Oxyacetylene ablation of (Hf0.2Ti0.2Zr0.2Ta0.2Nb0.2)C at 1350 − 2050 °C

Ablation resistance of a multi-component carbide (Hf_0.2Ti_0.2Zr_0.2Ta_0.2Nb_0.2)C (HTZTNC) was investigated using an oxyacetylene flame apparatus. When the surface temperature of the HTZTNC was below 1800 °C, (Nb, Ta)₂O₅, (Hf, Zr)TiO₄, and (Hf, Zr)O₂ were found to be the main oxidation products, while at higher temperature, formation of (Hf, Zr, Ti, Ta, Nb)O_x was favored and its content gradually increased with the increase in ablation temperature. Based on the ablation results and thermodynamic simulation analysis, a possible ablation mechanism of HTZTNC was proposed. Active oxidation of TiC and outward diffusion of TiO were demonstrated to occur during the ablation process, which constitute the critical steps for the ablation of HTZTNC. These results can contribute to the design of ablation resistant ultra-high-temperature ceramics.     

High-strength medium-entropy (Ti,Zr,Hf)C ceramics up to 1800°C

Medium-entropy (Ti,Zr,Hf)C ceramics were prepared by hot pressing a dual-phase medium-entropy carbide powder with low oxygen content (0.45 wt%). The results demonstrate that the medium-entropy (Ti,Zr,Hf)C ceramics sintered at 2100°C had a relative density of 99.2% and an average grain size of 1.9 ± 0.6 μm. The flexural strength of (Ti,Zr,Hf)C carbide ceramics at room temperature was 579 ± 62 MPa. With an increase in temperature to 1600°C, the flexural strength showed an increase up to 619 ± 57 MPa, and had no significant degradation even up to 1800°C. The high-temperature flexural strengths of (Ti,Zr,Hf)C were obviously higher than those of the monocarbide ceramics (TiC, ZrC, and HfC). The primary strengthening mechanism in (Ti,Zr,Hf)C could be attributed to the high lattice parameter mismatch effects between TiC and ZrC, which not only inhibited the fast grain coarsening of (Ti,Zr,Hf)C ceramics, but also increased the grain-boundary strength of the obtained ceramics.     

Wear behavior of (Mo–Nb–Ta–V–W)C high-entropy carbide

Wear characteristics of an (Mo–Nb–Ta–V–W)C high-entropy carbide were investigated using ball-on-flat technique. The experimental material with a high relative density of 99.0%, single phase, average grain diameter of 10.7 μm, and nanohardness of grains 28.6 GPa was prepared by ball-milling and two-step field-assisted sintering. The tribological test was realized during dry sliding in air with the SiC ball as tribological partner at applied loads 5, 25, and 50 N. The microstructure, deformation, and damage characteristics were studied using scanning electron microscopy and confocal electron microscopy. The friction coefficient values during the test with 5 and 25 N were very similar and stable, with a value of approximately .4, whereas during the test with 50 N, it decreased from the value of .48–.42. The specific wear rate increased with increasing load from 3.71 × 10^–7 mm³/N m at 5 N to 2.59 × 10^–6 mm³/N m at 50 N. The dominant wear mechanism was mechanical wear with intensive grains pullout, fracture, and powder formation, without visible tribochemical reactions and tribo-layer formation. The wear rate decreased due to the created rolling contacts among the tribopartners thanks to the hard and spherical nanopowders present.     

(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)2AlC–(Ti0.2V0.2Cr0.2Nb0.2Ta0.2)C high-entropy ceramics with low thermal conductivity

In recent years, the microstructure and physicochemical properties of high-entropy ceramics have received much interest by the combination of multiple principal elements. Herein, (Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)₂AlC–(Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)C high-entropy ceramics (M₂AlC-MC HECs) were prepared by the spark plasma sintering (SPS) technique, attributing to the structural and chemical diversity of MAX phases. The microstructure of M₂AlC-MC HECs was characterized from micron to atomic scales, and the phase composition of M₂AlC-MC HECs was analyzed by a combination of Maud and Rietveld analysis. The results indicate the successful solid solution of Ti, V, Cr, Nb, and Ta atoms in the M-site of the 211-MAX configuration, and all the samples show a classic layered structure. The weight percentage of (Ti_0.2V_0.2Cr_0.2Nb_0.2Ta_0.2)₂AlC in the M₂AlC-MC HECs was more than 90%. Furthermore, the thermoelectric properties of M₂AlC-MC HECs were investigated for the first time in this study, and the electrical conductivity and thermal conductivity of HECs are 3278 S cm⁻¹ and 2.78 W m⁻¹ K⁻¹at 298 K, respectively.     

Ablation behavior of high-entropy carbides ceramics (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C upon exposition to an oxyacetylene torch at 2000 °C

The ablation performance of a high-entropy ceramic carbide, (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C, was performed by oxyacetylene ablation flame, simulating the extreme service environment at 2000 ºC. Phase and microstructure characterization at multi-length scales was carried out. During ablation, a compositionally and microstructurally complex oxidation layer formed on the ablation surface, which consisted of a combination of (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇, Ti(Nb_xTa_1?x)₂O₇, and Ti_x(Zr_aHf_bNb_cTa_1?a-b-c)_1?xO₂. Based on the microstructure information, the ablation mechanisms were proposed considering the oxidation thermodynamics and kinetics. Comparable rates of O inward diffusion and Ti outward diffusion are suggested, and a particular innermost dense layer composed of isolated (Zr_xHf_1?x)₆(Nb_yTa_1?y)₂O₁₇ grains embedded in a continuous Ti(Nb_xTa_1?x)₂O₇ matrix is considered to be beneficial for a better ablation resistance.     

高熵(Zr,Hf, Nb,Ta)C微米长方体的制备、吸波性能和抗氧化性研究

针对高熵碳化物制备困难,本文采用ZrC、HfC、NbC和TaC粉为原料,Ni粉为熔剂,通过低温无压烧结工艺成功制备出三种不同成分的高熵(Zr, Hf, Nb, Ta)C粉体。结果表明,三种粉体均为微米长方体,且暴露(100)晶面。(Zr_1/4Hf_1/4Nb_1/4Ta_1/4)C微米长方体因具有高介电常数而展现出优异的吸波性能,在厚度为3.5 mm、频率为6.16 GHz时,最低反射损耗值可达-48.86 dB。高熵(Zr, Hf, Nb, Ta)C微米长方体在800～1 200℃下展示出优异的抗氧化性,且氧化产物均由正交相(Nb_xTa_1-x)₂O₅固溶体、单斜相(Zr_xHf_1-x)O₂固溶体和正交相HfO₂所组成,与氧化温度和过渡金属的物质的量比无关。Zr、Hf、Nb和Ta的协同作用导致其氧化机制与单组元碳化物截然不同,Hf的存...     

Reactive sintering behavior and enhanced densification of (Ti,Zr)B2–(Zr,Ti)C composites

Reactive hot pressing was used to prepare (Ti,Zr)B₂–(Zr,Ti)C composites from equimolar ZrB₂ and TiC powders. The reaction and solid-solution coupling effect and enhanced densification in ZrB₂-50 mol.% TiC were proposed as contrasted to conventional consolidation of TiB₂-50 mol.% ZrC. The (Ti,Zr)B₂–(Zr,Ti)C composite sintered at a temperature as low as 1750 °C exhibited negligible porosity and average grain sizes of 0.30 μm for (Ti,Zr)B₂ and 0.36 μm for (Zr,Ti)C. Complete reaction and rapid densification of ZrB₂-50 mol.% TiC was achieved at 1800 °C for only 10 min. The densification mechanism was mainly attributed to material transport through lattice diffusion of Ti and Zr atoms with an activation energy of 531 ± 16 kJ/mol. This study revealed for the first time novel insights into rapid densification of refractory fine-grained diboride–carbide composites by reactive hot pressing at relatively low temperatures.     

(Ti0.25Zr0.25Nb0.25Ta0.25)C高熵陶瓷的制备、力学性能及氧化行为研究

本文通过对碳化物粉末进行放电等离子烧结(SPS),成功制备了(Ti_0.25Zr_0.25Nb_0.25Ta_0.25)C高熵陶瓷(HECs),系统研究了HECs的微观结构演变、力学性能和氧化行为。结果表明,单相HECs的形成温度为1 800 ℃,低于已报道的HECs烧结温度。1 900 ℃烧结的陶瓷晶粒细小,平均晶粒尺寸约7.5 μm,元素分布均匀,相对密度高达99.2%。1 800 ℃和1 900 ℃烧结的HECs的室温显微硬度值分别为30.9 GPa和33.2 GPa,断裂韧性值分别为(4.6±0.24) MPa·m^1/2和(4.5±0.31) MPa·m^1/2,高于大多数已报道的HECs。原位高温纳米压痕试验结果表明,HECs的硬度随温度的升高而降低,当温度达到500 ℃时,1 800 ℃和1 900 ℃烧结的陶瓷硬度分别下降到21.9 GPa和22.2 GPa,具有突出的高温稳定性。此外,HECs在温度低于500 ℃时无明显氧化,当温度超过650 ℃时会发生明显氧化,氧化速率随温度升高而增加。     

硼热还原法合成MeB_2(Me=Zr,Hf,Ti,Ta)粉体研究进展

过渡金属硼化物陶瓷MeB_2 (Me=Zr、Hf、Ti、Ta)具有高熔点、高强度、高硬度、高热导和高电导等优异性能。合成高纯超细MeB_2粉体对于进一步提升陶瓷性能至关重要。在众多合成MeB_2粉体的方法中,硼热还原法避免了碳和金属杂质的引入,有望合成高品质MeB_2粉体。本文回顾了硼热还原法合成MeB_2粉体的研究进展,重点介绍了最近发展的两种合成亚微米ZrB_2和HfB_2粉体的硼热还原法:两步热处理结合中间水洗法和硼热还原结合原位固溶法。     

CVD制备MB_2(M=Ti,Zr,Hf)研究进展

MB2(M=Ti,Zr,Hf)是ⅣB族过渡金属的硼化物,化学气相淀积(CVD)是制备MB2类薄膜材料和保护涂层的最有效技术之一。对MB2类材料CVD制备的热力学进行了简要分析。阐述了反应前驱体的选择和应用。对近年来MB2类材料的CVD合成制备的最新研究报道进行了整理和总结,对MB2类材料的CVD制备发展进程进行了概括。从CVD过程的设备工艺到产物的表征,从显微结构的定性研究到晶格常数匹配的定量分析,均进行了简要的阐述。最后,对CVD工艺制备MB2类材料的发展和应用前景进行了展望。     

Ultra-high strength medium-entropy (Ti,Zr,Ta)C ceramics at 1800°C by consolidating a core-shell structured powder

We report for the first time the synthesis of a core-shell structured composite powder with a core of Zr(Ti,Ta)C and a shell of Ti,Ta(Zr)C at 1700°C and investigate the formation mechanism for the core-shell structure. The medium-entropy (Ti,Zr,Ta)C ceramics with fine grains (1.1 ± 0.4 μm) and relative density of 94.8% was prepared by hot-pressing at 2100°C. The flexural strength of (Ti,Zr,Ta)C at 1000°C (493 ± 21 MPa) was close to the room temperature (511 ± 52 MPa). As the temperature increased from 1600°C to 1800°C, the flexural strength was increased significantly, with an ultra-high flexural strength of 725 ± 32 MPa at 1800°C. The existence of the core-shell structure in the powder suppressed the grain growth due to the sluggish diffusion effect. The ultra-high strength of (Ti,Zr,Ta)C ceramics was attributed to its fine microstructures, high fracture toughness, and the reinforced the grain boundary strength.     

Effects of Zr content on microstructure and corrosion resistance of Ti–30Nb–Zr casting alloys for biomedical applications

Titanium alloys show attractive properties for biomedical applications where the most important factors are biocompatibility, corrosion resistance, low modulus of elasticity, very good strength-to-weight ratio, reasonable formability and osseointegration. The aim of this study was to investigate the effects of Zr content (7.5 and 15 wt%) on microstructure and corrosion resistance of Ti–30Nb–Zr as-cast alloy samples. The corrosion tests were carried out in a 0.9% NaCl (0.15 mol L^?1) solution at 25 °C and neutral pH range. A horizontal centrifuged solidification set-up was used to obtain as-cast samples with a fine dendritic arrangement. In order to evaluate the corrosion behavior, electrochemical impedance spectroscopy (EIS), polarization curves and an equivalent circuit analysis were used. It was found that the as-cast microstructure tends to have the dendrite arm spacings reduced with increasing Zr content. The results have shown that the addition of Zr to a Ti–30Nb alloy has been efficient to stabilize the beta phase. The resulting impedance parameters and passive current densities have shown that the Ti–30Nb alloys sample without Zr addition presents a higher surface reactivity which can induce a better osseointegration than those containing Zr.     

(Ti,Mo,W,Ta,V,Nb)(C,N)多元陶瓷相的价电子结构

运用固体与分子经验电子理论(EET理论)计算了(Ti,Mo,W,Ta,V,Nb)(C,N)多元陶瓷相的价电子结构.结果表明,价电子结构参数(nA)随碳化物添加量的增加而增加.不同碳化物对价电子结构参数的影响不同,其中VC的影响最为显著.价电子结构参数(nA)可以用来评价金属陶瓷的力学性能,提出了相关的判据关系式.     

The processing and properties of (Zr,Hf)B2–SiC nanostructured composites

(Zr, Hf)B₂–SiC nanostructured composites were fabricated by high energy ball milling and reactive spark plasma sintering (RSPS) of HfB₂, ZrSi₂, B₄C and C. Highly dense composites with homogeneously intermixed ultra-fine (Zr, Hf)B₂ and SiC grains (100–300 nm) were obtained after RSPS at 1600 °C for 10 min. The densification was promoted by high energy ball milling and ZrSi₂ additive. The additives were almost completely transformed into ZrB₂ and SiC during densification. The improvement of flexural strength and fracture toughness (641 MPa and 5.36 MPa m^1/2, respectively) was achieved. The relationships between the ultra-fine microstructure and mechanical properties were discussed.     

Pb(Zn,Nb)(Sn,Nb)(Zr,Ti)O3材料的介电性能研究

通过分析PZSN系材料的介电温度与介电频率曲线 ,发现PZSN系材料没有明确的相变温度 ,相变属弥散型相变。 12 5K与 4M处的介电频率峰对应O -Ti-O与O -Zr -O的偶极子的空间取向的响应行为 ,在顺电相T >Tcav时这种偶极子是不存在的 ,说明它们的行为与相变有关     

Zn1-xCax(Ti0.6Zr0.4)Nb2O8陶瓷的微波介电性能

采用传统固相反应法制备Zn_(1–x)Ca_xTi_(0.6)Zr_(0.4)Nb_2O_8(x=0.05,0.10,0.20,0.30)微波介质陶瓷,研究了不同Ca~(2+)取代量对Zn_(1–x)Ca_xTi_(0.6)Zr_(0.4)Nb_2O_8陶瓷的物相组成、显微结构及微波介电性能的影响,利用X射线衍射仪、扫描电子显微镜和网络分析仪等对其晶体结构、微观形貌及微波介电性能进行表征。结果表明:Ca~(2+)取代Zn~(2+)会导致Ca Nb_2O_6第二相的形成,且随Ca~(2+)含量的增加,ZnTiNb_2O_8相含量减少;Ca Nb_2O_6相的含量增加,导致Zn_(1–x)Ca_xTi_(0.6)Zr_(0.4)Nb_2O_8陶瓷的介电常数和品质因数减小,谐振频率温度系数向正方向移动。当x=0.3时,Zn_(1-x)Ca_xTi_(0.6)Zr_(0.4)Nb_2O_8陶瓷在1 140℃烧结并获得最佳微波介电性能:ε_r=30.42,Q×f=47 280 GHz,τ——f=–25.37×10~(–6)/℃。     

Electrophoretic deposition of Pb(Ni1/3Nb2/3)O3–Pb(Zr,Ti)O3 thick film on Pt wire

0.2PbNi_1/3Nb_2/3–0.8Pb(Zr,Ti)O₃ (PNN–PZT) thick films were deposited on Pt wire with the diameter of 50 μm by electrophoretic deposition (EPD) method. The EPD deposition times on the microstructures of PNN–PZT thick films were investigated. By optimizing the EPD process, the Pt wire can be uniformly wrapped with the PNN–PZT powders. During the sintering process, the as-deposited PNN–PZT/Pt wires were buried in the mixed powders of PbCO₃ and ZrO₂, and then sintered in the optimal temperature to get a dense microstructure. The piezoelectric properties of the thick films were characterized by scanning force microscopy (SFM) method. The results show that the PNN–PZT thick films prepared by EPD method have good piezoelectricity.     

钎料Zr含量对Ti–13Nb–13Zr/ZrO2钎焊接头界面组织及性能的影响

实现钛合金与Zr O₂陶瓷的生物相容性连接对于扩展其在生物医学领域的应用具有重要意义。采用Sn–Zr钎料成功实现了Ti–13Nb–13Zr合金和Zr O₂陶瓷的连接。运用SEM、EDS、XRD分析了TNZ/Zr O₂接头的界面组织与反应产物，通过剪切试验测试了接头的力学性能，并研究了Zr含量对钎焊接头的界面组织和性能的影响。研究表明，TNZ/Zr O₂接头典型界面微观组织为TNZ/Ti₆Sn₅/β-Sn+Zr(s,s)+Zr Sn₂/m-Zr O₂/Zr O₂，其中m-Zr O₂可以促进陶瓷与钎缝的冶金结合。随着Zr含量的增加，钎缝中Zr(s,s)含量逐渐增多，体积逐渐增大，Zr Sn₂相含量也相应增加，而β-Sn在钎缝中的占比逐渐减小。剪切试验表明，当Zr含量为6%(摩尔分数)时，钎焊接头的剪切强度达到最高，为25.6 MPa。接头的断裂路径分析...     

Liquid-phase sintering of Pb(Zr,Ti)O3 using PbO–WO3 additive

The objective of this work was to lower the sintering temperature of lead zirconate titanate (PZT) without reducing the piezoelectric performance. PZT was sintered using PbO–WO₃ additive of eutectic composition, which assists the densification process by liquid-phase formation. Sintering was carried out from 1075 to 1125 °C between 1 and 4 h. Density, dielectric properties and piezoelectric properties were measured. Microstructure and fracture mechanism have been studied by SEM. At the mildest sintering conditions, the additive has a positive effect on dielectric and piezoelectric properties. The liquid-phase sintering leads to a denser material without additional grain growth. PZT with PbO–WO₃ additive is mechanically weaker than pure PZT. The liquid phase leads to weaker grain boundaries and the material cracks in intergranular fracture, whereas pure PZT has a mixture of intergranular and transgranular fracture, and PZT sintered conventionally at 1260 °C has transgranular fracture.     

Preparation of Ni–Mo–C/Ti(C,N) coated powders and its influence on the microstructure and mechanical properties of Ti(C,N)-based cermets

Ni–Mo–C/Ti(C,N) coated powders, namely Ni–Mo alloy and Mo₂C coated Ti(C,N) composite powders, were synthesized by using a heterogeneous precipitation and thermal reduction method, then pressed and vacuum sintered to fabricate cermets. The chemical composition, microstructure and phases of the composite powders and the microstructure and properties of sintered cermets were experimentally investigated. The results show that a fine and uniform microstructure of (Ti,Mo)(C,N)-Ni cermets without the conventional core-rim structure is obtained. The phases formed during the preparation of the coated powders as well as the cermets were analyzed by means of a X-ray diffraction (XRD) technique. The XRD result confirms the formation of the Ni₃Ti phase in the cermets. Due to the formation of the non-magnetic Ni₃Ti and the dissolution of Mo in Ni binder phase, the magnetic properties are strongly retarded. The fracture of the cermets is mainly characterized by inter-granular and dimple fractures. Better mechanical properties can be obtained in comparison with conventionally fabricated ones.