Tubular shaft-furnace elements based on ceramics with protective coating for thermochemical treatment of high-purity niobium materials

A technology for fabrication of heat-resistant components (tubes of diameter and length of up to 0.3 m and weight of up to 12 kg) using a refractory laminated ceramic is developed. The laminated ceramic is a quartz-glass base made up of fragments cemented together by means of a two-sided protective coating of fused niobium pentaoxide. Areas for potential application of the technology based on refractory laminated ceramics are discussed.__________Translated from Novye Ogneupory, No. 12, pp. 19 – 23, December, 2004.     

硅橡胶防热涂层隔热与耐热性能研究

本文系统研究了自制DMGQ-2硅橡胶防热涂层的热物理性能。结果表明,固化剂用量对热化学参数无显著影响,热化学参数随温度的变化而变化,通过拟合方程,可以对特定温度下DMGQ-2涂层的导热系数λ、比热容Cp、热扩散系数α进行预估。采用有限元分析,相比于X涂层,DMGQ-2涂层的隔热性能更优。TGA分析了DMGQ-2涂层的热分解温度。200℃热空气老化性能表明,DMGQ-2涂层具有较好的抗热空气老化性能。     

Influence of phosphorus pentoxide on crystallisation,microstructure and mechanical properties of potassium fluorophlogopite glass ceramics

Abstract

Potassium fluorophlogopite glass ceramics were prepared. Differential thermal analysis showed that there were two exothermic peaks at ～750 and 950°C, corresponding to the Avrami exponent ～1·5 and 2·0. Spherical particles were found using a scanning electron microscope when the glass ceramics were heat treated at 750 and 800°C. With the increase in heat treatment temperature, a kind of lamellar crystals was gradually formed, and the shape of crystals changed from spherical to lamellar when the Avrami exponent changed from 1·5 to 2·0. Mechanical tests showed that material cutting and bending strength increased, while the Vickers hardness decreased with the increasing of P₂O₅ content and heat treatment temperature.     

Preparation,properties and high-temperature oxidation resistance of MoSi2-HfO2 composite coating to protect niobium using spent MoSi2-based materials

Industrial spent MoSi₂-based materials and HfO₂ were recycled as raw materials to fabricate MoSi₂-HfO₂ composite coating by spark plasma sintering (SPS). The microstructural evolution of the coatings was characterized and the 1500 °C oxidation behavior was explored. Cracks penetrated through the MoSi₂ coating while no cracks can be found in the HfO₂-containing composite coating owing to the reduction of the mismatch of thermal expansion coefficient (CTE). Good metallurgical bonding was exhibited since (Mo,Nb)₅Si₃ diffusion layer was found in the HfO₂-containing coating by the diffusion of Nb and Si across the interface without gaps. After 1500 °C oxidation of 20 h, cracks appeared in the surface of SiO₂ layer on MoSi₂ coating while the HfO₂-containing composite coating possessed crack-free oxide scale. HfSiO₄ with high temperature (>2900 °C) is formed during oxidation and it inlays in the silica oxide scale to improve the stability. Compared to MoSi₂ coating, Nb coated MoSi₂-HfO₂ has thinner oxide scale with lower mass gain during oxidation, thus presenting better high-temperature anti-oxidation properties.     

金属基耐高温陶瓷涂层抗热冲击性能的研究

为提高金属基陶瓷涂层的抗热冲击性能,以无机胶粘剂磷酸二氢铝、耐磨陶瓷骨料氧化铝、碳化硅和氧化镁混合后涂覆于金属表面制得陶瓷涂层。通过交替加热及冷却试验测试该陶瓷涂层的抗热冲击性能,并与其他人的研究数据进行比较。所得涂层抗热冲击次数超过10次,超过了其他人的实验数据,这是由于涂层与基体在界面处相互扩散形成过渡层。另外,宏观上的机械联锁有利于提高涂层与基体在界面处的结合,从而提高了其抗热冲击性能。     

Preparation and properties of anti-static coating on the 3Y-TZP ceramics

Anti-static coating on the 3Y-TZP ceramics was prepared through adding ZnO and SnO₂ to the glaze. Wettability and surface resistivity of the coating was studied. Dissolving, crystallization and distribution of ZnO and SnO₂ in the coating were analyzed. Static dissipative mechanism of the coating was explored. The results showed that the interface bonding and wetting between the coating and 3Y-TZP were perfect. The surface resistivity of the coating could be reduced to 10⁷ Ω/□. ZnO was helpful to improve the wettability of the coating and inhibit the crystallization of the SiO₂. ZnO and SnO₂ dissolved in the glaze and the semi-conductive composition rich melt layer around SnO₂ was formed. The melt layer contacted each other and the static dissipative network was established, which attributed to the anti-static properties of the coating.     

超高耐蚀性氯化钾镀镉防护层的制备

介绍了一种具有超高耐蚀性的氯化钾镀镉防护层的制备工艺,包括在金属基体上从内到外依次制备氯化钾镀镉层、铬酸盐低铬彩色钝化层和羟基石墨烯封闭层.在钢铁件上制备的这种防护层在中性盐雾试验中20160 h无白锈生成,27000 h无红锈生成.这为解决金属在热带海洋环境中的强腐蚀问题提供了新的解决方案.     

Kinetics of photodegradation and nanoparticle surface accumulation of a nanosilica/epoxy coating exposed to UV light

Temperature effect on the kinetics of photodegradation and surface accumulation of nanoparticles in an epoxy nanocoating exposed to ultraviolet light (UV) was investigated. A model epoxy coating containing 5% untreated nanosilica was selected. Exposed film specimens were removed at specified UV dose intervals for measurements of chemical degradation of the epoxy component, and nanosilica accumulation on specimen surface release as a function of UV dose for four temperatures. The chemical degradation was measured using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV–visible spectroscopy. Atomic force microscopy was employed to determine the kinetics of nanosilica accumulation on the nanocoating surface during UV exposure. The temperature dependence behaviors of kinetic parameters obtained by various measurement techniques will be used to better understand the degradation mechanism and surface accumulation of nanoparticles in exterior nanocoatings.     

Effect of doped niobium pentoxide on structure and properties of hafnium oxide films deposited by ion beam assisted deposition

Hafnium oxide is widely used in optical coating materials because of its high refractive index, broad band gap, high laser damage threshold and its high transparency in the wavelength range from ultraviolet to middle infrared (0.20–12 μm). However, hafnium oxide has some defects such as strong crystallinity and poor surface quality, which results in enhanced absorption and reflection of light wave and limits its application in optical field. As a pentoxide with high transparency and excellent optical properties in the wavelength range from ultraviolet to mid-infrared(0.35–9 μm), the doping of niobium pentoxide can effectively improve the surface quality and optical properties of the films. Mixed films with different chemical compositions were prepared by ion beam assisted deposition, and the microstructure, crystallinity, surface quality, optical properties and mechanical properties of the mixed films with different chemical compositions were investigated. The doping of niobium pentoxide inhibits the crystallization of hafnium oxide, optimizes the surface quality of the films, and improves the refractive index and mechanical properties of mixed films, effectively broadens the application of hafnium oxide in the field of optical films.     

Microstructures and EMI shielding properties of composite ceramics reinforced with carbon nanowires and nanowires-nanotubes hybrid

Carbon/ceramic composites are promising candidates as electromagnetic interference (EMI) shielding materials used at various harsh environments. The aim of present work is to prepare and investigate two kinds of composite ceramics reinforced with carbon nanowires (CNWs) and nanowires-nanotubes (CNWs-CNTs) hybrid, respectively. Results indicate that CNWs is highly curved and multi-defected, and CNWs-CNTs hybrid shows the best crystal structure at an optimal catalyst concentration of 5 wt%. When CNWs accounts for 5.15 wt%, the total shielding effectiveness (SE) of CNWs/Si₃N₄ reaches 25.0 dB with absorbed SE of 21.3 dB, meaning that 99.7% incident signal can be blocked, while it reaches 25.4 dB for CNWs-CNTs/Si₃N₄ as the carbon loading only increasing to 3.91 wt%. By contrast, CNWs/Si₃N₄ exhibits better electromagnetic attenuation capability with stronger absorption, mainly due to the unique microstructure of CNWs. Both of two composite ceramics have great potential to be designed as structural and multi-functional materials.     

多波段光吸收涂层的制备及性能研究

以有机吸光染料(牌号为N660、N720和N890)为光吸收剂、羟基丙烯酸酯为预聚体树脂、光学级聚酯薄膜为基材,采用湿法涂布的方式制备了能同时对可见光、近红外光多个波段有吸收作用的光吸收涂层。研究结果表明:当w(N660)=0.40%、w(N720)=0.60%和w(N890)=0.80%(均相对于涂布液质量而言)时,所得涂层在660、720、890 nm处的光线透过率均低于0.10%,并且在380~600 nm可见光区的平均透过率超过50%,而且其力学性能(硬度2H、附着力5B)良好。     

Microstructures and mechanical properties of 8Y-FSZ ceramics with BaTiO3 additive

The microstructure and mechanical properties of 8 mol% Y₂O₃ fully stabilized zirconia (8Y-FSZ) with BaTiO₃ additive were investigated. The introduction of BaTiO₃ additive would significantly increase the density and the grain size of 8Y-FSZ ceramics. XRD, Raman spectroscopy, and dielectric measurement were performed. A rhombohedral Ba(Ti_1−xZr_x)O₃ ferroelectric phase resulted in the composite with 5 mol% additive, while for those with higher additive content, the secondary phase changes to cubic Ba(Ti_1−xZr_x)O₃. The fracture toughness of the xBaTiO₃/(1−x)8Y-FSZ composites reached a maximum and then decreased with increasing the amount of additive. The highest value reached 6.1 MPa m^1/2 for 0.05BaTiO₃/0.95(8Y-FSZ) sintered at 1475 °C for 3 h, where the piezoelectric/ferroelectric secondary phase toughening played an important role. Moreover, the fracture toughness of the composites increased firstly and then decreased with increasing sintering temperature.     

Experimental study on insulative properties of intumescent coating exposed to standard and nonstandard furnace curves

This paper reports the results of an experimental study on two types of intumescent coating exposed to the ISO834 standard fire and three nonstandard fire curves. The nonstandard fires were all less severe than the standard fire. A total of 72 intumescent coating protected steel specimens were tested. The expanded thickness of intumescent char was measured, and the pore feature was observed. Constant thermal conductivity for each specimen was calculated based on the measured steel plate temperature. Thermogravimetric analysis (TGA) test was carried out, and the results show that more gas is trapped within the coating due to better matching of thermal behaviour between gas evolution and polymer viscosity as the rate of heating increases. The constant effective thermal conductivities for the intumescent coating under the nonstandard fires were 65% (type‐W) and 35% (type‐S) higher than that under the standard fire, which resulted in an overestimation of the coating failure time up to 15 and 11 minutes, respectively. Therefore, it is sometimes insecure to use results from standard fire tests guiding the design of coating thickness for steel elements under nonstandard fire conditions.     

Impact of sandblasting on the mechanical properties and aging resistance of alumina and zirconia based ceramics

Bioinert zirconia and alumina ceramic devices are widely used, both in orthopaedics and in dentistry. In order to improve their bonding with bone tissues or dental resin cements, their surfaces are often roughened at different scales. In this work, we have investigated the effects of the same sandblasting treatment on alumina, zirconia and a zirconia-toughened alumina, focusing on their mechanical performance and the interplay between surface defects and residual stresses. Additionally, we explored the impact of the treatment on the hydrothermal aging of the two zirconia-containing materials. Residual stresses generated during sandblasting were always predominant over surface defects but their effect varied with the material: while they had a weakening effect on alumina, they reinforced both zirconia-containing materials. Finally, we found that the monoclinic grains at the surface of sandblasted zirconia recrystallized into tetragonal nanograins after annealing and this led to an increased resistance to aging.     

Microstructures and optical properties of 6Li,Ce:YAG ceramics for thermal neutron detection

The microstructures and optical properties of 5%⁶Li: Ce_3xY_3(1-x)Al₅O₁₂ (x = 0.001, 0.003, 0.05, 0.01, 0.02) transparent ceramics prepared by solid-state reaction and vacuum sintering were investigated in this paper. The results revealed that the grain size of ⁶Li,Ce:YAG ceramics at this ration conditions is 4 μm–20 μm. With the doping of Ce³⁺, the transmittance of ⁶Li,Ce:YAG ceramics decreases from 82% (x = 0.001) to 67% (x = 0.02) at 800 nm, and the intensity of transmittance peak at 340 nm and 460 nm increases. The emission peaks show red shift at around 530 nm with the increasing of Ce³⁺ concentration.     

Microstructures and ablation resistance of WSi2/ZrSi2/ZrxHf1-xC/SiC coating based on a pattern strengthening one-step method

To improve the ablation performance of carbon/carbon (C/C) composite materials, a WSi₂–ZrSi₂ composite-reinforced Zr_xHf_1–xC/SiC coating was prepared on the substrate surface via patterning strengthening method. The results show that this coating can protect the substrate from failure for 300 s under an oxyacetylene flame at 2600 °C. Owing to the presence of W, an extremely dense oxide layer was formed on the surface of the coating during initial ablation, which progressively led to the expulsion of the product of oxidation (WO₃) from the inner layer, as well as to holes and cracks on healing the coating surface, thereby significantly improving the ablation performance of the C/C composites. In addition, the excellent ablative performance and mechanism of the coating were analysed using volatility diagrams.     

Effect of the mass fraction of a binder in a thermal protective coating on the heat and mass exchange with a high-enthalpy gas flow

The properties of two modifications of a thermal protective coating based on phenolformaldehyde resin and carbon fabric with various portions of the components were studied experimentally. The results are used for mathematical simulation of the ablation of the materials during thermochemical degradation in a high-enthalpy gas flow. It is shown that a decrease in the mass fraction of the polymer binder has an ambiguous effect on the rate of ablation because of changes in thermal characteristics and increase in the thickness of the reacting layer under long-duration external thermal action. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 4, pp. 48–55, November–December 1998.     

Microstructures and mechanical properties of TiB2 composite ceramics with co-addition of Ti and TiC fabricated by SPS

TiB₂ composite ceramics containing different amounts of Ti and TiC were fabricated via spark plasma sintering (SPS), and effects of their addition contents on the microstructure and mechanical properties were discussed. The newly formed phases of TiB with a cubic lattice structure in the composite ceramics were observed. At a relatively low temperature of 1510 °C, pressure of 50 MPa, and holding time of 5 min, the TiB₂ composite ceramic with 30 wt% TiC and 10 wt% Ti additions acquired an excellent strength of 727 MPa and a high toughness of 7.62 MPa m^1/2. The improvement in strength and toughness was attributed to the mixed fracture mode, second phase strengthening, and increased energy consumption for crack propagation caused by the newly formed phases and fine TiC particles. In addition, the significant effects of the Ti and TiC addition contents on the densification temperature and mechanical properties of the composite ceramics were determined using analysis of variance (ANOVA).     

Effect of boron addition on microstructure,mechanical properties and oxidation resistance of TaC ceramics

TaC ceramics with 0.03–0.60?wt% of boron additions were prepared by hot pressing at 2100?°C for 1?h under a pressure of 40?MPa. Effects of boron content on densification, phase composition, microstructure, mechanical properties and oxidation resistance of the TaC ceramics were investigated. When the boron content was 0.12?wt% and above, full density was obtained due to reactions between boron and oxygen impurity at presence of TaC. Minor phases of TaB₂ and C were formed in the 0.24 and 0.60?wt% B compositions after gas-out of the oxygen impurity. Microstructure of the TaC ceramics was refined with increasing in boron content. The TaC ceramic with 0.24?wt% of boron showed the best mechanical properties with a Vickers hardness, flexural strength and fracture toughness of 17.7?GPa, 534?MPa and 4.6?MPa?m^1/2, respectively. When more boron was added, interfacial bonding of the TaC grains was strengthened causing a decrease in fracture toughness. Oxidation resistance of the TaC ceramics increased with boron content. Particularly, the 0.60?wt% B composition showed a weight gain of 0.0018?g/cm² after oxidization at 800?°C in air for 3?h.