Ultrahigh thermal conductive graphite film via the in-situ construction of aligned nanographene skeleton using chemical vapor deposition

Both high thermal conductivity(K)and large cross-sectional area are essential for thermal dissipation materials to maximize their heat transfer capability.However,the drastic decrease of K values with the increased thickness makes the existing graphite/graphene films less favored for practical applications.In this work,graphite film with both large thickness and high K value is produced based on an in-situ com-position strategy between nanographene(G)and pyrocarbon(PyC)via chemical vapor deposition(CVD)using CH_(3) OH/C_(2)H_(5)OH mixed precursors.It’s found that an optimized O/C ratio of precursors facilitates the construction of ordered G skeletons within the deposited G/PyC composites.Such G/PyC compos-ites can be completely graphitized at a lower temperature than the existing products.After 2400℃ an-nealing,dense,thick,and highly aligned graphite films were prepared.Their K values reach 1350 and 1010 W m^(-1) K^(-1) at the thickness of 40 and 120μm,respectively,surpassing the existing records with similar thicknesses.More importantly,the proposed method is insensitive to the deposition substrates,and the G/PyC can be infiltrated into large-size fiber preforms as a matrix for preparing centimeter-thick high K materials.Besides,the G/PyC also exhibits better mechanical and electromagnetic shielding per-formances than the existing products,indicating a promising multifunctional application prospect.

High-aspect-ratio ZrC whiskers:Synthesis,growth mechanism and electromagnetic wave absorption properties

Stealth materials with high dependability at elevated temperatures and outstanding mechanical properties are urgently needed for practical applications.As one-dimensional ultrahigh temperature ceramic(UHTC)materials,zirconium carbide whiskers(ZrCw)have attracted a great deal of attention due to their desirable mechanical and ablation resistance performance in high-temperature environments.We have successfully synthesized ZrCw using a carbothermal reduction technique without the introduction of metal catalytic in this paper.ZrCw shows a typically prismatic structure with the diameter of 1e2 mm and the aspect ratio of up to 250.The growth of ZrCw is controlled by a solid-liquid-solid(SLS)and vaporsolid(VS)compound mechanism in conjunction with the auxiliary action of mesophase Na3ZrF7.The ZrCw/paraffin hybrids achieve the minimum reflection loss(RL(min))of
25.77 dB at 13.28 GHz under the thickness of 1.25 mm,and reach an effective absorption bandwidth(EAB)of 3.04 GHz(14.96 e18.00 GHz)with a thickness of only 1.0 mm.This work presents a promising approach for large-scale producing high-purity whiskers,and verifies that ZrCw has extensive application prospects in the field of stealth materials.

High-entropy(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))2Hf_(2)O_(7) ceramic: A promising thermal barrier coating material

Thermal barrier coating(TBC)materials perform an increasingly important role in the thermal or chemical protection of hot components in a gas turbine.In this study,a novel high entropy hafnate(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) was synthesized by solution combustion method and investigated as a potential TBC layer.The as-synthesized(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) possesses a pure single disordered fluorite phase with a highly homogeneous distribution of rare earth(RE)cations,exhibiting prominent phase stability and excellent chemical compatibility with Al_(2)O_(3) even at 1300°C.Moreover,(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) demonstrates a more sluggish grain growth rate than Y_(2)Hf_(2)O_(7).The thermal conductivity of(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7)(0.73-0.93 W m^(-1)K^(-1))is smaller than those of components RE_(2)Hf_(2)O_(7) and many high entropy TBC materials.Beside,the calculated thermal expansion coefficient(TEC)of(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7)(10.68×10^(-6)/K,1100°C)is smaller than that of yttriastabilized zirconia(YSZ).Based on the results of this work,(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) is suitable for the next generation TBC materials with outstanding properties.

Thermophysical properties of a novel high entropy hafnate ceramic

High-entropy oxides(HEOs)are considered promising thermal barrier coating(TBC)materials due to their unique thermophysical performances induced by the entropy effects.In this work,(La_(0.2)Ce_(0.2)Pr_(0.2)Sm_(0.2)Eu_(0.2))_(2)Hf_(2)O_(7)high entropy hafnate,as a thermal barrier coating(TBC)material,was successfully synthesized by solution combustion method for the first time.From the X-ray diffraction,scanning electron microscopy,and transmission electron microscopy results,it is confirmed that(La_(0.2)Ce_(0.2)Pr_(0.2)Sm_(0.2)Eu_(0.2))_(2)Hf_(2)O_(7)has pure single-phase ordered pyrochlore structure with highly homogeneous composition at both micrometer and nanometer scales.The synthesized(La_(0.2)Ce_(0.2)Pr_(0.2)Sm_(0.2)Eu_(0.2))2 Hf2O7 possesses excellent phase stability at 1600℃and demonstrates a low thermal conductivity(1.0-1.24 W·m^(-1)·K^(-1))which is lower than those of rare earth hafnates(RE2Hf2O7,RE=La,Ce,Pr,Sm,Eu).Therefore,it provides a new perspective and potential to prompt the next generation TBC materials with better performance.

Biomedical applications of the powder-based 3D printed titanium alloys:A review

3D printing technology is a new type of precision forming technology and the core technology of the third industrial revolution.The powder-based 3D printing technology of titanium and its alloys have received great attention in biomedical applications since its advantages of custom manufacturing,costsaving,time-saving,and resource-saving potential.In particular,the personalized customization of 3D printing can meet specific needs and achieve precise control of micro-organization and structural design.The purpose of this review is to present the most advanced multi-material 3D printing methods for titanium-based biomaterials.We first reviewed the bone tissue engineering,the application of titanium alloy as bone substitutes and the development of manufacturing technology,which emphasized the advantages of 3D printing technology over traditional manufacturing methods.What is more,the optimization design of the hierarchical structure was analyzed to achieve the best mechanical properties,and the biocompatibility and osseointegration ability of the porous titanium alloy after implantation in living bodies was analyzed.Finally,we emphasized the development of digital tools such as artificial intelligence,which provides new ideas for the rational selection of processing parameters.The 3D printing titanium-based alloys will meet the huge market demand in the biomedical field,but there are still many challenges,such as the trade-off between high strength and low modulus,optimization of process parameters and structural design.We believe that the combination of mechanical models,machine learning,and metallurgical knowledge may shape the future of metal printing.

Methane pyrolysis in preparation of pyrolytic carbon:Thermodynamic and kinetic analysis by density functional theory

The density functional theory has been successfully applied in analyzing pyrolytic carbon deposition by methane pyrolysis from the view of thermodynamics and kinetics based on a total number of 39 elementary reactions.M06-2X/def2-TZVP level was employed to optimize species structures and locate the transition states.The enthalpy changes and Gibbs free energy changes of all the reactions in the temperature range of 298.15–1800K were derived with optimized species.Results show that the reacting temperature should be above 1200 K based on the equilibrium constant analysis,which is consistent with the typical reaction temperature adopted in experiments.Potential energy surface profiles report that radical attacking reactions have lower energy barriers than those direct decomposition reactions,especially hydrogen radical attacking reactions.The energy barriers of the first steps,dehydrogenations of methane and ethylene,are 272.4 kJ·mol^(-1)and 288.9 kJ·mol^(-1)at 1200 K,which are very close to the experimental activation energy for methane pyrolysis.The most favorable decomposition reaction path is the path of hydrogen radical attacking reactions.The highest energy barrier of the path at 1200K is 185.7kJ·mol^(-1)presented by the C–H bond breaking in ethynyl attacked by hydrogen radical.

Microstructure and Mechanical Properties of Multilayertextured 2D Carbon/Carbon Composites

Two-dimensional（2D） carbon/carbon（C/C） composites with multilayered texture, especially with different thickness of high-textured（HT） pyrocarbon layer, were prepared by isothermal, isobaric chemical vapor infiltration（CVI） technique. The influence of matrix microstructure on mechanical properties of C/C composites was investigated by polarized light microscopy, scanning electron microscopy and three-point bending test. The results show that the samples with multilayer-textured pyrocarbon matrix own a higher flexural strength than the one with pure medium-textured structure, which is attributed to multiple crack deflection and interfacial sliding between different textured pyrocarbon layers and between sub-layers within HT layer. The increase in thickness of HT pyrocarbon layer improves the plasticity of the samples and renders the fracture in pseudo-plastic behavior.

甲烷化学气相沉积不同织构热解碳的动力学分析（英文）

本文建立了一种甲烷化学气相沉积不同织构热解碳的动力学模型.该模型包括甲烷气相热解的详细基元反应及热解碳沉积的集总表面反应,模拟了1323–1398 K立式热壁反应炉中热解碳沉积过程中不同气体的组分浓度及热解碳的沉积速率,且模拟结果优于已有报道的模型、与实验结果吻合良好.结果表明,热解碳的主要沉积机理为碳源气体在表面活性位的生长机理,其中C_1物质为主要碳源气体.随着温度升高,甲烷热解混合气体趋于成熟,乙炔、苯和多环芳香烃逐渐成为重要的碳源气体.基于以上碳源气体的热解碳形成机理分析指出热解混合气体的成熟有利于形成由六元环及五元环-七元环组合而成的高定向平面结构,即高织构热解碳,与实验结论吻合良好.