Extrusion-based additive manufacturing of Ti3SiC2 and Cr2AlC MAX phases as candidates for high temperature heat exchangers

High Temperature Heat Exchangers (HTHXs) are used in many industrial processes and are likely to become key components in green power generation. For the development of HTHXs, novel designs and new materials need to be explored. Additive manufacturing (AM) opens many possibilities for novel designs. Extrusion-based AM in general, and composite extrusion modelling (CEM) in particular, offers the opportunity of using new binder systems, that cannot be employed in other AM techniques. MAX phases, due to their ceramic-metallic properties combination, are great candidates for HTHXs. In this work, the printability of Ti₃SiC₂ and Cr₂AlC, through CEM with an innovative sustainable binder is explored. For this purpose, rheological properties of the feedstocks and the influence of the printing parameters are studied for each MAX phase feedstock. Microstructural analysis and final sample characterisation is performed, in order to determine the suitability of this technique to obtain near-net shape MAX phase parts.     

Yttrium incorporation in Cr2AlC: On the metastable phase formation and decomposition of (Cr,Y)2AlC MAX phase thin films

Herein we report on the synthesis of a metastable (Cr,Y)₂AlC MAX phase solid solution by co-sputtering from a composite Cr–Al–C and elemental Y target, at room temperature, followed by annealing. However, direct high-temperature synthesis resulted in multiphase films, as evidenced by X-ray diffraction analyses, room-temperature depositions, followed by annealing to 760°C led to the formation of phase pure (Cr,Y)₂AlC by diffusion. Higher annealing temperatures caused a decomposition of the metastable phase into Cr₂AlC, Y₅Al₃, and Cr-carbides. In contrast to pure Cr₂AlC, the Y-containing phase crystallizes directly in the MAX phase structure instead of first forming a disordered solid solution. Furthermore, the crystallization temperature was shown to be Y-content dependent and was increased by ∼200°C for 5 at.% Y compared to Cr₂AlC. Calculations predicting the metastable phase formation of (Cr,Y)₂AlC and its decomposition are in excellent agreement with the experimental findings.     

Phase formation and microstructure evolution of plasma sprayed Cr2AlC MAX phase coatings under post annealing

In this study, thick Cr₂AlC coatings were first synthesized via plasma spraying of Cr₃C₂–Al–Cr agglomerated powders and post annealing. The microstructure evolution and mechanical properties of the Cr₂AlC coatings annealed at 500–1000 °C were investigated. The as-sprayed coatings exhibited a lamellar structure, primarily consisting of Cr₂AlC, Cr₇C₃, Cr₂₃C₆, and (Cr, Al)C_x solid solutions. The short residence time during spraying led to incomplete reactions in the Cr₃C₂@Al–Cr agglomerates, resulting in the formation of (Cr, Al)C_x. Post annealing provided sufficient energy for the transition of (Cr, Al)C_x → Cr₂AlC. With an increase in the annealing temperature (<900 °C), gradual transition of the (Cr, Al)C_x phase led to a slight increase in the Cr₂AlC content, and thus, the as-annealed coatings maintained high hardness (>1000 HV_0.2) with improved fracture toughness. Higher annealing temperatures (>900 °C) promoted clear enhancement of the Cr₂AlC content, thus reducing the coating hardness. The transition phase (Cr, Al)C_x and high temperature annealing were the primary factors to promoting the formation of the Cr₂AlC phase in sprayed coatings. This study indicates that the Cr₃C₂@Al–Cr agglomerates can be effective alternatives to expensive MAX phase powders as feedstock for plasma spraying of Cr₂AlC coatings.     

Synthesis and thermal expansion of chalcogenide MAX phase Hf2SeC

Thermal expansion of MAX phases along different directions tends to be different because of the anisotropy of hexagonal crystals. Herein, a new Hf₂SeC phase was synthesized and confirmed to be relatively isotropic, and the coefficients of thermal expansion (CTEs) along a and c directions were determined to be 9.73 μK^?1 and 10.18 μK^?1, respectively. The strong MS bond endowed Hf₂SC and Zr₂SC lower CTEs than those of Hf₂SeC and Zr₂SeC. The relationship between the thermal expansion anisotropy and the ratio of elastic stiffness constant c₁₁ and c₃₃ was established. This straightforward approximation can be used to roughly predict the thermal expansion anisotropy of MAX phases.     

Cold spray deposition of Cr2AlC MAX phase for coatings and bond-coat layers

Highly pure Cr₂AlC powders were synthesized and deposited for the first time by cold spray technology on stainless steel substrates. The Cr₂AlC coatings were relative dense, up to 91%, and present high purity (> 98%) since only small traces of Cr₂Al, Al₂O₃ and Cr₂O₃ were detected by XRD, SEM and EDX. The microstructure of the coatings is homogeneous, although some preferential orientation in the basal plane was observed by XRD pole figures. The adhesion between the coating and the substrate is strong, and compressive residual stresses up to 300 MPa in the coating were determined by XRD. Furthermore, a conventional YSZ Thermal Barrier Coating (TBCs) was deposited by Atmospheric Plasma Spray (APS) on top of the cold sprayed Cr₂AlC coating in order to demonstrate the processing feasibility of Cr₂AlC MAX phases as a bond-coat layer.     

High‐temperature oxidation and compressive strength of Cr2AlC MAX phase foams with controlled porosity

Cr₂AlC foams have been processed for the first time containing low (35 vol%), intermediate (53 vol%), and high (75 vol%) content of porosity and three ranges of pore size, 90‐180 μm, 180‐250 μm, and 250‐400 μm. Sacrificial template technique was used as the processing method, utilizing NH₄HCO₃ as a temporary pore former. Cr₂AlC foams exhibited negligible oxidation up to 800°C and excellent response up to 1300°C due to the in‐situ formation of an outer thin continuous protective layer of α‐Al₂O₃. The in‐situ α‐Al₂O₃ protective layer covered seamlessly all the external surface of the pores, even when they present sharp angles and tight corners, reducing significantly the further oxidation of the foams. The compressive strength of the foams was 73 and 13 MPa for 53 vol% and 75 vol% porosity, respectively, which increased up to 128 and 24 MPa after their oxidation at 1200°C for 1 hour. The increase in the compressive strength after the oxidation was caused by the switch from inter‐ to transgranular fracture mode. According to the excellent high‐temperature response, heat exchangers and catalyst supports are the potential application of these foams.     

Thermal stability enhancement of Cr2AlC coatings on Zr by utilizing a double layer diffusion barrier

Decomposition of Cr₂AlC deposited onto a Zr substrate and vacuum-annealed is observed at 800 °C as Al diffuses from the MAX phase into the Zr substrate. A double layer of ZrN and AlN has been predicted by CALPHAD calculations to act as diffusion barrier between the Zr substrate and Cr₂AlC. Experimental thermal stability investigations corroborate this prediction by confirming that the proposed double layer diffusion barrier coatings suppress the decomposition of Cr₂AlC for one hour at temperatures of up to 1000 °C.     

非均匀热通量对热电器件性能的影响

提高冷热两端温度是提高热电器件发电效率的有效途径之一。但当施加于热端表面的热通量不均匀时,热端表面温度不均匀,系统性能受到影响。基于此,建立了热电器件的热电转换耦合数学模型,分析热电材料物性参数,非均匀热通量等参数对热电器件的功率输出特性的影响。数值模拟结果表明:材料物性参数随温度的变化对系统输出功率的影响不可忽略,热通量4 W·cm^-2时物性参数对系统最高温度的影响接近4%;非均匀热通量对热电器件输出特性影响也十分显著,热通量均匀度越小,热端表面温度分布不均匀性越大,极值温度越高,高温区越小,断路电压越低。     

Molten salt shielded synthesis (MS3) of Ti2AlN and V2AlC MAX phase powders in open air

Synthesis of high-temperature carbide and nitride MAX phases requires an inert atmosphere to prevent oxidation. Energy and cost expensive conventional synthesis processes are not useful for large scale productions. Molten salt synthesis is simple but generally carried out in argon atmosphere. Molten salt shielded synthesis (MS³) is carried out in the air but needs salt encapsulation to prevent oxidation below melting point of the salt as reported earlier. In the present work, MS³ method without salt encapsulation in open air has been explored by using low melting eutectic salt mixture to provide oxidation shielding of the metal precursors at high temperature as well as to produce the required phase at lower temperature with shorter soaking time. In this paper, V₂AlC and Ti₂AlN have been synthesized using MS³ to obtain satisfactory phase purity. Therefore; this method can be extended to large scale industrial production through its eco-friendliness, sustainability and energy-efficient nature.     

Experimental and numerical analysis of heat transfer including viscous dissipation in a scraped surface heat exchanger

Viscous dissipation plays an important role in the dynamics of fluids with strongly temperature-dependent viscosity because of the coupling between the energy and momentum equations. The heat generated by viscous friction causes a local temperature increase in the high shearing zone with a consequent decrease of the viscosity which may dramatically change the temperature and velocity distribution. These processes are mainly controlled by the Brinkman number, the rotating velocity and the thermal boundary conditions. This work analyses forced convection heat transfer including the viscous dissipation in a scraped surface heat exchanger (SSHE). In this study the increase of the temperature due to the viscous dissipation is analysed both experimentally and numerically for Newtonian and non-Newtonian fluids. Heat transfer simulations including viscous dissipation were carried out by means of the CFD code of the software Fluent, version 6.3, with solving momentum and energy equations. Two thermal boundary conditions were considered: pseudo-adiabatic wall and constant temperature on the stator wall exchange. In the case of Newtonian fluid (pure HV45), for both considered thermal boundary conditions, an important increase of the temperature was obtained. In the case of non-Newtonian shear thinning fluid (2 wt% CMC solution), viscous dissipation is neglected. The developed numerical model agrees well with experimental results. The validated numerical model was then used to study the effect of index and consistency behaviour of shear thinning fluid using power-law rheological behaviour on the viscous dissipation, and correlation using dimensionless analysis expressed with different dimensionless process numbers is proposed for Newtonian and non-Newtonian shear thinning fluid.     

Effects of solid lubricant and laser surface texturing on tribological behaviors of atmospheric plasma sprayed Al2O3-ZrO2 composite coatings

Atmospheric plasma-sprayed (APS), low-friction Al₂O₃-ZrO₂ coatings were deposited with solid lubricants, and their effects on the laser surface texturing (LST) under a lubricated condition (commercial engine oil, 5W30) were evaluated. All specimens showed a 19.6% dimple density, and the contact angle of the APS coatings after LST processing decreased by ∼19–50%. The hydrophilicity of the APS coatings improved after LST processing, and the tribological behaviors of the APS coating containing different solid lubricants were mainly observed in the abrasive grooves, spalling, microcracks, residual pores and trapped wear debris on the worn surface. The coefficient of friction for the APS coatings was affected by the solid lubricants and the LST processing. The coefficient of friction for the APS coatings (except for h-BN) when adding solid lubricants and LST processing decreased to 3.7–7.9% and 2.3–9.5%, respectively.     

MHD flow and heat transfer of a micropolar fluid over a nonlinear stretching surface with variable surface heat flux and heat generation

An analysis has been carried out to study magnetohydrodynamic boundary layer flow and heat transfer of an electrically conducting micropolar fluid over a nonlinear stretching surface with variable wall heat flux in the presence of heat generation/absorption and a non‐uniform transverse magnetic field. The governing system of partial differential equations is first transformed into a system of ordinary differential equations using similarity transformation. The transformed equations are solved numerically. Results for the dimensionless velocity, micro‐rotation, and temperature profiles are displayed graphically delineating the effects of various parameters characterising the flow. The results show that the velocity profile decreases as the magnetic parameter and the velocity exponent increase, while it increases as the material parameter increases. The results show also that the temperature profile increases as the magnetic parameter, the velocity exponent, and the heat generation parameter increase. Furthermore, the temperature profile decreases as the material parameter, the heat absorption parameter, and the Prandtl number increase.     

超亲水表面上滞止区水喷流沸腾的临界热通量

引言二氧化钛(TiO2)作为一种光催化剂,由于其独特的特性近年来引起了人们的重视.研究者发现,TiO2有着极其特殊的超亲水特性[1].当具有TiO2涂层的表面受到紫外线的辐射时,水在其表面的接触角会一直减小,最后接近于0°.对于水沸腾这类有气液相变现象的传热过程来说,这一特性有着重要的应用价值.利用超亲水传热表面,相变传热过程可以被大大强化.高温壁面上液体喷流和钢铁工业中应用十分广泛的一种高效方法.有关滞止区内喷流沸腾的研究,以往主要集中在对沸腾区域内非稳态换热特性的研究上,而对滞止区内喷流沸腾的稳态实验研究则很少[2～4].最…     

管壳式换热器管束振动分析及防振措施

针对管壳式换热器常见的振动破坏形式 ,全面分析了管束振动的机理 ,并提出了相应的抗振措施     

Influence of laser surface texturing on the flexural strength of Al2O3 and Si3N4

Laser surface texturing (LST) is well known to be capable of improving the tribological performance and reducing the friction of ceramic surfaces. However, the influence of LST on the flexural strength of ceramics has rarely been researched.In this study, we examine the influence of LST on high purity (> 99.5 wt%) dense-sintered fine Al₂O₃ and hot-pressed fine Si₃N₄ with polished and laser-textured surfaces based on the biaxial ball-on-3-balls (B3B) test. A heat transfer simulation of the LST process is performed to understand the occurrence of residual stress. In addition, the B3B strength of Al₂O₃ and Si₃N₄ texture groups is calculated by adapting the previous formula based on the finite element (FE) simulation. Subsequently, the stress distribution in the FE simulation is used to calculate the effective volume and effective surface to study the size effect on both ceramics. It is found that LST improves the strength of Al₂O₃ and Si₃N₄ due to two reasons: it induces compressive residual stress on the tensile-loaded surface of ceramic specimens; more importantly, it reduces the effective volume and effective surface remarkably, thus improving the component strength significantly.     

恒热流条件下亲疏水表面液滴蒸发特性

以恒热流方式结合高速摄影技术同步观察记录3 μl的小液滴在不同亲疏水表面的蒸发过程。通过一系列的对比实验观察记录了不同亲疏水表面液滴蒸发时接触角、接触直径、蒸发时间等的动态特性。从实验分析中可以看出亲水表面液滴蒸发速率比疏水表面上液滴蒸发速率快,并且随着热通量的增大,液滴蒸发速率增大。在恒热流条件下亲疏水表面液滴蒸发以CCR模式为主,在蒸发后期呈现混合蒸发模式,全程未出现CCA模式。     

Effects of ZrC content on the synthesis of MAX phase and mechanical properties of Cf-C-SiC-Ti3SiC2-ZrC composites

In this research synthesis of Ti₃SiC₂ nano-laminate, effects of Al and ZrC on the amount and morphology of the synthesized MAX phase and mechanical properties of the C_f-C-SiC, C_f-C-SiC-Ti₃SiC₂ and C_f-C-SiC-Ti₃SiC₂-ZrC composites, fabricated by LSI method, were investigated. The infiltration process was conducted at 1500?°C for 30?min and then the samples were annealed at 1350?°C. X-ray diffraction (XRD) technique and scanning electron microscopy (SEM) were utilized in order to investigate the phase composition and microstructure of the samples, respectively. The results showed that the sample containing Al, had the largest amount of synthesized MAX phase and also addition of ZrC led to the decrease of intensities of MAX phase peaks. Among the samples, C_f-C-SiC-Ti₃Si(Al)C₂ had the best mechanical properties compared to the others. Bending strength, interlaminar shear strength and fracture toughness of this sample were 505?MPa, 34?MPa and 19.1?MPa?m^1/2 respectively. The results confirmed that the mechanical properties were decreased by addition of ZrC. Among ZrC-containing samples, the sample containing 10?vol% ZrC has shown the least decrease properties including the bending strength of 369.11?MPa, interlaminar shear strength of 26?MPa and fracture toughness of 16.9?MPa?m^1/2. Addition of ZrC phase caused pseudo-plastic behavior appearance in the force-displacement curve and led to fibers pull-out and also displacement enhancement. Microstructural observations confirmed the plate-like morphology of synthesized MAX phases. Furthermore, the distance between layers decreased and MAX phase size increased respectively by addition of Al. Also MAX phase size decreased by increasing the ZrC content. It was confirmed that the MAX phase-containing samples can tolerate various micro-deformation mechanisms including: crack deflection, bending and delamination of lamellae, kink boundary and laminate fracture. These mechanisms led to the toughening of the composites.     

Unsteady flow and heat transfer on an accelerating surface with blowing or suction in the absence and presence of a heat source or sink

The problem of unsteady flow and heat transfer in the laminar boundary layer on a linearly accelerating surface with suction or blowing in the absence and presence of a heat source or sink is considered. The governing partial differential equations for this investigation are transformed into the non-dimensional equations by using pseudo-similarity time and pseudo-similarity coordinate. The resulting two points boundary-value problem is solved numerically by the central finite difference method associated with Newton's iteration from the initial stage (ξ=0) to a steady state (ξ=1) completely. A parametric study is performed to illustrate the effects of Prandtl number, power-law surface temperature (PLST) or power-law heat flux (PLHF), heat sink or heat source, and suction or blowing parameter on the dynamic velocity and temperature fields as well as the transient development of the skin-friction coefficients and the Nusselt number. These results are depicted graphically to display special aspects of unsteady flow and heat transfer characteristics in all time.     

管壳式换热器壳程流动与传热的数值模拟与验证

针对目前管壳式换热器中微小流路建模和分析的缺失,采用“分段建模,整体综合”的模拟方法成功地开发了小间隙流路A和E的建模技术,建立了既包含主体流路（B、C）,也包含微小泄漏流道（A、E）的全流路管壳式换热器流动与传热模型,得到了与实际换热器相适应的几何模型。通过应用CFD软件Fluent进行分段模型的流动与传热研究,对各流道在折流空间中对传热和流动的影响进行了分析讨论。同时,采用整体综合技术,将数值模拟获得的局部流动与传热数据综合整理得到了换热器传热和阻力系数的整体法关联式。并将模拟结果与几种著名的壳程计算方法（Donohue、Kern和Bell-Delaware,流路分析）进行了对比,结果发现数值模拟与Bell-Delaware法和流路分析法的结果吻合良好,最大偏差小于20%。     

TPMS多孔铝-石蜡复合相变材料蓄热过程数值模拟及实验

传统相变材料受限于自身热导率小,其相变蓄热效率难以提升,通过在相变材料中添加具有高热导率的金属多孔结构是强化传热的重要手段之一。本文建立了三周期极小曲面（triply periodic minimal surface,TPMS）多孔铝-石蜡复合相变材料的三维、瞬态包含自然对流的相变蓄热模型,利用数值仿真结合实验的方法研究了TPMS多孔铝-石蜡复合相变材料在蓄热过程中的固液相界面演变规律、实时温度变化、热传输特性以及蓄热性能。结果表明,在纯石蜡中添加primitive杆状（primitive sheet,PS）、primitive壳状（primitive network,PN）两种TPMS多孔铝结构后,石蜡相变温度范围内出现明显的相变温度平台,PS-石蜡、PN-石蜡复合相变材料的相变起始时间较纯石蜡分别减少了74.1%与91.4%,竖直方向上的最大温度梯度由纯石蜡的1605.7℃/m分别下降至PS-石蜡、PN-石蜡复合相变材料的840℃/m、943.8℃/m,蓄热速率较纯石蜡分别提高3.10倍、4.69倍。最后,通过选区激光熔化（selective laser melting,SLM）技...