= 7 ×10⁻⁴−18 × 10⁻⁴), a low aging rate (0.2% per time decade), and good dielectric strength (8 V/μm at a thickness of 0.6 mm). Microstructural investigations revealed the presence of precipitates and transformation twins that should be eliminated during additional processing optimization.     

Fibrous Monolithic Ceramics: I, Fabrication, Microstructure, and Indentation Behavior

Monolithic ceramics have been fabricated from coated green fibers to create fibrous microstructures. The fibrous monoliths consist of high aspect ratio polycrystalline regions (cells) of a primary phase regions (cell boundaries) designed to improve fracture resistance. The cells are the remnants of the green fiber which consists of ceramic powder and a polymer binder. The coating applied on the green fiber forms the cell boundaries. Fabrication and microstructure are described for fibrous monoliths in the SiC/graphite, silicon nitride/BN, alumina/alumina–zirconia, alumina/aluminum titanate, alumina/nickel and Ce-TZP/alumina–Ce–zirconia systems. The SiC/graphite fibrous monolith displays noncatastrophic failure in flexure, with shear delamination along the weak graphite layers. Indentations in SiC/graphite cause cells to spall, with crack arrest and extrusion of graphite from the cell boundaries. Crack deflection and spalling of cells are also observed in alumina/alumina–zirconia fibrous monoliths. In the Ce-TZP/alumina system, transformed regions around indentations are significantly modified by the alumina-containing cell boundaries.     

Permeability and Infiltration of Partially Sintered Ceramics

For infiltration of a molten salt into porous zirconia (relative densities = 0.56 to 0.88), the infiltration depth was found to be a function of both infiltration time and initial compact density. The intrinsic liquid permeability of the porous Y-TZP was determined by extrapolation of gas permeability data. Permeability values, which ranged from 0.5 × 10⁻¹⁸ to 25 × 10⁻¹⁸ m² (∼0.5 to 25 microdarcy), could be used to predict the trend in infiltration depth with compact density, though they underestimated the absolute values. The Carman-Kozeny relationship, which relates the permeability to measurable microstructural parameters, was also evaluated. There was good agreement at relative densities <0.75, but not at the higher densities, likely due to the increasing tortuosity of the flow path as pores shrink and close.     

Fabrication of Microconfigured Multicomponent Ceramics

Microfabrication by coextrusion is a novel ceramic processing technique capable of creating complex ceramic-metal structures. These structures are fabricated by multiple pass coextrusion of a feedrod comprised of several powder-filled thermoplastic compounds. The compounds contain either ceramic, metal, or fugitive powders. To illustrate the capabilities of microfabrication, a demonstration part containing lead manganese niobate-lead titanate ceramic and silver palladium was fabricated. The final part was microconfigured, with a fenestrated structure containing 3110 repeat units per square centimeter. The repeat unit feature sizes were 15 and 5 µm for the ceramic and electrode, respectively. Microfabrication by coextrusion is proposed as a fabrication technique for the production of smart structures and materials.     

微晶刚玉瓷的生产

材料主晶相的细微化是提高材料性能的有效途径.在相关理论的指导下,笔者结合科研实验和长期的生产实践,总结出一些制备微晶刚玉瓷的经验,以适应微晶瓷产业化的客观实际需要.     

Low-Temperature Fabrication of Anorthite Ceramics

Fabrication of anorthite ceramics suitable for low temperature, sinterable, multilayered substrates is described. Anorthite ceramics were synthesized from relatively pure kaolin and calcites of varying particle sizes. Mixed powders were uniaxially pressed and fired between 900° and 1200°C. Firing at 1000°C yielded a dense anorthite ceramic when the finest calcite powder was used. The relative density and water absorption of the anorthite ceramic were 94% and almost zero, respectively. Mixtures containing coarser calcite had lower densities and higher water absorption.     

Freeform Fabrication of Ceramics via Stereolithography

Ceramic green bodies can be created using stereolithography methods where a ceramic suspension consisting of 0.40–0.55 volume fraction ceramic powder is dispersed within an ultraviolet-curable solution. Three ceramic materials were investigated: silica for investment casting purposes, and alumina and silicon nitride for structural parts. After mixing the powders in the curable solution, the ceramic suspension is photocured, layer by layer, fabricating a three-dimensional ceramic green body. Subsequent binder removal results in a sintered ceramic part. Three-dimensional objects have been fabricated from a 0.50 volume fraction silica suspension.     

Er,Yb:CaF2透明陶瓷的制备及其性能研究

首先以(Ca(NO3)2·4H2O、Er(NO3)3·5H2O、Yb(NO3)3·5H2O和KF·2H2O等试剂为原料,选择化学沉淀法合成了Er,Yb:CaF2纳米粉体,采用XRD和FE-SEM技术对纳米粉体特性进行了测试与表征.以合成的纳米粉体为原料,采用真空热压烧结技术,在800℃、30 MPa条件下制备了Er,Yb:CaF2透明陶瓷,并对陶瓷样品的元素组成与分布、显微结构及热导率等特性进行了测试与表征.结果表明:合成的Er,Yb:CaF2纳米粉体为单相立方萤石结构,平均尺寸在20～40 nm,并易形成尺寸为3～5μm的团聚体;Er与Yb元素在陶瓷样品中分布均匀,陶瓷的平均晶粒尺寸为1μm左右,并由于不均匀烧结,样品中存在大尺寸的孔洞结构,随着Yb掺杂浓度的增加,陶瓷样品的热导率逐渐降低.     

New High-Permittivity AgNb1-xTaxO3 Microwave Ceramics: Part I, Crystal Structures and Phase-Decomposition Relations

The crystal structure of monophasic AgNb_1/2Ta_1/2O₃ was investigated via Rietveld refinement using powder X-ray diffractometry (XRD) data. The study revealed a monoclinic unit cell of the P 2/ m space group with the following unit-cell parameters: a = 3.9286(3) Å, b = 3.9259(2) Å, c = 3.9302(3) Å, and β= 90.49(1)°. High-temperature XRD studies of the decomposition of AgNb_1/2Ta_1/2O₃ showed that the kinetics of the decomposition are influenced by the volatilization of silver. Suppression of silver volatilization by enclosing the sample in a corundum tube and applying an oxygen atmosphere yielded sintered, almost-monophasic AgNb_1/2Ta_1/2O₃ ceramics with >97% of the theoretical density. The Ag₈(Nb,Ta)₂₆O₆₉ phase, which regularly appeared as a result of partial decomposition of Ag(Nb_{1− x} Ta _x )O₃, was studied by energy-dispersive spectroscopy analysis and XRD. The Ag₈(Nb,Ta)₂₆O₆₉ phase exhibited a bronzelike orthorhombic structure with the following unit-cell parameters: a = 37.116(5) Å, b = 12.432(3) Å, and c = 7.826(2) Å. Indexed powder diffraction data for Ag₈(Nb,Ta)₂₆O₆₉ have been reported.     

Oxide and Alumina Ceramics (Review). Part 3. Russian Manufacturers of Alumina Ceramics1

Refractories and Industrial Ceramics - This review provides the analysis of aluminum oxide, including the properties associated with its use, types of commercial products, production methods...     

Fabrication and Scintillation Performance of Nonstoichiometric LuAG:Ce Ceramics

Nonstoichiometric LuAG:Ce Ceramics ([Lu_(1–x)Ce_x]₃Al₅O₁₂, x = 0.005) with different excess of Lu³⁺ were designed on the basis of Lu₂O₃‐Al₂O₃ phase diagram and fabricated by a solid‐state reaction method. Without using any traditional sintering aids, pure phase and good optical performance were obtained in such a Lu‐rich LuAG:Ce ceramics. In addition, scintillation efficiency and light yield of 1% excess of Lu³⁺ ceramic sample were found 16 times and 1.82 times higher than that of commercial Bi₄Ge₃O₁₂ (BGO) single crystals, respectively. Such values are comparable or even better than those in most of LuAG:Ce single crystals. However, antisite defects were also induced by excess of Lu doping, whose luminescence was found at 350–410 nm in Lu‐rich LuAG:Ce ceramics. The relationship of excess content of Lu and the microstructure, optical quality, and scintillation performance were clarified and discussed. Furthermore, by utilizing X‐ray absorption near edge spectroscopy technique, the charge state stability of cerium in Lu‐rich LuAG:Ce ceramics was examined. It appears that the excess of isovalence Lu³⁺ doping has a negligible effect on the cerium valence instability and creation of stable Ce⁴⁺ center.     

Fabrication of Silicon Carbide-Mullite Composite by Melt Infiltration

The melt infiltration method was used to fabricate a SiC-mullite composite at high temperature. Mullite was successfully obtained from a SiO₂ and Al₂O₃ powder mixture by melting above 1830°C in a BN crucible with a lid. When infiltrated into a porous SiC preform, the mullite significantly reacted with SiC to form gaseous SiO and CO, even at the lowest investigated temperature of 1830°C, consuming SiO₂ and leaving Al₂O₃ and silicon phases in the sample. The relevant reactions were studied in detail. A closed system was adopted to suppress the reaction, and a dense composite was successfully obtained.     

PTCR Characteristics and Fabrication of Porous, Sb-Doped BaTiO3 Ceramics

Sb-doped BaTiO₃ ceramics containing corn-starch were prepared by sintering at 1350°C for 1 h in air. In this study, the effect of corn-starch on positive temperature coefficient of resistivity (PTCR) characteristics and microstructures of Sb-doped BaTiO₃ ceramics was investigated. It was found that the porosity and pore size increased and the grain size slightly decreased with increasing corn-starch content. XRD results showed the presence of BaTiO₃ peaks only in the Sb-doped BaTiO₃ ceramics with and without corn-starch. The PTCR jump of the Sb-doped BaTiO₃ ceramics with corn-starch was over 10⁶ and 1–2 orders higher than that of samples without corn-starch. The increase in the room-temperature resistivity with increasing corn-starch content was attributed mainly to the increase in the electrical barrier height of grain boundaries and the porosity as well as the partial decrease in the donor concentration of grains and the grain size. It was also noticed that the grain boundary resistivity contributed largely to the total resistivity of the Sb-doped BaTiO₃—corn-starch ceramics.     

Fabrication of Open-Cell, Microcellular Silicon Carbide Ceramics by Carbothermal Reduction

A novel processing route for developing open-cell, microcellular SiC ceramics has been developed. The strategy adopted for making microcellular SiC ceramics involved the following: (i) fabricating a formed body from a mixture of polysiloxane, phenol resin (used as a carbon source), polymer microbeads (used as sacrificial templates), and Al₂O₃–Y₂O₃ (an optional sintering additive); (ii) cross-linking the polysiloxane in the formed body; (iii) transforming the polysiloxane and phenol resin by pyrolysis into silicon oxycarbide and C, respectively; and (iv) synthesizing SiC by carbothermal reduction. By controlling the microbead and additive contents, it was possible to adjust the porosity so that it ranged from 60% to 95%.     

Fabrication of Translucent Magnesium Aluminum Spinel Ceramics

A precursor for magnesium aluminum spinel powder, composed of crystalline ammonium dawsonite hydrate (NH₄Al(OH)₂CO₃·H₂O) and hydrotalcite (Mg₆Al₂(CO₃)(OH)₁₆·4H₂O) phases, was synthesized via precipitation, using ammonium bicarbonate as the precipitant. The precursor was characterized by differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. Reactive spinel powder, which could be densified to translucency under vacuum at 1750°C in 2 h without additives, was obtained by calcining the precursor at 1100°C for 2 h.     

Fabrication of High-Curie-Point Barium-Lead Titanate PTCR Ceramics

High-Curie-point semiconducting barium-lead titanate positive temperature coefficient of resistivity (PTCR) ceramics of composition Ba_0.897Pb_0.1La_0.003TiO₃ and Ba_0.5Pb_0.5La_0.003TiO₃ were prepared. The starting powders were synthesized by reacting commercial BaTiO₃, PbO, and TiO₂. To avoid the nonstoichiometry due to the volatilization of Pb during the sintering process, a lead atmosphere sintering approach with PbTiO₃ as packing powder was used. The samples being fabricated by this method show a PTCR effect of 3 to 4.5 orders of magnitude above the Curie point. The curie points were about 180°C for Ba_0.897Pb_0.1La_0.003TiO₃ and about 360°C for Ba_0.497Pb_0.5La_0.003TiO₃.