Porous high-entropy rare-earth phosphate(REPO_(4),RE=La,Sm,Eu,Ce,Pr and Gd)ceramics with excellent thermal insulation performance via pore structure tailoring

Thermal insulation materials play an increasingly important role in protecting mechanical parts functioning at high temperatures.In this study,a new porous high-entropy(La_(1/6)Ce_(1/6)Pr_(1/6)Sm_(1/6)Eu_(1/6)Gd_(1/6))PO_(4)(HE(6RE_(1/6))PO_(4))ceramics was prepared by combining the high-entropy method with the pore-forming agent method and the effect of different starch contents(0–60vol%)on this ceramic properties was systematically investigated.The results show that the porous HE(6RE_(1/6))PO_(4)ceramics with 60vol%starch exhibit the lowest thermal conductivity of 0.061 W·m^(-1)·K^(-1)at room temperature and good pore structure stability with a linear shrinkage of approximately1.67%.Moreover,the effect of large regular spherical pores(>10μm)on its thermal insulation performance was discussed,and an optimal thermal conductivity prediction model was screened.The superior properties of the prepared porous HE(6RE_(1/6))PO_(4)ceramics allow them to be promising insulation materials in the future.

Preparation of Thermal Insulation Ceramics Using Felsic Tailings as Main Raw Material and Soda-ash Dregs as Flux

Low-cost thermal insulation porous ceramics with uniform pore diameter and low bulk density were prepared with soda-ash dregs and felsic tailings.We investigated the effect of temperature,foaming agent,fluxing agent,Al_(2)O_(3)and CaO content on the pore structure and crystal phase of porous ceramics.The effect of Ca^(2+)in soda-ash dregs on the preparation of quartz-feldspar based porous ceramics was studied.The results showed that the contribution of Ca^(2+)to the preparation of porous ceramics in this system was mainly to accelerate the Si-O bond fracture and reduce the sintering temperature at the initial stage of sintering,which destroyed the needle-like feldspar in the high temperature melt and reduced the melt viscosity,thus reduced the foaming resistance and promoted the porous products with uniform pore size distribution.The Ca^(2+)content on the high side can participate in the formation of crystals in sintering.The generated needle-like diopside and augite,which have small length-diameter ratio,will negligibly change in the viscosity of melt at high temperatures,and their inhibition effect on pores is not as good as that of feldspar with large length-diameter ratio,resulting in the merger and collapse of pores.But the increase of diopside and augite can improve the compressive strength of porous products to some extent.Porous ceramic products containing needle-like feldspar phase can be prepared by using two kinds of solid waste,which can improve the compressive strength of the products and reduce the raw material cost and energy consumption while comprehensively utilizing the double solid waste.The optimal product has a bulk density of 0.45 g/cm^(3),a compressive strength of 3.17 MPa,and a thermal conductivity of 0.11 W/(m·K).

Thermal Insulation Materials of Alumina Based Porous Ceramics

Significant energy saving effects can be made through the improvement of furnace refractories,especially the thermal insulation refractories. In this study,the preparation and the application of different alumina based porous ceramics were briefly introduced. Alumina based porous ceramics were prepared combined foaming method with gelcasting,sol- gel process or cement curing process. The influences of different preparation methods on the sintering shrinkage, porosity, phase composition, microstructure, compressive strength and thermal conductivity were discussed. Alumina based porous ceramics with relatively high strength and low thermal conductivity could be obtained through the above mentioned methods. Compared with the traditional lining materials,about 40% energy could be saved when they were used as the furnace wall.

Study of Leakage Current Behaviour on Artificially Polluted Surface of Ceramic Insulator

This paper presents the results of the study concerning to the leakagecurrent behaviour on artificially polluted ceramic insulator surface. From the present study it wasobserved that there is a reasonably well-defined inception of current i.e. scintillations at afinite voltage, The corresponding voltages for extinction of the current are in the range of 0.8 kVto 2.1 kV. Obviously, the dry band formed in the immediate vicinity of the pin prevents smoothcurrent flow as the voltage rises from zero. Only when the voltage is adequate it causes a flashoverof the dry band and current starts flowing. As is common in similar current extinction phenomena,here also, the extinction voltages are significantly lower than the inception voltages. Further, thevoltage-current curves invariably show hysteresis - the leakage currents are lower in the reducingportion of the voltage. This is obviously due to drying of the wet pollutant layer therebyincreasing its resistance. It is believed that this is the first time that such a directquantitative evidence of drying in individual half cycles is experimentally visualized.

Wet flashover voltage improvement of the ceramics with dielectric barrier discharge

Surface modification techniques with plasma are widely investigated to improve the surface insulation capability of polymers under dry conditions,while the relationship between treatment method,surface physical and chemical properties,and wet flashover voltage is still unclear for inorganic ceramics.In this work,the surface insulation properties of ceramics under wet conditions are improved using nanosecond-pulsed dielectric barrier discharge with polydimethylsiloxane(PDMS)as the precursor.The relationships between PDMS concentration and the water contact angle,dry and wet flashover voltages are obtained to acquire the optimal concentration.The surface charge dissipation test and surface physio-chemical property measurement with SEM,AFM,XPS are carried out to further explore the mechanism of surface insulation enhancement.The results show that film deposition with micron thickness and superhydrophobicity occurs at the PDMS concentration of 1.5%.The dry flashover voltage is increased by 14.6%due to the induction of deep traps,while the wet flashover voltage is increased by 66.7%.The gap between dry-wet flashover voltage is decreased by 62.3%compared with the untreated one due to the self-cleaning effect.

Biomimetic porous silicon oxycarbide ceramics with improved specific strength and efficient thermal insulation

Considering the challenge of aerodynamic heating,the development of high-performance insulating ce-ramic materials with lightweight and low thermal conductivity is crucially important for aerospace vehi-cles to achieve flight at high speed for a long time.In this work,macro-porous silicon oxycarbide(SiOC)ceramics with directional pores(DP-SiOC)(mean pore size of 88.1μm)were prepared using polysiloxane precursors via freeze casting and photocrosslinking,followed by pyrolysis.The DP-SiOC samples were lightweight(density∼0.135 g cm^(-3))with a porosity of 90.4%,which showed good shapability through the molding of polysiloxane precursors.The DP-SiOC samples also exhibited an ultra-low thermal con-ductivity of 0.048 W(m K)^(-1)at room temperature,which can also withstand heat treatment at 1200°C for 1 h.In addition,scaffolds with triply periodic minimal surfaces(TPMS)were fabricated using digital light processing(DLP)printing,which was further filled with polysiloxane precursors for increasing the strength of DP-SiOC.The TPMS scaffolds filled with macro-porous SiOC ceramics(TPMS-DP-SiOC)showed good integration between TPMS and macro-pore structures,which had a porosity∼75%and high specific strength of 9.73×10^(3)N m kg^(-1).The thermal conductivity of TPMS-DP-SiOC samples was 0.255 W(m K)^(-1)at room temperature.The biomimetic TPMS-DP-SiOC ceramics developed in this study are likely used for thermal protection systems.

Hierarchical porous(Ta_(0.2)Nb_(0.2)Ti_(0.2)Zr_(0.2)Hf_(0.2))C high-entropy ceramics prepared by a self-foaming method for thermal insulation

To meet the emerging demands for thermal protection materials for hypersonic aircraft,developing porous ultrahigh-temperature ceramics with both robust mechanical properties and superior thermal insulation performance is a critical challenge.Herein,we report novel porous(Ta_(0.2)Nb_(0.2)Ti_(0.2)Zr_(0.2)Hf_(0.2))C high-entropy carbide(PHEC)ceramics fabricated by a self-foaming method using commercially available metal chloride and furfuryl alcohol(FA)as precursors.The PHEC ceramics are constructed of microspheres with a size of 2µm,leading to a high porosity of 91.3%and an interconnected frame.These microspheres consist of high-entropy carbide grains(20 nm),resulting in abundant interfaces and nanosized pores in the PHEC ceramics.Due to its unique hierarchical structure,the prepared PHEC ceramics have outstanding compressive strength(28.1±2 MPa)and exceptionally low thermal conductivity(κ_(T),0.046 W·m^(-1)·K^(-1))at room temperature.This makes it a promising thermal insulation materials for ultrahigh temperature applications.This work provides a cost-effective and facile strategy for producing porous ultrahigh-temperature ceramics.

Lightweight Si_(3)N_(4)@SiO_(2) ceramic foam for thermal insulation and electromagnetic wave transparency

Increasing porosity is one of the most direct ways to improve the thermal insulation and dielectric properties of materials.Until now,many wet methods for preparing Si_(3)N_(4) ceramic foams usually face the problems of complex rheology,long period,and expensive cost,and the reported pore sizes of Si_(3)N_(4) ceramic foams are typically micron-grade,resulting in a lack of competitiveness in thermal insulation and wave-transparent applications.In this paper,the Si_(3)N_(4)@SiO_(2) ceramic foams were prepared using an efficient dry-method,which combined three processes of low temperature chemical vapor deposition(LTCVD),template,and isostatic pressing.The method has the advantages of simple operation and short preparation period,and can realize near-net size molding and mass production.In addition,the evolution mechanisms of honeycomb microstructure and composition of Si_(3)N_(4)@SiO_(2) ceramic foam during sintering were studied by chemical reaction thermodynamics.The as-prepared Si_(3)N_(4)@SiO_(2) ceramic foam possesses low density(0.377 g·cm^(-3)),high compressive strength(7.5 MPa),low thermal conductivity(0.0808 W·m^(-1)·K^(-1)),and excellent dielectric properties(ε<1.32,tanδ<0.009)in the frequency range of 8-18 GHz,and its maximum working temperature in air can reach up to 1100℃.It will be recommended to be applied in the interlayer of Si_(3)N_(4) ceramic radome to improve its thermal insulation and electromagnetic wave transparency performances.

Three-dimensional machining of insulating ceramics materials with electrical discharge machining

The insulating ceramics were processed with sinking and wire cut electrical discharge machining(EDM). The new technology was named as the assisting electrode method. In the machining, the electrical conductive material was adhered on the surface of insulating workpiece as the starting point of electrical discharge. As the processing operated in oil, the electrical conductive product composed of decomposition carbon element from working oil adhered on the workpiece during discharge. The discharges generated continuously with the formation of the electrical conductive layer. So, the insulating ceramics turn to the machinable material by EDM. We introduced the mechanism and the application of the machining of insulating ceramics such as Si3N4 and ZrO2.

Resistance Measurements and Interpretations Relating to Flashover of Artificially Contaminated Ceramic Insulator Strings

The flashover of insulator strings occurring at normal working voltages undercontaminated/polluted conditions, obviously deserves serious consideration. Though much researchhas been gone into pollution-induced flashover phenomena but grey areas still exist in ourknowledge. In the present experimental study the breakdown (flashover) voltages across gaps oninsulator top surfaces and gaps between sheds (on the underside of an insulator), also the flashoverstudies on a single unit and a 3-unit insulator strings were carried out. An attempt has been madeto correlate the values obtained for all the cases. From the present investigation it was found thatresistance measurement of individual units of a polluted 3-unit string before and after flashoverindicates that strongly differing resistances could be the cause of flashover of ceramic discinsulator strings.

Optimization of Compressed Ceramics Fiber Insulating Layers

Compressed thin layers of ceramic fiber insulation are used as high temperature insulating layers as well as mechanical support for catalyst coated ceramic monoliths in automotive emission control devices. Minimization of energy losses, choice of material and thickness of com- pressed insulating layer are based on knowledge of their thermal physical properties. Currently, consistent meas- urements of materials in a compressed state, as they would be in emission control applications, are absent due to the absence of suitable methods for s,wh tests. A test method was developed for measurement of the thermal conductivity of compressed thin fiber layers. This paper summarizes the results of thermal conductivity and diffu- sivity measurements of 27 compressed fiber alumina -sili- ca -vermiculite materials in the range of 200 -950℃. Thermal physical properties as a function of temperature, density/mechanical pressure, thickness and composition of insulating layers are presented. The whole set of exper- imental data is generalized on 3D surface plots and de- scribed by polynomial functions. The possible heat trans- fer mechanisms governing apparent thermal conductivity of pressed insulation layers are discussed.

EFFECTS OF STATIC ELECTRIC FIELD ON THE FRACTURE BEHAVIOR OF PIEZOELECTRIC CERAMICS

The paper gives an overview on experimental observations of the failure behavior of electrically insulating and conducting cracks in piezoelectric ce- ramics.The experiments include the indentation fracture test,the bending test on smooth samples,and the fracture test on pre-notched(or pre-cracked)compact ten- sion samples.For electrically insulating cracks,the experimental results show a com- plicated fracture behavior under electrical and mechanical loading.Fracture data are much scattered when a static electric field is applied.A statistically based frac- ture criterion is required.For electrically conducting cracks,the experimental results demonstrate that static electric fields can fracture poled and depoled lead zirconate titanate ceramics and that the concepts of fracture mechanics can be used to mea- sure the electrical fracture toughness.Furthermore,the electrical fracture toughness is much higher than the mechanical fracture toughness.The highly electrical fracture toughness arises from the greater energy dissipation around the conductive crack tip under purely electric loading,which is impossible under mechanical loading in the brittle ceramics.

Corrosion performance of Al–Al2O3 cold sprayed coatings on mild carbon steel pipe under thermal insulation

This paper focused on the corrosion resistance of cold spray Al–Al_2O_3composite coatings used on carbon steel pipe surfaces under thermal insulation. Al–Al_2O_3coatings were produced on the carbon steel pipe surface by cold spray(CS) technology. Experimental apparatus was built to test the corrosion resistance of coatings beneath mineral wool insulation under isothermal, thermal cycling and wet/dry conditions. The results showed that when α-Al_2O_3 was added in spraying powder, the coating could obtain higher hardness and a denser microstructure. From corrosionunder-insulation(CUI) tests, Al–Al_2O_3CS coatings were proven to be efficient in protecting carbon steel pipe from CUI mainly owning to lamellar microstructures of coatings. There was no evidence to show that α-Al_2O_3 might bring any negative effect on corrosion resistance. Al–Al_2O_3CS coatings were sensitive to the chloride ion concentration. When these coatings were exposed to higher concentrations of NaC l, the coating's exhibited faster degradation.

Synergistic regulation of temperature resistance and thermal insulation performance of zirconia-based ceramic fibers

ZrO_(2) fiber is a promising high-temperature resistant and heat-insulating fiber material.However,the decrease in mechanical properties caused by grain growth at high temperatures seriously affects its application.How to achieve the synergy of its temperature resistance and the thermal insulation performance is still the focus of the current industry.In this work,we started with doping inequivalent elements and studied the phase composition,temperature resistance,and thermal insulation properties of Y_(2)O_(3)-ZrO_(2) ceramic fibers by adjusting the Y/Zr molar ratio.The results showed that Y_(2)O_(3) could enter the crystal lattice of ZrO_(2) and form a solid solution.With the increase in Y_(2)O_(3) content,the structure of fibers changed from a tetragonal phase to a cubic phase,and the configurational entropy of the system increased.The larger configuration entropy in the sample could produce a robust steric hindrance effect,inhibiting grain growth.After heat treatment at 1300℃,the grain size of Y_(2)Zr_(2)O_(7)(Y5Z5)fibers was only 61.8%that of Y_(0.1)Zr_(0.9)O_(1.95)(Y1Z9)fibers.The smaller grain size made the Y5Z5 fibers still have excellent flexibility and deformation recovery performance after heat treatment at 1300℃and could still return to the original state after 85%compression or folded in half.In addition,due to the larger configurational entropy,the mean free path of phonon scattering was shortened,thereby improving the thermal insulation performance of the fiber.In short,this work achieves the synergistic effect of temperature resistance and thermal insulation properties of zirconia-based fiber materials only through simple inequivalent element doping.

The Investigation of Replacing Ceramic Bushings with Silicon Bushings in the Distribution Transformers

In recent years, the deficiencies of ceramic insulators along with their high maintenance costs have resulted in the replacement of ceramic insulators with silicon type in the pollution area. This idea has been employed for more than two decades in the polluted areas. Humidity of the weather in the south of Iran and the presence of pollutants in the air have made special conditions for construction and maintenance of some equipment including transformer bushings. Usual ceramic bushings, due to their ability in absorbing pollution and the rapid reduction of creepage distance (in a limited time period), have reduced transformer disconnection severely as a result of earth fault. Additionally, they are costly to be washed regularly. Therefore, using intelligent materials in designing bushing can increase the reliability of network and consequently reduction of costs. In this regard, this paper investigates the use of silicon bushings in the distribution systems and proposes operational ideas for the optimal operation of these devices in the polluted areas. The sample bushing was evaluated based on the IEC60137 standard test.

Carbonaceous ceramic nanofibrous aerogels for high-temperature thermal superinsulation

Ultralight ceramic aerogels are attractive thermal superinsulating materials,but display a formidable tradeoff between low and high temperature thermal conductivity(κ)due to their low-density features.Embedding carbon species as infrared opacifier in ultralight ceramic aerogels can substantially reduce the thermal radiation heat transfer without compromising the ultralow solid conduction.However,the oxidation resistance of embedded carbon species still remains inadequate to prevent thermal etching at high temperatures.Herein,we report a carbonaceous design and synthesis of ceramic nanofibrous aerogels with amorphous carbon embedded in the yttrium-stabilized zircon nanofibers to achieve a high-temperature thermal superinsulating performance with robust thermomechanical stability.The aerogels display one of the lowestκof 95 mW·m^(−1)·K^(-1)at 1,000℃in air among ultralight material family,as well as robust mechanical flexibility with up to 95%compressive strain,30%non-linear fracture strain,and 99%bending strain,and high thermal stability with ultralow strength degradation less than 1%after sharp thermal shocks(240℃·s^(-1))and working temperature up to 1,200℃.The combined high-temperature thermal superinsulating and thermomechanical properties offer an attractive material system for robust thermal insulation under extreme conditions.

数据驱动的中温隔热陶瓷管件结构优化设计及性能评估

随着中温区电加热的电子产品和家电用品不断涌现,低成本的高效保温隔热材料研究日益成为热点。如何确保管路内部快速升温并避免外表温度大幅波动,有效提高能源利用率,是保温隔热材料领域的挑战。据此,本研究以低熔点玻璃助烧结的镁橄榄石、莫来石、珍珠岩和磷酸锌等复合物陶瓷为对象,从数据驱动的壁内多孔结构隔热管件优化设计出发,开展陶瓷隔热管件三维打印制造,通过显微结构和力学性能分析,并围绕管件的导热、隔热影响因素评估,系统考察了该类新型隔热管件的理化、力学和隔热性能及影响关系。结果表明,通过对沿管壁环绕的矩形大孔高度调控(400~1 200μm)、管腔内热源区的管壁倍增(1.2 mm×2 mm)以及有机微球造孔剂引入(~9%),同时通过烧结温度控制(700~750℃),可以显著改善热流出口区的温度水平(~20℃),并显著优于不锈钢真空隔热管的出口温度。研究结果表明,增材制造复合陶瓷管件具有优良的综合性能以及较低的原料成本,使得多孔陶瓷隔热管件在中温隔热领域具有广阔的应用前景。

TiO_(2)含量对Al_(2)O_(3)基绝缘涂层综合性能的影响

在GCr15轴承钢的基体材料上制备了不同TiO_(2)含量的Al_(2)O_(3)基绝缘涂层,研究了TiO_(2)含量对Al_(2)O_(3)基绝缘涂层性能的影响,结果表明:孔隙率基本随着TiO_(2)含量的增加而增大;结合力随着TiO_(2)含量的增加先增大后基本不变,TiO_(2)含量为1.5%左右达到峰值;TiO_(2)含量越高,耐冲击性能越好;随着TiO_(2)含量的增加,高温、常湿和常温、高湿环境下涂层绝缘电阻均呈先升高后降低的趋势,TiO_(2)含量为1.5%左右达到峰值。对绝缘涂层进行封孔处理,试验结果表明,封孔处理可以显著提高涂层的结合力、环境适应性、抗冲击性能。TiO_(2)含量为1.5%~3.0%时的涂层在未封孔和封孔后均具备较好的综合性能。

先进绝缘材料电场辅助性能调控及制备研究进展

电工材料作为电工装备的基础,其特性直接决定极限运行参数,开发具有新功能、高性能的先进电工材料有助于电工装备挑战更高参数极限。受学科发展战略引导、重大科学问题启发,该文从电场辅助制备的物理机制、几类典型绝缘材料性能调控及制备机理等方面综述了电场辅助先进绝缘材料性能调控及制备的研究进展。首先,总结电场辅助制备电工材料的物理理论,详细阐述电泳、介电泳、自组装等机械效应和焦耳热、弛豫损耗等能量效应的机理。其次,以机械效应为主探讨电场辅助在以智能绝缘材料、导热绝缘材料、高密度储能材料、纤维增强材料和介电功能梯度材料为代表的先进绝缘材料性能调控领域的应用现状;以能量效应为主探讨电场辅助在热塑(固)性聚合物熔融(固化)、功能陶瓷快速烧结领域的研究进展。最后,对先进绝缘材料电场辅助性能调控及制备研究趋势进行展望,可为革新电工材料制造工艺、开拓传统高电压学科研究新方向提供一定理论参考。

可陶瓷化EPDM绝热材料的烧蚀特性及陶瓷化机理

绝热层中炭化层的特性与其耐烧蚀性能紧密相关。通过对可陶瓷化三元乙丙(EPDM)绝热材料氧乙炔烧蚀特性和成分进行表征分析,得到了线烧蚀率与炭化层不同参数之间的相关性,分析了类陶瓷炭化层的形成机理。结果表明,当可陶瓷化硅树脂填料含量在35~40 phr时,炭化层结构更致密,硬度更高,孔隙率更小,陶瓷产物更多,有效提高了EPDM绝热层的耐烧蚀性能;线烧蚀率与炭化层的结构与成分之间存在强相关性;类陶瓷炭化层结构的形成主要分为高分子聚合物的热分解、小分子物质的化学气相沉积、烧蚀表面SiC的原位生成和熔融SiO_(2)在炭化层中的填充粘结四个阶段,最终形成一种Si—O—C的复相陶瓷结构。