(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。     

(Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Fe2O4高熵陶瓷的制备与吸波性能

采用固相反应法制备了单相块体(Mg_0.2Co_0.2Ni_0.2Cu_0.2Zn_0.2)Fe₂O₄高熵尖晶石陶瓷。结合X射线衍射,扫描电子显微镜和能谱仪对制备过程中的物相组成、显微结构和元素分布进行分析。随烧结温度的升高陶瓷材料体积密度增大,气孔率降低,1 200℃烧结所得致密高熵尖晶石陶瓷材料呈单相,元素均匀分布,其弯曲强度和断裂韧性分别达43.00 MPa和1.30 MPa·m^1/2。所制备高熵尖晶石陶瓷对电磁波兼具介电损耗和磁损耗能力,其在3.0 mm处可获得最大的有效吸收带宽为12.37 GHz,是具有一定承载能力和优异宽频吸波性能的陶瓷材料。     

碳纳米管掺杂的SiCOAl陶瓷的制备及吸波性能研究

为了得到一种新型的耐高温吸波材料,利用乙酰丙酮铝(AlAC)对聚硅乙炔树脂(PSA)进行改性,得到了一种新型的含铝树脂PSAl,再利用先驱体转化陶瓷法制备出了硅碳氧铝陶瓷(SiCOAl)。SiCOAl陶瓷呈非晶态,伴生大量的碳纳米管(CNTs)和微纳米孔洞,增强了界面极化,利于电磁波的吸收。随着铝元素的增加(Al AC添加量增多),SiCOAl陶瓷产率从82%降低至45%;当Al含量为0.5%(Al-1)时,SiCOAl中的碳纳米管量最多,吸波能力最强;计算得到的反射损耗最低可达到-21 dB,有效吸收频宽可达到3.3 GHz。结果表明,制备的碳纳米管掺杂的SiCOAl陶瓷具有很好的吸波性能,有望应用在电磁波吸收领域。     

Nb2C/MnO2复合材料的制备及吸波性能研究

二维过渡金属碳化物（MXenes）由于其多层结构、优异的导电性和较大的层间距,在电磁波吸收领域具有广阔的发展前景。本采用静电自组装法制备了一种多层结构的Nb2C纳米片和一维纳米棒形貌的MnO2组成的复合体系。通过研究发现,Nb2C/MnO2复合材料质量比为1∶1时,最小反射损耗值在16.98GHz处可达到-29.22d...     

(Fe,Co,Ni,Cu,Zn)CrxOy高熵复相陶瓷的制备及其吸波性能

以高熵合金的研究为背景,将构型熵稳定单相的概念引入无机非金属材料,而逐步发展出一种新的陶瓷材料体系——高熵陶瓷。高熵陶瓷的优点是成分和结构的多样性,这使得其有潜能成为广泛应用的功能材料。本工作采用简单易行的固相烧结法合成了具有尖晶石结构和钙钛矿结构的高熵复相陶瓷,并进一步研究了其物相组成、显微结构、元素含量及价态、和电磁波吸收性能,探究了高熵复相陶瓷的吸波性能随烧结温度的变化规律。结果表明：高熵复相陶瓷可成功制备成型,通过高熵效应能够烧结出2种晶体结构(尖晶石结构和钙钛矿结构)。在1 300℃的烧结温度下,存在最大的介电常数,在频率范围为X波段8.2～12.4 GHz时,具备最佳的电磁波吸收性能。     

壳核结构SiC/C纤维的制备与吸波性能的研究

采用活性碳纤维转换法制备了壳核结构SiC/C纤维,采用拉曼光谱、SEM、XRD以及热重分析等测试方法对比研究了生成SiC的厚度对壳核结构SiC/C纤维样品的热重及吸波性能的影响.结果表明:包裹SiC壳层后样品吸波性能得到提高,样品厚度为3.0 mm时,保温4 h样品的最小反射损耗在8.24 GHz处达到-17.22 d...     

非等摩尔比Sr(Ti,Zr,Y,Sn,Hf)O3-σ高熵钙钛矿氧化物的制备及电学性能研究

本研究采用非等摩尔混料结合两步法烧结制备了一系列非等摩尔比高熵钙钛矿氧化物Sr(Ti,Zr,Y,Sn,Hf)O3-σ。通过XRD、TG-DSC、SEM及TEM分析了其物相转变、表面形貌及晶体结构,并且采用电化学交流阻抗谱(EIS)对其电导率进行分析。研究结果表明Sr(Ti,Zr,Y,Sn,Hf)O3-σ在1480℃左右形成了单相钙钛矿结构,并且各元素分布均匀。在测试温度范围为300~750℃的条件下,Sr(Ti,Zr,Y,Sn,Hf)O3-σ单相钙钛矿结构稳定,其电导率符合阿伦尼乌斯方程,电导机理保持稳定。相较于其他三种高熵钙钛矿氧化物,Sr(Ti0.20Zr0.20Y0.20 Sn0.20Hf0.20)O3-σ(HEOY1)表现出最高的电导率,其750℃的电导率为3.55×10-3S/cm,与文献报道中的高熵钙钛矿氧化物的电导率(2.41×10-3S/cm)相比有较大提升。     

三元复合材料Fe@(ZnO/C)的制备和吸波性能EI北大核心CSCD

以间苯二酚、二水乙酸锌和七水硫酸亚铁为原料,用湿化学法和高温热处理制备了的Fe@(ZnO/C)复合材料。结果表明:引入了纳米球链Fe粉后的Fe@(ZnO/C)三元复合材料具有更优的阻抗匹配和更高的衰减系数,当频率为10.91 GHz匹配厚度为2.73 mm时,样品的最小反射损耗(r_(RL_(min)))达到-56.01 dB,有效吸收带宽(r_(RL_(min))<-10 d B)达到5 GHz(8.54~13.54 GHz),表现出优异的电磁波吸收性能。     

(Zr,Sn)TiO4系微波介质陶瓷的掺杂改性研究现状

(Zr,Sn)TiO4系微波介质陶瓷因具有适中的介电常数εr,较高的品质因数Q和近零的谐振频率温度系数,r r,被广泛的用于制造谐振器、滤波器等微波元器件.本文综述了不同添加剂对(Zr,Sn)TiO4的掺杂改性机理,并指出了其研究方向.     

Mn掺杂(MgCoNiCuZn)0.2O高熵陶瓷的制备与摩擦性能的评价

采用固相烧结法制备了Mn²⁺掺杂(MgCoNiCuZn)_0.2O高熵陶瓷材料,并利用X射线衍射、摩擦性能测试手段对样品的晶体结构以及润滑摩擦系数进行了表征。XRD结果表明Mn²⁺掺杂量为0%、2.5%、5%的(MgCoNiCuZn)_0.2O高熵陶瓷材料都从900℃开始形成了形成了单一相的岩盐结构。三种不同Mn²⁺掺杂量的(MgCoNiCuZn)_0.2O高熵陶瓷材料均能降低蒸馏水平均摩擦系数,特别是5%Mn²⁺掺杂的(MgCoNiCuZn)_0.2O陶瓷材料作为添加剂效果最好,使得蒸馏水的平均摩擦系数降低20.65%,说明样品颗粒作为添加剂可以增强水的减摩效果。     

High-temperature oxidation behavior of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramics in air

High-entropy metal carbides have recently been arousing considerable interest. Nevertheless, their high-temperature oxidation behavior is rarely studied. Herein the high-temperature oxidation behavior of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy metal carbide (HEC-1) was investigated at 1573-1773 K in air for 120 minutes. The results showed that HEC-1 had good oxidation resistance and its oxidation obeyed a parabolic law at 1573-1673 K, while HEC-1 was completely oxidized after isothermal oxidation at 1773 K for 60 minutes and thereby its oxidation followed a parabolic-linear law at 1773 K. An interesting triple-layered structure was observed within the formed oxide layer at 1673 K, which was attributed to the inward diffusion of O₂ and the outward diffusion of Ti element and CO or CO₂ gaseous products.     

Low-temperature sintered (Ti,Zr, Nb,Ta, Mo)C-based composites toughened with damage-free SiCw

The high sintering temperature would have a great tendency to damage the morphology and thus properties of the silicon carbide whisker (SiC_w) in high entropy carbide-silicon carbide whisker (HEC-SiC_w) composites, which, in turn, would impact the effectiveness of the operative toughening mechanisms. The objective of this study was to achieve full contributions to the toughening effects of SiC_w by preparing (Ti, Zr, Nb, Ta, Mo)C-SiC_w composites at low temperature (1600 ℃) using cobalt as additives. Results showed that the fracture toughness of the (Ti, Zr, Nb, Ta, Mo)C bulk reinforced with 20 vol% SiC_w and 5 vol% Co was 7.2 MPa?m^1/2, which was much higher than that of the (Ti, Zr, Nb, Ta, Mo)C bulk only sintered with 5 vol% Co (3.4 MPa?m^1/2). Meanwhile, it was also higher than that of the reported HEC-20 vol% SiC_w composite sintered at 2000 ℃ (4.3 MPa?m^1/2). For the fracture toughness of HEC-SiC_w composites, it was significantly increased by the introduction of damage-free SiC_w.     

First-principles study,fabrication, and characterization of (Hf0.2Zr0.2Ta0.2Nb0.2Ti0.2)C high-entropy ceramic

The formation possibility of (Hf_0.2Zr_0.2Ta_0.2Nb_0.2Ti_0.2)C high-entropy ceramic (HHC-1) was first analyzed by the first-principles calculations, and then, it was successfully fabricated by hot-pressing sintering technique at 2073 K under a pressure of 30 MPa. The first-principles calculation results showed that the mixing enthalpy and mixing entropy of HHC-1 were −0.869 ± 0.290 kJ/mol and 0.805R, respectively. The experimental results showed that the as-prepared HHC-1 not only had an interesting single rock-salt crystal structure of metal carbides but also possessed high compositional uniformity from nanoscale to microscale. By taking advantage of these unique features, it exhibited extremely high nanohardness of 40.6 ± 0.6 GPa and elastic modulus in the range from 514 ± 10 to 522 ± 10 GPa and relatively high electrical resistivity of 91 ± 1.3 μΩ·cm, which could be due to the presence of solid solution effects.     

Synthesis,mechanical, and thermophysical properties of high-entropy (Zr,Ti,Nb,Ta,Hf)C0.8 ceramic

Two high-entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C_0.8, were prepared from monocarbides and ZrH₂. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high-entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry.     

Mechanisms responsible for enhancing low-temperature oxidation resistance of nonstoichiometric (Zr,Ti)C

A series of (Zr,Ti)C_x (x = 0.7–1.0) samples were fabricated by a modified spark plasma sintering apparatus to investigate the effects of carbon concentration and Ti substitutions on the oxidation behavior. Crushed powders of (Zr,Ti)C_x were oxidized in lab air (N₂–20-vol.% O₂) from room temperature to 900°C. The results indicated that Zr_0.8Ti_0.2C_0.8, with a nominal carbon concentration x = 0.8, displayed good oxidation resistance, which was attributed to the formation of dense t-(Zr,Ti)O₂ oxide solid solution. During the oxidation of (Zr,Ti)C_x, Ti substitutions for Zr enhanced the outward diffusion of carbon, enabling a uniform carbon layer and a Zr–Ti–C–O layer on the surface of carbides. The formed carbon layer improved the oxidation resistance of (Zr,Ti)C_x below 550°C, where carbon is relatively oxidation resistant. Increasing the Ti concentration was found to enhance the oxidation resistance of (Zr,Ti)C_x with an increased oxidation onset temperature (672 ± 2°C for Zr_0.8Ti_0.2C_0.8).     

Single-phase (Hf0.84Ta0.16)C solid solution nanowires growth via catalyst-assisted chemical vapor deposition

Fabricating one-dimensional (1D) ceramic nanostructure with superior performance is an important approach to broaden their applications. In this paper, the synthesis of single-phase (Hf_0.84Ta_0.16)C solid solution nanowires via catalyst-assisted chemical vapor deposition was proposed for the first time. The [0 1 1̄] growth direction of (Hf_0.84Ta_0.16)C solid solution nanowires was discussed in detail and its corresponding growth mechanism was identified to be top-type vapor–liquid–solid mechanism. More impressively, our work could widen the application ranges of ceramic nanowires and propose the new thinking of fabricating ultrahigh melting point nanostructure.     

(K,Na) (Nb,Ta,Sb)O3粉体制备及陶瓷介电性能研究

Ta/Sb掺杂的K0.5Na0.5Nb0.7TaxSb0.3-xO3(KNNSTx,x＝0.06,0.09,0.12,0.15,0.18,0.21, 0.24)粉体经水热合成,Ta、Sb对粉体物相、微观结构的影响被系统研究,烧结后陶瓷的微观结构、介电性能表明:陶瓷物相均具有纯钙钛矿结构;随着Ta含量的增加陶瓷晶粒尺寸逐渐增大,x＝0.09、0.12时较小粒径颗粒均匀分布在大颗粒的空隙之间,陶瓷密度增加;样品的介电常数随着Ta含量的增加x＝0.06～0.12逐渐降低,x＝0.15～0.24逐渐增加,居里温度均在350 ℃左右,且x＝0.09、0.12时陶瓷介电损耗较小.     

Regulation mechanism of in-situ synthesized (Nb,Ta)C/Ni composite cladding coatings by laser shock peening: Microstructure evolution and electrochemical corrosion behavior

The surface microstructure of (Nb,Ta)C/Ni composite cladding coatings containing in-situ synthesized composite carbides (ISSCC-LCed coatings) was regulated by laser shock peening (LSP). The regulation mechanism of laser shock waves (LSW) on the microstructure evolution and electrochemical corrosion behavior of the ISSCC-LCed coatings was emphatically revealed. Correspondence between corrosion morphology and microstructure was established. Meanwhile, the phase evolution and residual stress variation laws were also tested and analyzed. The obtained results indicated that the LSW-induced surface structure exhibited interphase distribution characteristics of large and small grains through plastic deformation mechanisms such as extrusion, slip, folding, fracture, and cracking. Plastic deformation caused the relative movement of (Nb,Ta)C at the subsurface out of the surface, and the large (Nb,Ta)C fractured and extended over the entire carbide. Furthermore, element interdiffusion phenomenon existed between (Nb,Ta)C and Fe–Ni alloy. The coupling effect of LSW-induced surface fine grains and residual compressive stress inhibited corrosion propagation and improved electrochemical corrosion resistance.     

Thermal and electrical properties of a high entropy carbide (Ta,Hf, Nb,Zr) at elevated temperatures

The thermal and electrical properties were measured for a high entropy carbide ceramic, consisting of (Hf, Ta, Zr, Nb)C. The ceramic was produced by spark plasma sintering a mixture of the monocarbides and had a relative density of more than 97.6%. The resulting ceramic was chemically homogeneous as a single-phase solid solution formed from the constituent carbides. The thermal diffusivity (0.045–0.087 cm²/s) and heat capacity (0.23–0.44 J/g•K) were measured from room temperature up to 2000°C. The thermal conductivity increased from 10.7 W/m•K at room temperature to 39.9 W/m•K at 2000°C. The phonon and electron contributions to the thermal conductivity were investigated, which showed that the increase in thermal conductivity was predominantly due to the electron contribution, while the phonon contribution was independent of temperature. The electrical resistivity increased from 80.9 μΩ•cm at room temperature to 114.1 μΩ•cm at 800°C.