Microstructure and Oxidation Behavior of ZrB_(2)-SiC Ceramics Fabricated by Tape Casting and Reactive Melt Infiltration

ZrB_(2)-based ceramics typically necessitate high temperature and pressure for sintering,whereas ZrB_(2)-SiC ceramics can be fabricated at 1500℃using the process of reactive melt infiltration with Si.In comparison to the conventional preparation method,reactive synthesis allows for the more facile production of ultra-high temperature ceramics with fine particle size and homogeneous composition.In this work,ZrSi_(2),B4C,and C were used as raw materials to prepare ZrB_(2)-SiC via combination of tape casting and reactive melt infiltration herein referred to as ZBC ceramics.Control sample of ZrB_(2)-SiC was also prepared using ZrB_(2) and SiC as raw materials through an identical process designated as ZS ceramics.Microscopic analysis of both ceramic groups revealed smaller and more uniformly distributed particles of the ZrB_(2) phase in ZBC ceramics compared to the larger particles in ZS ceramics.Both sets of ceramics underwent cyclic oxidation testing in the air at 1600℃for a cumulative duration of 5 cycles,each cycle lasting 2 h.Analysis of the oxidation behavior showed that both ZBC ceramics and ZS ceramics developed a glassy SiO_(2)-ZrO_(2) oxide layer on their surfaces during the oxidation.This layer severed as a barrier against oxygen.In ZBC ceramics,ZrO_(2) is finely distributed in SiO_(2),whereas in ZS ceramics,larger ZrO_(2) particles coexist with glassy SiO_(2).The surface oxide layer of ZBC ceramics maintains a dense structure because the well-dispersed ZrO_(2) increases the viscosity of glassy SiO_(2),preventing its crystallization during the cooling.Conversely,some SiO_(2) in the oxide layer of ZS ceramics may crystallize and form a eutectic with ZrO_(2),leading to the formation of ZrSiO_(4).This leads to cracking of the oxide layer due to differences in thermal expansion coefficients,weakening its barrier effect.An analysis of the oxidation resistance shows that ZBC ceramics exhibit less increase in oxide layer thickness and mass compared to ZS ceramics,suggesting superior oxidation resistance of ZBC ceramics.

High temperature oxidation behavior of electroconductive TiN/O′-Sialon ceramics prepared from high titania slag-based mixture

The oxidation behavior of electroconductive TiN/O′-Sialon ceramics prepared using high titania slag as main starting material was studied at 1 200-1 300 °C in air. The isothermal and non-isothermal oxidation processes were investigated by DTA-TG. Phase compositions and morphologies of the oxidized products were analyzed by XRD, SEM and EDS. The results indicate that the oxidation of TiN and O′-Sialon occurs at about 500 °C and 1 050 °C, respectively. After oxidation at 1 200-1 300 °C, a protective scale that consists of Fe2MgTi3O10, SiO2 and TiO2 is formed on the surface of the materials, which effectively prevents the oxidation process. The formation of a protective scale is relative to TiN content and apparent porosity of the samples, the amount of SiO2 and amorphous phase in the oxidation product. At the initial oxidation stage, the oxidation kinetics of the materials follows perfectly the linear law with the apparent activation energy of 1.574×105 J/mol, and at the late-mid stage, the oxidation of the samples obeys the parabolic law with the apparent activation energy of 2.693×105 J/mol. With the increase of TiN content, mass gain of the materials increases significantly.

Oxidation behavior of boron-containing(Zr,Ti)C_(x)B_(y)solid solution ceramics at 1600℃in air

Multicomponent boron-containing carbide coatings(i.e.,(Zr,Ti)C_(x)B_(y))on a C/C composite show good ablation resistance.However,the high-temperature oxidation behavior of this new type of boron-containing(Zr,Ti)C_(x)B_(y)solid solution ceramics has not been clarified yet.The present work fabricated(Zr,Ti)C_(x)B_(y)solid solution block ceramics by spark plasma sintering,and their oxidation behavior at 1600℃in air(N2–20-vol%O2)was investigated for the first time.The effects of boron on the oxidation resistance of(Zr,Ti)C_(x)B_(y)ceramics were examined.The results indicate that the(Zr,Ti)C_(x)B_(y)ceramics display good oxidation resistance with the parabolic rate law describing the oxidation process.After the trace solution of boron(0.5 wt%)into(Zr,Ti)Cx,the oxidation resistance of carbide ceramics is significantly enhanced,leading to a decrease of 30%in the oxidation rate constant.The formed oxide scale in the(Zr,Ti)C_(x)B_(y)ceramics is dense,and the interlayer shows stronger ability to inhibit inward diffusion of oxygen.In addition,the introduction of boron leads to more negative binding energy of(Zr,Ti)C_(x)B_(y)and improves the oxidation resistance of carbides.

Molten salt synthesis, formation mechanism, and oxidation behavior of nanocrystalline HfB2 powders

Nanocrystalline Hf B2 powders were successfully synthesized by molten salt synthesis technique at 1373 K using B and Hf O2 as precursors within KCl/Na Cl molten salts.The results showed that the as-synthesized powders exhibited an irregular polyhedral morphology with the average particle size of 155 nm and possessed a single-crystalline structure.From a fundamental aspect,we demonstrated the molten-salt assisted formation mechanism that the molten salts could accelerate the diffusion rate of the reactants and improve the chemical reaction rate of the reactants in the system to induce the synthesis of the high-purity nanocrystalline powders.Thermogravimetric analysis showed that the oxidation of the as-synthesized Hf B2 powders at 773–1073 K in air was the weight gain process and the corresponding oxidation behavior followed parabolic kinetics governed by the diffusion of oxygen in the oxide layer.

Oxidation behavior of non-stoichiometric(Zr,Hf,Ti)C_(x) carbide solid solution powders in air

Multi-component solid solutions with non-stoichiometric compositions are characteristics of ultra-high temperature carbides as promising materials for hypersonic vehicles.However,for group IV transition-metal carbides,the oxidation behavior of multi-component non-stoichiometric(Zr,Hf,Ti)C_(x)carbide solid solution has not been clarified yet.The present work fabricated four kinds of(Zr,Hf,Ti)C_(x)carbide solid solution powders by free-pressureless spark plasma sintering to investigate the oxidation behavior of(Zr,Hf,Ti)C_(x)in air.The effects of metallic atom composition on oxidation resistance were examined.The results indicate that the oxidation kinetics of(Zr,Hf,Ti)C_(x)are composition dependent.A high Hf content in(Zr,Hf,Ti)C_(x)was beneficial to form an amorphous Zr-Hf-Ti-C-0 oxycarbide layer as an oxygen barrier to enhance the initial oxidation resistance.Meanwhile,an equiatomic ratio of metallic atoms reduced the growth rate of(Zr,Hf,Ti)O_(2)oxide,increasing its phase stability at high temperatures,which improved the oxidation activation energy of(Zr,Hf,Ti)C_(x).

Design and preparation of an ultra-high temperature ceramic by in-situ introduction of Zr_(2)[Al(Si)]_(4)C_(5) into ZrB_(2)-SiC:Investigation on the mechanical properties and oxidation behavior

Novel ZrB_(2)-matrix composites were designed and prepared by in-situ introducing SiC and Zr_(2)[Al(Si)]_(4)C_(5) simultaneously for the first time.The obtained composites were dense and showed good mechanical properties,especially the strength and toughness,706 MPa and 7.33 MPa·m^(1/2),respectively,coupled with high hardness of 21.3 GPa,and stiffness of 452 GPa.SiC and Zr_(2)[Al(Si)]_(4)C_(5) constituted a reinforcing system with synergistic effects including grain refinement,grain pull-out as well as crack branching,bridging,and deflection.Besides,the oxidation results of the composites showed that the oxidation kinetics followed the parabolic law at 1600℃,and the oxidation rate constants increased with the increase of Zr_(2)[Al(Si)]_(4)C_(5) content.The formation and evolution model of the oxidation structure was also investigated,and the oxide scale of the composite exhibited a three-layer structure.

Effect of Rare Earth on the Crystallization Behavior of Li_2O-Al_2O_3-SiO_2 Glass-ceramic

X-ray powder diffraction and Fourier transform infrared spectroscopy were applied for characterization of Li2O-Al2O3-SiO2 glass-ceramic powders doped with Eu2O3,Gd2O3 and Er2O3,respectively,in the conditions of different heat-treatment temperatures and with various amounts.The powders were derived from the polyacrylamide gel method.The results show that,the wet gels prepared by polyacrylamide perform a unique crystallization behavior in the process of drying,comparing with some customary preparation such as melt processing.The main crystal phase and crystallization sequence of Li2O-Al2O3-SiO2 micro-powders have no distinct with addition of Eu2O3,Gd2O3 or Er2O3,while the crystallization temperature of the β-spodumene decreased and the amount of the β-spodumene increased.

锆钛酸铅压电陶瓷元件高温老化行为及其微观机制

长时高温作用会显著加速压电陶瓷元件电学性能的退化,极大地影响压电元件使用寿命。本工作在85、125和150℃下对PZT压电陶瓷元件进行长时老化处理,研究并分析了高温老化过程中PZT陶瓷元件电学性能退化规律及其微观机制。压电常数与介电常数随老化周期延长均退化明显,初期退化速率较慢,后期退化加快并逐渐趋于稳定,介电常数和压电常数退化均符合Boltzmann变化规律。85℃和125℃下老化49 d时,元件介电常数退化率达到最大,分别为31.01%和36.48%;压电常数退化率达到最大,分别为11.57%和19.02%。PZT压电元件介电性能退化主要由表面Ag电极内结构退化、Ag/PZT界面弱化及PZT陶瓷退极化三方面所贡献,其中界面弱化和陶瓷退极化占主导。长时高温作用促使Ag电极逐渐缓慢氧化生成Ag2S,晶粒收缩产生大量孔洞,导致Ag电极致密度下降和Ag/PZT界面分层,降低了Ag/PZT的电容率;此外,长时高温作用加速了PZT陶瓷电畴转向无序状态,促进氧空位位移钉扎电畴壁,导致了PZT压电元件介电性能退化。

Y_(2)O_(3)改性SiC陶瓷高温水氧行为研究

连续碳化硅纤维增强碳化硅陶瓷基复合材料(SiCf/SiC)因高温水氧侵蚀致使力学性能急剧下降,限制了其热端部件在航空发动机中的长寿命服役。研究证实,该侵蚀主要表现为SiC基体和纤维与水、氧气反应生成气相Si(OH)_(4),导致质量耗散。因此,从组分、结构调控角度提升SiC耐水氧侵蚀性能已成为研究热点。对比研究了添加和未添加Y_(2)O_(3)对SiC陶瓷水氧侵蚀行为的影响。与SiC陶瓷相比,SiC-Y_(2)O_(3)陶瓷的氧化速率及挥发速率均明显下降,且随着氧化温度的升高,两者差距愈发明显,可见添加Y_(2)O_(3)后SiC陶瓷的耐水氧侵蚀性能得到明显改善。氧化后的陶瓷微观结构表明,SiC-Y_(2)O_(3)陶瓷氧化层明显更薄且更致密,进一步分析表明这主要由Y组分在陶瓷表面的迁移以及聚集所致。在侵蚀过程中,陶瓷内部的Y_(2)O_(3)会向SiC陶瓷表面迁移,并与SiO_(2)反应形成β-Y_(2)Si_(2)O_(7),并逐渐聚集形成连续的Y_(2)Si_(2)O_(7)层。因此,在氧化层与SiC陶瓷交界处形成了一个富Y_(2)Si_(2)O_(7)/富Y_(2)O_(3)层。多层含Y氧化层在陶瓷表面形成水氧阻挡屏障,有效抑制了水氧介质向陶瓷内部的渗透与侵蚀。该研究结果为SiC_(f)/SiC复合材料的耐水氧结构设计与调控提供了重要思路。

超高温陶瓷及其复合材料的稀土改性研究进展

超高温陶瓷及其复合材料因具有耐超高温、轻质和抗氧化烧蚀等优点,目前已成为航空航天领域热结构材料研究的热点和前沿。基于稀土化合物在热障涂层等领域的优异性能和成功应用,研究人员将稀土化合物引入超高温陶瓷及其复合材料中,改善氧化层的结构和性质,以期解决超高温陶瓷基复合材料氧化层增长速度偏快和宽温域高低温循环氧化层易剥落等问题。本文综述了稀土改性超高温陶瓷及其复合材料的研究现状,分析探讨了改性机理,并展望了未来的研究发展方向。

Simultaneous generation of electricity, ethylene and decomposition of nitrous oxide via protonic ceramic fuel cell membrane reactor

Ethylene,one of the most widely produced building blocks in the petrochemical industry,has received intense attention.Ethylene production,using electrochemical hydrogen pump-facilitated nonoxidative dehydrogenation of ethane(NDE)to ethylene,is an emerging and promising route,promoting the transformation of the ethylene industry from energy-intensive steam cracking process to new electrochemical membrane reactor technology.In this work,the NDE reaction is incorporated into a BaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1)O_(3-δ)electrolyte-supported protonic ceramic fuel cell membrane reactor to co-generate electricity and ethylene,utilizing the Nb and Cu doped perovskite oxide Pr_(0.6)Sr_(0.4)Fe_(0.8)Nb_(0.1)Cu_(0.1)O_(3-δ)(PSFNCu)as anode catalytic layer.Due to the doping of Nb and Cu,PSFNCu was endowed with high reduction tolerance and rich oxygen vacancies,showing excellent NDE catalytic performance.The maximum power density of the assembled reactor reaches 200 mW cm^(-2)at 750℃,with high ethane conversion(44.9%)and ethylene selectivity(92.7%).Moreover,the nitrous oxide decomposition was first coupled in the protonic ceramic fuel cell membrane reactor to consume the permeated protons.As a result,the generation of electricity,ethylene and decomposition of nitrous oxide can be simultaneously obtained by a single reactor.Specifically,the maximum power density of the cell reaches 208 mW cm^(-2)at 750℃,with high ethane conversion(45.2%),ethylene selectivity(92.5%),and nitrous oxide conversion(19,0%).This multi-win technology is promising for not only the production of chemicals and energy but also greenhouse gas reduction.

Single-source-precursor synthesis and air-plasma ablation behavior of(Ti,Zr,Hf)C/SiC ceramic nanocomposites at 2200℃

Dense monolithic(Ti,Zr,Hf)C/SiC ceramic nanocomposites with four different molar ratios of metallic elements in the(Ti,Zr,Hf)C phase(i.e.,Ti:Zr:Hf=1:1:1,2:3:5,2:3:3,and 1:2:1)were prepared upon pyrolysis of novel(Ti,Zr,Hf)-containing single-source precursors(SSPs),followed by spark plasma sintering(SPS).A thorough characterization was conducted to elucidate the synthesis of the SSPs,polymer-to-ceramic transformation,chemical/phase compositions,and microstructure of the SiTiZrHfC-based ceramics.The results revealed the feasibility of synthesizing nanocomposites with high(Ti,Zr,Hf)C contents using the SSP method.These nanocomposites were characterized by a unique microstructure with in situ generated(Ti,Zr,Hf)C@C core-shell nanoparticles homogeneously mixed withβ-SiC.The ablation behavior of the nanocomposites was evaluated on an air-plasma device for 60 s.Impressively,the nanocomposites exhibited excellent ablation resistance,and the lowest linear ablation rate reached−0.58μm/s at 2200°C.Notably,the ablation resistance can be dramatically improved by precisely tailoring the atomic ratios of metal elements within the(Ti,Zr,Hf)C phase via the molecular design of the SSPs.The formation of a multiple-oxide layer with both a high-meltingpoint phase((Ti,Zr,Hf)O_(2))and low-melting-point phases((Zr,Hf)TiO_(4))and glassy SiO_(2),as well as their structure,played a critical role in the enhanced ablation resistance.The uniform distribution of the high-melting-point(Ti,Zr,Hf)O_(2)nano/microparticles throughout the glassy SiO_(2)matrix significantly enhanced the viscosity and stability of the oxide layer by the pinning effect,offering superior protection against the ingress of oxygen atoms and excellent resistance to mechanical erosion.

可切削加工陶瓷材料研究进展

从玻璃陶瓷、氧化物陶瓷和非氧化物陶瓷三个方面比较全面地回顾了可加工陶瓷的研究进展，介绍了其制备工艺、可加工机制及强韧化机制，评述了国内外研究现状．并对可加工陶瓷研究的发展趋势提出了作者的观点．

纯铝微弧氧化陶瓷膜组织及耐腐蚀性能

利用微弧氧化技术,在碱性硅酸盐电解液中对纯铝进行表面改性处理,制备均匀致密的陶瓷膜。利用扫描电子显微镜(SEM)观察氧化陶瓷膜表面形貌及横截面组织结构,利用纳米压入硬度测试仪测量陶瓷膜的显微硬度和杨氏模量的分布,运用电化学方法测量陶瓷膜的耐腐蚀性能。结果表明,铝合金微弧氧化陶瓷膜的表面硬度高达25.3GPa,纳米硬度和杨氏模量在陶瓷膜的横截面分布相似,从膜基结合处向膜层表面呈下降趋势。从极化曲线中的腐蚀电势和腐蚀电流来看,微弧氧化处理后,纯铝的抗腐蚀能力得到很大的提高。

陶瓷过滤器材料及其制备技术进展

对陶瓷过滤器的材料及其生产工艺进行了综述 因非氧化物陶瓷、氧化物陶瓷及其混合物因具有很高的耐热性、抗热震冲击性、高强度、良好的抗氧化性。

光聚合陶瓷先驱体裂解制备陶瓷涂层及其抗氧化性能

炭材应用广泛,但在高于500℃的有氧气氛中氧化迅速,其结构和性能受到严重影响。为此,采用光聚合巯基/乙烯基聚硅氮烷(PSN-1)陶瓷先驱体在炭材料表面裂解制备抗氧化陶瓷涂层,采用偏光显微镜和X射线衍射技术探讨了涂层对炭材抗氧化性能的影响。结果表明:先驱体溶液浓度为30%时浸渍效果最好;加入质量比(Ti/PSN-1)为1/10～1/5的钛粉时,制备的涂层抗氧化性能最佳,恒温氧化120min后,失重率仅为18%;最佳的浸渍/裂解循环次数为3次,恒温氧化120min,失重率为15%;聚硅氮烷经高温裂解后最终生成氮化硅陶瓷。

非氧化物陶瓷材料的氧化性

从研究方法、非氧化物陶瓷材料的氧化性以及主要理论成果等几个方面 ,综合评述了目前国内外对非氧化物陶瓷材料氧化性行为的研究状况。综合得出 ,研究氧化性的主要方法为高温TGA分析和材料氧化后称重两种方法 ,利用XRD、SEM、WDX、XPS、EDS和TEM等手段对氧化后的试样成分、相组成、结构等进行分析 ;评述了硼化物、碳化物、氮化物等几种具有代表性非氧化物陶瓷材料的氧化性行为 。

Elucidating the role of preferential oxidation during ablation:Insights on the design and optimization of multicomponent ultra-high temperature ceramics

Multicomponent ultra-high temperature ceramics(UHTCs)are promising candidates for thermal protection materials(TPMs)used in aerospace field.However,finding out desirable compositions from an enormous number of possible compositions remains challenging.Here,through elucidating the role of preferential oxidation in ablation behavior of multicomponent UHTCs via the thermodynamic analysis and experimental verification,the correlation between the composition and ablation performance of multicomponent UHTCs was revealed from the aspect of thermodynamics.We found that the metal components in UHTCs can be thermodynamically divided into preferentially oxidized component(denoted as MP),which builds up a skeleton in oxide layer,and laggingly oxidized component(denoted as ML),which fills the oxide skeleton.Meanwhile,a thermodynamically driven gradient in the concentration of MP and ML forms in the oxide layer.Based on these findings,a strategy for pre-evaluating the ablation performance of multicomponent UHTCs was developed,which provides a preliminary basis for the composition design of multicomponent UHTCs.

多孔氮化硅高温氧化特性分析

采用TG-DSC、XRD、SEM、ICP等分析手段,对某一典型多孔氮化硅样品进行4个不同温度点的静态和微动态连续氧化试验,最高氧化温度为1 400℃。结果表明:多孔氮化硅在0.1 MPa静态空气气氛下,800℃之前,氧化反应非常微弱,800℃以上可见明显的氧化反应,1 000℃以上氧化反应加剧,增重速率加快,并优先发生在表面与外部孔壁处,之后再发生在样品的内部孔隙处,氧化反应受界面处的化学动力学控制,以被动氧化为主,主要生成物是SiO2,属吸热反应。当生成的SiO2将氮化硅表面和孔壁处覆盖时,在其界面处,随着温度的进一步升高或时间的延长,会生成Si2N2O,且需要注意防范样品可能出现脆性断裂情况。此外,同等温度下,动态氧化气氛将加速氮化硅的氧化,特别是多孔和粉末状样品。

TC4钛合金表面微弧氧化陶瓷涂层的摩擦学行为

在硅酸盐体系碱性溶液中,采用交流微弧氧化法在TC4钛合金基体上制备出氧化物陶瓷膜,并通过X射线衍射、扫描电镜分析了膜层的物相组成、微观结构和显微形貌。用HIT-2型球-盘摩擦磨损试验机对其耐磨性能进行了测试,分析陶瓷涂层的摩擦学行为。结果表明:微弧氧化陶瓷膜层同GCr15对摩钢球干摩擦时的摩擦系数仅为0.09左右,表面的磨损痕迹较轻微,磨损量较少,其磨损机制主要是磨粒磨损与黏着磨损;且由于在磨损过程中对摩材料转移到微弧氧化陶瓷层表面,在磨损后期呈现钢-钢对磨的规律;微弧氧化技术能够改善TC4钛合金基体的表面耐磨性。