(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5) high-entropy ceramic with low thermal conductivity,tunable thermal expansion coefficient,and excellent resistance to CMAS corrosion

Low thermal conductivity,compatible thermal expansion coefficient,and good calcium–magnesium–aluminosilicate(CMAS)corrosion resistance are critical requirements of environmental barrier coatings for silicon-based ceramics.Rare earth silicates have been recognized as one of the most promising environmental barrier coating candidates for good water vapor corrosion resistance.However,the relatively high thermal conductivity and high thermal expansion coefficient limit the practical application.Inspired by the high entropy effect,a novel rare earth monosilicate solid solution(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)was designed to improve the overall performance.The as-synthesized(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)shows very low thermal conductivity(1.07 W·m-1·K-1 at 600℃).Point defects including mass mismatch and oxygen vacancies mainly contribute to the good thermal insulation properties.The thermal expansion coefficient of(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)can be decreased to(4.0–5.9)×10^(-6)K^(-1)due to severe lattice distortion and chemical bonding variation,which matches well with that of SiC((4.5–5.5)×10^(-6)K^(-1)).In addition,(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)presents good resistance to CMAS corrosion.The improved performance of(Ho_(0.25)Lu_(0.25)Yb_(0.25)Eu_(0.25))_(2)SiO_(5)highlights it as a promising environmental barrier coating candidate.

New class of high-entropy pseudobrookite titanate with excellent thermal stability,low thermal expansion coefficient,and low thermal conductivity

As a type of titanate,the pseudobrookite(MTi_(2)O_(5)/M_(2)TiO_(5))exhibits a low thermal expansion coefficient and thermal conductivity,as well as excellent dielectric and solar spectrum absorption properties.However,the pseudobrookite is unstable and prone to decomposing below 1200℃,which limits the practical application of the pseudobrookite.In this paper,the high-entropy pseudobrookite ceramic is synthesized for the first time.The pure high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) with the pseudobrookite structure and the biphasic high-entropy ceramic composed of the highentropy pseudobrookite(Cr,Mn,Fe,Al,Ga)_(2)TiO_(5) and the high-entropy spinel(Cr,Mn,Fe,Al,Ga,Ti)_(3)O_(4) are successfully prepared by the in-situ solid-phase reaction method.The comparison between the theoretical crystal structure of the pseudobrookite and the aberration-corrected scanning transmission electron microscopy(AC-STEM)images of high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) shows that the metal ions(M and Ti ions)are disorderly distributed at the A site and the B site in high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5),leading to an unprecedentedly high configurational entropy of high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5).The bulk high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) ceramics exhibit a low thermal expansion coefficient of 6.35×10^(−6) K^(−1) in the temperature range of 25-1400℃ and thermal conductivity of 1.840 W·m^(−1)·K^(−1) at room temperature,as well as the excellent thermal stability at 200,600,and 1400℃.Owing to these outstanding properties,high-entropy(Mg,Co,Ni,Zn)Ti_(2)O_(5) is expected to be the promising candidate for high-temperature thermal insulation.This work has further extended the family of different crystal structures of high-entropy ceramics reported to date.

High Performance ZrNbAl Alloy with Low Thermal Expansion Coefficient

Thermal expansion is a common phenomenon in both metals and alloys, which is important for metallic material applications in modern industry, especially in nuclear and aerospace industries. A lower thermal expansion coefficient may cause lower thermal stress and higher accuracy. A new Zr-based alloy is developed and presented.The XRD diffraction results demonstrate that only a close-packed hexagonal phase（α or α＇ phase） exists in the microstructure. The thermal expansion and mechanical properties are studied. According to the experimental results, the new Zr-based alloy presents a low thermal expansion coefficient and good mechanical properties.Also,its thermal expansion coefficient is stable through solution treatment.

Mechanical, electrical, and thermal expansion properties of carbon nanotube-based silver and silver–palladium alloy composites

The mechanical, electrical, and thermal expansion properties of carbon nanotube（CNT）-based silver and silver–palladium（10：1, w/w） alloy nanocomposites are reported. To tailor the properties of silver, CNTs were incorporated into a silver matrix by a modified molecular level-mixing process. CNTs interact weakly with silver because of their non-reactive nature and lack of mutual solubility. Therefore, palladium was utilized as an alloying element to improve interfacial adhesion. Comparative microstructural characterizations and property evaluations of the nanocomposites were performed. The structural characterizations revealed that decorated type-CNTs were dispersed, embedded, and anchored into the silver matrix. The experimental results indicated that the modification of the silver and silver–palladium nanocomposite with CNT resulted in increases in the hardness and Young＇s modulus along with concomitant decreases in the electrical conductivity and the coefficient of thermal expansion（CTE）. The hardness and Young＇s modulus of the nanocomposites were increased by 30%？40% whereas the CTE was decreased to 50%-60% of the CTE of silver. The significantly improved CTE and the mechanical properties of the CNT-reinforced silver and silver–palladium nanocomposites are correlated with the intriguing properties of CNTs and with good interfacial adhesion between the CNTs and silver as a result of the fabrication process and the contact action of palladium as an alloying element.

DEVELOPMENT OF INCONEL~ ALLOY 783，A LOW THERMAL EXPANSION，CRACK GROWTH RESISTANT SUPERALLOY

Low thermal expansion superalloys have been used for a number of years in a variety of applications, including gas turbine engines. The low thermal expansion characteristics of the most widely used class of materials are derived from the ferromagnetic characteristics of Ni, Fe, and Co-based austenitic matrices containing little or no Cr.Alloy developments have been aimed at improving the oxidation resistance and stress accelerated grain boundary oxygen (SAGBO) attack.INCONEL alloy 783 is an oxidation resistant, low coefficient of thermal expansion superalloy developed for gas turbine applications. Alloy 783 represents a culmination in the development, of an alloy system with very high alumtnum content that, in addition to forming γ′,causes βaluminide phase precipitation in the austenitic matrix.This type of structure can be processed to resist both SAGBO and general oxidation,while providing low thermal expansion and useful mechanical properties up to 700℃.Key aspects of the alloy's development are presented.

Estimation of thermal expansion properties of quasicrystalline alloys

By investigating the thermal expansion properties of three quasicrystalline alloys Al 65 Cu 20 Cr 15 quenched, Al 65 Cu 20 Cr 15 cast and Al 65 Cu 20 Fe 15 cast particles reinforced Al matrix composites from 25 ℃ to 500 ℃, the thermal expansion coefficients of three quasicrystalline alloys were theoretically estimated. The results show that the thermal expansion coefficients of the composites are much lower than that of pure Al, and the thermal expansion coefficients of the composites reinforced by Al Cu Cr quasicrystalline particles are lower than those of the composites reinforced by Al Cu Fe quasicrystalline particles. According to estimating, quasicrystalline alloys have negative thermal expansion coefficients, and the thermal expansion coefficients of Al Cu Cr quasicrystalline alloys are lower than those of Al Cu Fe quasicrystalline alloys. In the alloys, the more the qusicrystalline content, the lower the thermal expansion coefficient.

(La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4: A high-entropy rare-earth phosphate monazite ceramic with low thermal conductivity and good compatibility with Al2O3

Low thermal conductivity, matched thermal expansion coefficient and good compatibility are general requirements for the environmental/thermal barrier coatings(EBCs/TBCs) and interphases for Al2O3 f/Al2O3 composites. In this work, a novel high-entropy(HE) rare-earth phosphate monazite ceramic (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is designed and successfully synthesized. This new type of HE rare-earth phosphate monazite exhibits good chemical compatibility with Al2O3, without reaction with Al2O3 as high as 1600℃ in air. Moreover, the thermal expansion coefficient(TEC) of HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4(8.9 × 10^-6/℃ at 300–1000℃) is close to that of Al2O3. The thermal conductivity of HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 at room temperature is as low as 2.08 W·m^-1·K^-1, which is about 42% lower than that of La PO4. Good chemical compatibility, close TEC to that of Al2O3, and low thermal conductivity indicate that HE (La0.2Ce0.2Nd0.2Sm0.2Eu0.2)PO4 is suitable as a candidate EBC/TBC material and an interphase for Al2O3 f/Al2O3 composites.

Synthesis and thermal expansion of 4J36/ZrW_2O_8 composites

Gas atomized 4J36 alloy powder was milled for 72 h then mixed with ZrW2O8 powder and sintered at 600℃ for 4 h under argon atmosphere. 4J36/ZrW2O8 composites containing 10 vol.%, 20 vol.%, 30 vol.%, and 40 vol.% ZrW2Os were fabricated, the relative density of which ranged from 70% to 80%. Thermal expansion coefficients of the composites decreased as the amount of ZrW2O8 increased, in agreement with the rule of the mixture. The coefficient of thermal expansion of the 4J36/40 vol.%ZrW2O8 composite in 25-100℃ is 0.55 × 10^＋6/℃.

Effects of cryogenic treatment on the thermal physical properties of Cu_(76.12) Al_(23.88) alloy

The thermal diffusion coefficient, heat capacity, thermal conductivity, and thermal expansion coefficient of Cu76.12Al23.88 alloy before and after cryogenic treatment in the heating temperature range of 25℃ to 600℃ were measured by thermal constant tester and thermal expansion instrument. The effects of cryogenic treatment on the thermal physical properties of CU76,12A123,88 alloy were investigated by comparing the variation of the thermal parameters before and after cryogenic treatment. The results show that the variation trend of the thermal diffusion coefficient, heat capacity, thermal conductivity, and thermal expansion coefficient of CU76.12Al23.88 alloy after cryogenic treatment was the same as before. The cryogenic treatment can increase the thermal diffusion coefficient, thermal conductivity, and thermal expansion coeffi- cient of Cu76.12Al23.88 alloy and decrease its heat capacity. The maximum difference in the thermal diffusion coefficient between the before and after cryogenic treatment appeared at 400℃. Similarly, thermal conductivity was observed at 200℃.

Evaluation of the microstructure,thermal and mechanical properties of Cu/SiC nanocomposites fabricated by mechanical alloying

Nano-sized silicon carbide(SiC:0wt%,1wt%,2wt%,4wt%,and 8wt%)reinforced copper(Cu)matrix nanocomposites were manufactured,pressed,and sintered at 775 and 875℃in an argon atmosphere.X-ray diffraction(XRD)and scanning electron microscopy were performed to characterize the microstructural evolution.The density,thermal expansion,mechanical,and electrical properties were studied.XRD analyses showed that with increasing SiC content,the microstrain and dislocation density increased,while the crystal size decreased.The coefficient of thermal expansion(CTE)of the nanocomposites was less than that of the Cu matrix.The improvement in the CTE with increasing sintering temperature may be because of densification of the microstructure.Moreover,the mechanical properties of these nanocomposites showed noticeable enhancements with the addition of SiC and sintering temperatures,where the microhardness and apparent strengthening efficiency of nanocomposites containing 8wt%SiC and sintered at 875℃were 958.7 MPa and 1.07 vol%^(−1),respectively.The electrical conductivity of the sample slightly decreased with additional SiC and increased with sintering temperature.The prepared Cu/SiC nanocomposites possessed good electrical conductivity,high thermal stability,and excellent mechanical properties.

Effects of Cryogenic Treatment on the Thermal Physical Properties of TC4 Titanium Alloy

A cryogenic treatment test was carried on TC4 titanium alloy,and the effects of cryogenic treatment on the thermal physical properties of TC4 titanium under the temperature range of 25~600°C were investigated through comparing the variation of the thermal conductivity and thermal expansion coefficient of the alloy before and after cryogenic treatment.The results show that the thermal conductivity of the cryogenic treated alloy is first less and then greater than that of the untreated alloy,however,its thermal expansion coefficient is first greater and then less than that of the untreated alloy with the increase of temperature.When the temperature is up to 600°C,the thermal conductivity and thermal expansion coefficient of the cryogenic treated alloy are 7.24 W·m-1·K-1 and 11.71×10-6°C-1,and increased and decreased by 3.18%and 1.65%than that of the untreated alloy at the same temperature,respectively.

Thermal Properties of Al-50%Si Alloys

In order to prepare a hypereutectic Al-Si alloy with low coefficients of thermal expansion (CTE), Al-50%Si alloy was produced by powder metallurgy (P/M) and ingot metallurgy (I/M). P/M specimen was prepared by mechanical alloying (MA) and pulsed electric-current sintering (PECS). The microstructures of specimens were characterized by optical microscopy and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). Vickers microhardness and CTE measurements were performed. The grains in the P/M specimen were refined with increasing MA time. Primary Si and eutectic Si in the I/M specimen were remarkably refined by adding minute amounts of Sr. The CTE of P/M and I/M specimens were estimated as T.SxlO"6 and 10.7xlO"6, respectively. These values were as same as a CTE of AlaOs ceramics.

Microstructure and thermal properties of copper matrix composites reinforced with titanium-coated graphite fibers

Milled form of mesophase pitch-based graphite fibers were coated with a titanium layer using chemical vapor deposition technique and Ti-coated graphite fiber/Cu composites were fabricated by hot-pressing sintering. The composites were characterized with X-ray diffraction, scanning/transmission electron microscopies, and by mea- suring thermal properties, including thermal conductivity and coefficient of thermal expansion （CTE）. The results show that the milled fibers are preferentially oriented in a plane perpendicular to the pressing direction, leading to anisotropic thermal properties of the composites. The Ti coating reacted with graphite fiber and formed a continuous and uniform TiC layer. This carbide layer establishes a good metallurgical interracial bonding in the composites, which can improve the thermal properties effectively. When the fiber content ranges from 35 vol% to 50 vol%, the in-plane thermal conductivities of the composites increase from 383 to 407 W.（m.K）-~, and the in-plane CTEs decrease from 9.5 x 10-6 to 6.3 10-6 K-1.

Effects of boron on the microstructure and thermal properties of Cu/diamond composites prepared by pressure infiltration

Diamond reinforced copper（Cu/diamond） composites were prepared by pressure infiltration for their application in thermal management where both high thermal conductivity and low coefficient of thermal expansion（CTE） are important.They were characterized by the microstructure and thermal properties as a function of boron content,which is used for matrix-alloying to increase the interfacial bonding between the diamond and copper.The obtained composites show high thermal conductivity（660 W/（m·K）） and low CET（7.4×10-6 K-1） due to the formation of the B13C2 layer at the diamond-copper interface,which greatly strengthens the interfacial bonding.Thermal property measurements indicate that in the Cu-B/diamond composites,the thermal conductivity and the CTE show a different variation trend as a function of boron content,which is attributed to the thickness and distribution of the interfacial carbide layer.The CTE behavior of the present composites can be well described by Kerner＇s model,especially for the composites with 0.5wt% B.

Thermal and Mechanical Properties Optimization of ABO4 Type EuNbO4 By the B-Site Substitution of Ta

Ferroelastic ABO4 type RETaO4 and RENbO4 ceramics(where RE stands for rare earth)are being investigated as promising thermal barrier coatings(TBCs),and the mechanical properties of RETaO4 have been found to be better than those of RENbO4.In this work,B-site substitution of tantalum(Ta)is used to optimize the thermal and mechanical properties of EuNbO4 fabricated through a solid-state reaction(SSR).The crystal structure is clarified by means of X-ray diffraction(XRD)and Raman spectroscopy;and the surface microstructure is surveyed via scanning electronic microscope(SEM).The Young’s modulus and the thermal expansion coefficient(TEC)of EuNbO4 are effectively increased;with respective maximum values of 169 GPa and 11.2×10^-6 K^-1(at 1200℃).The thermal conductivity is reduced to 1.52 W·K^-1·m^-1(at 700℃),and the thermal radiation resistance is improved.The relationship between the phonon thermal diffusivity and temperature was established in order to determine the intrinsic phonon thermal conductivity by eliminating the thermal radiation effects.The results indicate that the thermal and mechanical properties of EuNbO4 can be effectually optimized via the B-site substitution of Ta,and that this proposed material can be applied as a high-temperature structural ceramic in future.

Pressureless infiltration processing and thermal properties of SiCp/Al composites with high SiC particle content

SiCp/1060Al, SiCp/ZL101,SiCp/ZL102 composites with SiCp volume fraction of 55% were fabricated by pressureless infiltration. The microstructure was examined and thermal properties were characterized for SiCp/Al composites. The results show that the composites are dense and macroscopically homogeneous. With the increase of temperature, the mean linear coefficient of thermal expansion(CTE) at 25-200℃of the composites increases and ranges from 7.23×10-6 to 10.4×10-6K-1, but thermal conductivity declines gradually at the same time. With the increase of Si content in the Al matrix, CTE of the composites declines and thermal conductivity also declines but not linearly, when Si content is up to 7%, the average thermal conductivity is 140.4 W/(m·K), which is close to that of the SiCp/1060Al composite (144.6 W/(m·K)). While Si content is 11.7%, the average thermal conductivity declines markedly to 87.74 W/(m·K). The annealing treatment is better than the solution aging treatment in reducing CTE and improving thermal conductivity of the composites. Compared with conventional thermal management materials, SiCp/Al composites are potential candidate materials for advanced electronic packaging due to their tailorable thermo-physical properties.

高熵稀土氧化物热障涂层材料研究进展

热障涂层(Thermal barrier coating,TBC)材料在航空发动机和燃气轮机的热防护中具有保护高温合金基底免受氧化及腐蚀,并降低高温合金的工作温度的重要作用。新型热障涂层材料中存在许多高熵稀土氧化物,能够实现比单一主成分稀土氧化物更优异的热学、力学、高温相稳定性以及抗烧结、耐腐蚀等性能。但是目前对高熵稀土氧化物的研究仍然停留在初步阶段,其中稀土元素对材料性能的作用尚未完全明确,且没有形成统一标准。简要概述了热障涂层的基本结构,并重点总结了高熵锆酸盐、铈酸盐、铪酸盐、钽酸盐和铌酸盐等5种高熵稀土酸盐的晶体结构、热物理性能与力学性能。对比分析了其与相应的单一组分稀土酸盐的差异,并探讨了影响其性能优劣的多种因素。相比于单一组分稀土氧化物,高熵稀土氧化物的热导率、热膨胀系数和相稳定性均有明显改善。最后,展望了未来高熵稀土热障涂层的发展方向。

Sm_(2)Y_(1-x)Al_(x)TaO_(7)陶瓷材料的热物理性能

以高纯度Al_(2)O_(3)、Y_(2)O_(3)、Sm_(2)O_(3)和Ta_(2)O_(5)为原材料,采用高温烧结方法制备了Sm_(2)Y_(1-x)Al_(x)TaO_(7)系列陶瓷材料,对其晶体结构、显微组织、元素组成、热导率和热膨胀性能进行了系统研究。结果表明,除Sm_(2)Y_(0.5)Al_(0.5)TaO_(7)具有焦绿石结构之外,其余三种氧化物均具有单一的萤石晶体结构。其块体材料相对致密度均在90%以上,晶界清晰,各元素摩尔比与其化学式基本一致。由于Al^(3+)和Y^(3+)之间较大的离子半径和原子量差别,增大了对声子的散射,其热传导性能随着Al_(2)O_(3)含量的增加而降低。虽然Al_(2)O_(3)含量增加降低了其热膨胀系数,其大小与YSZ基本相当,所制备陶瓷在室温至1400℃范围内具有良好的相稳定性能。

Effect of Microstructure on Thermal Expansion Coeffcient of 7A09 Aluminum Alloy

The relationship between microstructure evolution and coefficient of thermal expansion （CTE） of 7A09 aluminum alloy was investigated in this paper. Differential scanning calorimetry （DSC） was combined with transmission electron microscopy （TEM） and high-resolution transmission electron microscopy （HRTEM） to investigate microstructure evolution taking place in 7A09 aluminum alloy during heating and cooling process. The corresponding CTE curves of the 7A0g alloy were recorded by thermal dilatometer. Results indicated that GPII zones and Ur phase were main precipitates in the highest strength tempered （T6） 7A09 alloy. The r/phase was the main participate in 7A09 alloy during the cooling process. The nonlinear dependency existed between CTE and temperature in both changing temperature processes. During the heating process, obvious additional contraction of alloy volume was directly caused by phase transition, such as dissolution of η＇ phase, transition from η＇ to η phase and dissolution of η phase. The additional contraction could slow down the increase of CTE greatly and be expressed in the nonlinearity of CTE curve. Volume and energy changes of alloy system influenced the variation trend of CTE directly, which was caused by the precipitation of U phase during the cooling process. These effects were revealed by the corresponding nonlinear change of CTE.

Cold spray additive manufacturing of Invar 36 alloy:microstructure,thermal expansion and mechanical properties

In this work,the Invar 36 alloys were manufactured using cold spray(CS)additive manufacturing technique.The systematic investigations were made on the microstructural evolution,thermal expansion and mechanical properties under as-sprayed(AS)and heat-treated(HT)conditions.XRD(X-ray diffraction)and ICP-AES(inductively coupled plasma atomic emission spectroscopy)analyses show that no phase transformation,oxidation,nor element content change have occurred.The X-ray computed tomography(XCT)exhibited a near fully dense structure with a porosity of 0.025%in the helium-produced sample under as-sprayed condition,whereas the nitrogen-produced samples produced at 5 MPa and 800℃show more irregular pore defects.He-AS sample shows a more prominent grain refinement than that of nitrogen samples due to the more extensive plastic deformation.The post heat-treatment exhibited a promoted grain growth,inter-particle diffusion,as well as the formation of annealing twins.Between25℃and 200℃,the nitrogen samples possessed lower CTE(coefficient of thermal expansion)values(1.53×10^(-6)/℃)compared with those produced by casting and laser additive manufacturing.The He-AS samples exhibited a noticeable negative CTE value between 25℃and 200℃,which may due to the significant compressive residual stress(-272 MPa)compensating its displacement with temperature increase during CTE test.The N2-HT and He-HT Invar 36 samples present a notable balance between strength and ductility.In conclusion,the CS technique can be considered as a potential method to produce the Invar36 component with high thermal and mechanical performance.