Microwave Processing of Actively Seeded Precursor for Fabrication of Polymer Derived Ceramics

An innovative, faster and nonconventional processing technique is demonstrated for preparing silicon carbide‐based ceramics derived from polymer precursor. The technique is based on microwave‐induced pyrolysis of an actively seeded, high‐purity preceramic polymer that leads to rapid fabrication of silicon carbide components. It is successfully demonstrated that it is feasible to carry out microwave‐induced pyrolysis by seeding the polymer precursor with very low volume fractions of micrometer and nanometer‐sized metal and dielectric fillers. This process allows for rapid, net‐shape, and potentially low‐cost fabrication of silicon carbide‐based materials. Mechanical properties and microstructure of the silicon carbide‐based composites fabricated using this process are characterized.     

Microwave Dielectric Properties of Gallium-Doped Hexagonal Barium Titanate Ceramics

High-permittivity and low-loss ceramics with composition BaTi_0.92Ga_0.08O_2.96 have been prepared in the BaO–Ga₂O₃–TiO₂ system using the mixed-oxide route. This compound forms as the hexagonal polymorph (6 H ) of BaTiO₃ with the space group P 6₃/ mmc . The dielectric properties of dense ceramics have been studied, at microwave frequencies, with the ceramics fired at 1450°C under flowing oxygen gas; the results are a relative permittivity, ɛ_r, of ∼74 and a quality factor, Q · f _r, of ∼7815 at 5.5 GHz. The quality factor is strongly influenced by the sintering conditions (temperature and atmosphere), whereas the relative permittivity is not influenced significantly by ceramic processing for pellets ≥93% of the theoretical X-ray density. To our knowledge, this is the first report of microwave dielectric resonance in a perovskite-type BaTiO₃-based ceramic.     

LNG项目的隔热结构设计

结合广东LNG项目的实例,综合分析了几种隔热材料的性能,从中进行选择,进而确定隔热结构,为同类型工程项目的隔热设计工作提供了参考。     

Insulating Properties of Lanthanum-Doped BaTiO3 Ceramics Prepared by Low-Temperature Synthesis

Samples of composition Ba_{1− x} La _x Ti_{1− x /4}O₃, x = 0, 0.003, 0.03, and 0.10, were prepared by an alkoxide sol–gel route with final firing of ceramics at 1100°C, 2 h in air. All samples showed bulk insulating behavior with no evidence of semiconductivity caused by either direct donor doping or oxygen loss.     

Nepheline and Carnegieite Ceramics from A-Type Zeolites by Microwave Heating

The thermal disintegration processes of A-type zeolites and the properties of the materials that were formed after microwave heating (at 2.45 GHz) were studied. The thermal disintegration of 4A-type zeolite proceeded as follows: disintegration at ∼880°C, followed by transformations to high-carnegieite 1 at ∼920°C, nepheline (in nepheline(4A) form) at ∼990°C, and high-carnegieite 2 at ∼1254°C (further heating led to melting at ∼1526°C). High-carnegieite 1, nepheline(4A), and high-carnegieite 2 efficiently absorbed microwaves. Cooling of the molten material yielded low-carnegieite that contained some impurities. The change of 3A-type zeolite proceeded as follows: disintegration at ∼910°C and transformation to nepheline (in nepheline(3A) form) at ∼1006°C. In 3A-type zeolite, in contrast to 4A-type zeolite, carnegieite did not form before the crystallization of nepheline(3A). Nepheline of high purity and carnegieite with some impurities were synthesized easily by heating 3A- and 4A-type zeolites, respectively, using microwaves. These results suggest that the microwave heating of zeolites can be used to prepare some ceramic materials.     

Microwave Heating of Grain Boundaries in Ceramics

It has been suggested that enhanced sintering of ceramics by microwave heating may be the result of accelerated grainboundary diffusion caused by the grain boundaries being heated to temperatures significantly greater than the bulkspecimen temperature. Heat flow calculations show that the temperature difference between the grain boundary and grain interior will be a small fraction of a degree, even under extreme conditions.     

Influence of Copper(II) Oxide Additions to Zinc Niobate Microwave Ceramics on Sintering Temperature and Dielectric Properties

The effect of CuO additions on the firing temperature of ZnNb₂O₆ ceramics was investigated using dilatometry, transmission electron microscopy, and X-ray diffractometry. A 5 wt% CuO addition to ZnNb₂O₆ ceramics significantly lowered the firing temperature from 1150° to ∼900°C. The presence of a CuO-rich intergranular phase in the specimen was observed and was evidence of the formation of a liquid phase during sintering. The composition of the liquid phase was (ZnCu₂)Nb₂O₈. In particular, the low-fired ZnNb₂O₆ ceramics had good microwave dielectric characteristics— Q × f = 59 500, ɛ_r= 22.1, τ_f=–66 ppm/^oC. These properties were correlated with the formation of a second phase, (ZnCu₂)Nb₂O₈.     

工艺管道防冻及保温设计分析

厂区内管道的保温和防冻设计与节约能源和工厂安全密切相关。管道由于受冻破裂可能直接给工厂带来或大或小的经济损失,保温厚度不合理可能造成能源的浪费等。从工程中碰到的情况来对管道的保温和防冻设计进行了分析。     

Low‐Temperature Synthesis of CoNb2O6 Microwave Dielectric Ceramics

Low‐fired cobalt niobate (CoNb₂O₆) microwave dielectric ceramics were prepared through a developed sol–gel process using Nb₂O₅·nH₂O as starting source. A metal‐dioxo‐bridged complex precursor was described on the basis of FT‐IR spectrum. The crystalline phases of calcined powders were characterized by X‐ray diffraction. Nanosized CoNb₂O₆ particles with orthorhombic α‐PbO₂‐type structure were obtained above 750°C. There was no subsequent phase change upon sintering, and all compounds sintered to at least 94% of theoretical density. At 1000°C/4 h, CoNb₂O₆ ceramics exhibited ε_r ~ 21.9, Q × f ~ 66 140 GHz (at 8.9 GHz) and τ_f ~ ?39.7 ppm/°C, having a good potential for low‐temperature cofired ceramic applications.     

SiO2气凝胶隔热保温涂料研究及保温结构优化设计

石油化工行业是国家节能减排重点工程之一,如果相关设备设施保温不当,不仅会造成严重的热量散失,还会对油水分离和输送等产生影响。本文系统介绍了传统的以中空玻璃微珠或陶瓷微珠为主要隔热功能填料的隔热保温涂料的保温机理、性能及其存在的问题;以 SiO₂气凝胶为主要功能填料的新型隔热保温涂料研究现状及研究中的技术难点;针对不同应用环境,开发兼具防腐、防开裂、隔热保温功能的复合保温结构设计。本文为研制新型耐高温、高效水性无机隔热保温涂料的研究提供了参考方向,为隔热保温涂料在石油化工领域的工程应用提供分析。     

浅谈浮法玻璃熔窑保温结构设计

浮法玻璃熔窑日益趋近大型化,实行窑炉全保温是一项全方位的系统工程。为能达到既节约能源又延长窑炉使用期限的效果,要设计合理的窑炉结构,选用优质的相匹配的耐火材料。本文依据笔者设计实践,介绍了大中型浮法玻璃熔窑全保温结构设计。     

大碹保温设计与实践

通过论述大碹保温设计和实践,比较了热保与冷保的利弊,提出了适合国情的大碹保温方法.     

Temperature Measurements during Microwave Processing: The Significance of Thermocouple Effects

Reliable and accurate temperature measurement during microwave processing of ceramic bodies is controversial. Although thermocouples are routinely used in conventional thermal furnaces, their presence in microwave furnaces can locally distort the electromagnetic field, conduct heat away from the sample, induce thermal instabilities and microwave breakdown, and lead to serious measurement errors. These thermocouple effects have been studied and found to be more pronounced in low- and medium-loss ceramic materials. To decrease the thermocouple effects during the processing of advanced ceramic materials, an optical, noncontact temperature sensing system has been developed, calibrated, and incorporated into a computer-controlled microwave furnace.     

绝热设计与绝热施工

本文对绝热设计和绝热施工中遇到的几个问题进行了补步的探讨。     

大型LNG单容罐保冷结构设计及绝热验算

对大型液化天然气储罐的绝热保冷材料及其保冷结构进行了详细的研究,提出了用于LNG储罐的保冷材料所应具有的性能及LNG储罐保冷结构特点,并以一台10 000 m~3LNG单容罐的保冷设计为例,验证其自然漏热量和日蒸发率,为大型LNG储罐保冷设计提供了依据。     

Fabrication of Ceramic Photonic Crystals with Diamond Structure for Microwave Applications

A process for fabricating three-dimensional photonic crystals composed of SiO₂–TiO₂-based ceramics with a diamond structure was investigated. An epoxy structure having an inverse diamond configuration was fabricated by stereolithography, a rapid prototyping method. The epoxy structure was infiltrated with a ceramic slurry and then cold isostatically pressed. After sintering at 670°C for 5 h in air, the epoxy was burned off, leaving behind the desired structure of a ceramic photonic crystal. The calculated band diagram indicated that an absolute photonic band gap for all wave vectors existed. The measurement of transmission in the 〈100〉 direction from 10 to 20 GHz showed that a complete band gap formed at about 14.7–18.5 GHz. The magnitude of the maximum attenuation was as large as 30 dB at 17 GHz, which indicated that the fabricated structure worked well as a photonic crystal.     

Electric-Field-Controlled Permittivity Ferroelectric Composition for Microwave LTCC Modules

A low-temperature cofired ferroelectric ceramic composition for electrically tunable radio frequency devices is introduced. A sintering temperature of 950°C for BaSrTiO₃–MgO was obtained following the combined addition of B₂O₃ and Li₂CO₃. The effects of these sintering aids on densification, microstructure, and ferroelectric properties were investigated. The permittivity and dissipation factor values were 234 and 0.0010, respectively, measured at 286.3 K, 1 kHz. The electrically tunable ferroelectric structure for microwave measurement was fabricated throughout the tape-casting process. Measured permittivity was 130 at 26 GHz and tunability >15% (4 V/μm).     

保冷选材和施工浅析

从保冷材料的性能和经济等选用原则入手,重点介绍了-30℃以下的情况下选用泡沫玻璃和硬质聚氨酯复合保冷结构的优点,并结合实践经验,论述了在施工中常见保冷失效的主要原因。     

Low-Fire Processing of ZrO2–SnO2–TiO2 Ceramics

Effects of a liquid-phase-sintering aid, BaCuO₂+ CuO (BCC), on densification and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5–5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST. The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO₂, SnO₂, CuO, and BaTi₈O₁₆ which reduce the amount of BCC flux available for liquid-phase sintering. The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST. For samples with 2.5–5 wt% BCC, a high relative sintered density is obtained at 1000°C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35–38 and 2800–5000, respectively.