Functionally Graded Materials VIII

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Authors: Jian Ping Wang, Shu Yong Yang, Li Sheng Liu
Abstract: The functionally graded thermal barrier coating (TBC) serves in high temperature and/or high temperature gradient environment for a long time. According to the experimental and theoretical research, in the metal substrate and the metal-rich interlayer creep deformation will appear under high temperature environment. In order to design and optimize the compositional distribution of FGM, it is necessary to analyze the stress and strain responses taking into account the creep phenomenon of the materials. In this article, the thermo-mechanical responses of ceramic/metal functionally graded TBC in work environment are analyzed by a finite element method. The creep phenomenon of the metal and the interlayers are taken into account. The numerical results indicate that the creep behavior of all interlayers, even for the ceramic-rich interlayer, cannot be neglected in analysis. It is suggested that the creep phenomenon of the material is important in the functionally graded TBC systems.
Authors: Noriyuki Hayashi, Tomohiko Onoda, Yasushi Sakamoto, Shin-ichiro Tsuru, Toshinori Kawabe, Masanori Hara
Authors: S. Walczak, Winfried Seifert, Eckhard Müller
Abstract: Commercialization of Peltier coolers has progressed during last years and special efforts have been undertaken to enhance the efficiency of thermoelectric (TE) devices. Along with the continued search for advanced TE materials, the concept of FGM offers a strategy of gradual improvement of device performance. In reality a functional gradient in a TE material means a related spatial variation of all TE properties – Seebeck coefficient, electrical, and thermal conductivity – whereas the most relevant effect is linked to the gradient of the Seebeck coefficient. Due to the spatial dependence of the Seebeck coefficient, Peltier heat is absorbed or released inside the TE element under current flow (distributed Peltier effect) which can be exploited to shape the internal temperature profile in a desired manner. Starting from the first principles of thermoelectricity, a differential equation governing the coupling of thermal and electrical transport is derived within the frame of a one-dimensional model. It is shown that this approach can be also used to model multi-segment Peltier cooling devices. Temperature profiles T(x) have been calculated for a segmented TE element within the framework of a constant parameters theory. The work presents an analytical model for performance evaluation of multiply-segmented Peltier elements. The problem is treated in a one-dimensional approach for a p-type stack containing N segments of different properties. Assuming constant TE material properties in each of the segments, the differential equation of TE transports has been solved to obtain the temperature profile T(x) in each segment. With the material properties values in each segment representing volume average values this model gives an excellent approximation also for continuously graded elements. The boundary conditions of the TE problem set-up, as conservation of heat at any intermediate junction between the segments, and fixed temperature at the cold and hot end of the element, lead to a linear equation system, which can be easily solved by means of standard methods. From the solution, all desired performance parameters can be deduced. Based on realistic material data exemplary calculations are presented for stacked and continuously graded elements. To demonstrate the developed numerical algorithm, gradients of the Seebeck coefficient are mainly considered. Calculations have been performed for N = 2, 5, 10, and continuous gradients. As target parameters, the C.O.P. and the cooling power have been calculated as functions of the electric current. As well, the minimum temperature of the cold side has been determined for various shape of the Seebeck gradient. It is shown that the TE FGM effect can be almost completely utilized already by a stack of two to five homogeneous segments. The results allow for giving an estimation on the order of magnitude of performance improvement of both discontinuously and continuously graded Peltier cooling devices. The model calculation was implemented with the software tool MATHEMATICA. The code provides an easy to handle convenient instrument for performance estimation of non-homogeneous Peltier pellets. Technological studies for controlled fabrication of those pellets are underway.
Authors: Li Sheng Liu, Qing Jie Zhang, Peng Cheng Zhai, Chang Chu Zhu
Abstract: In the paper, propagation of two-dimensional viscous-plastic waves in graded layer media is investigated by numerical method. Firstly, an infinite model is used to analysis wave’s propagation in FGMs, this model has a finite gradient layer and non-reflecting boundary condition is used to simplify the model to be finite. Then the characteristic of viscous-plastic wave’s propagation in graded media is investigated on the base of this model. The following contents is researched by this model: (1) The refracted effect of functionally gradient layers on incident wave; (2) the effect of incident angle on the bottom layer’s stress ; (3) the effect of gradient layer’s thickness on the bottom layer’s stress. Finally, some interesting, reasonable and important results are obtained by our researching work.
Authors: Shui Gen Huang, Lin Li, Jef Vleugels, Omer Van der Biest
Abstract: The thermodynamic properties of the Co-V-C and Co-V8C7 systems are of interest for superfine cemented carbide applications. The model parameters for the Gibbs energy of the individual phases have been evaluated using the CALPHAD method by combining the recently optimized phase diagram information of the V-C, Co-C and Co-V system. The isothermal sections of ternary system Co-V-C at 1400 °C and 1600 °C, as well as the vertical section of Co-V8C7 system were extrapolated. The calculated results, especially the liquid forming temperature of Co-V8C7 system was validated with experiments by using differential scanning calorimetry (DSC) analysis. Through controlling the carbon activity, various vertical sections with different carbon activity in Co-V8C7 system are presented.
Authors: Rafael Schouwenaars, Víctor H. Jacobo, Sara M. Cerrud, Armando Ortiz
Abstract: Soft anti-friction alloys are metal-metal composites used in journal bearings as a functionally graded system. The physical processes occurring during severe wear of these products are analysed and placed within the framework of theories on tribolayer formation. A simplified analysis of the thermodynamics of the phenomena permits analysing the anti-friction properties of the alloys in terms of the activity coefficients of their constituents and indicates that during wear, a wear-resistant contact layer is formed which retards further damage.
Authors: G. Miyamoto, Y. Tomio, Tadashi Furuhara, Tadashi Maki
Abstract: Microstructures of the plasma nitrided ferritic Fe-M (M = 0.5, 0.7, 1.0mass%Ti, 1.0mass%V, 1.0mass%Al) binary alloys were studied mainly by TEM. Metastable clusters are uniformly formed in the Ti added alloys. Near the surface of the specimen, transition from the clusters to the equilibrium B1-type TiN is observed. The microstructure formed in the 1%V alloy is similar to that of the Ti added alloy. In contrast, no cluster is formed in the 1%Al alloy and B1-type AlN is observed to nucleate on dislocations.
Authors: Gang Chen, Peng Cheng Zhai, Ai-Jun Shao
Abstract: The numerical simulation for the Small Punch creep (SP-C) tests is conducted using a Finite Element method. The objective of the present study is to obtain the deformation states of the SP-C specimen and to estimate the feasibility of SP-C test method for high-temperature creep properties. The emphasis is put on the relationship between the equivalent creep strain and the central deflection of the SP-C specimen. The time history of central deflection and equivalent creep strain is obtained by finite element method and the effects of the load, temperature and material properties on the relationship of central deflection and equivalent creep strain are discussed. From the numerical results, the relationship between the central deflection and the equivalent creep strain is approximately independent of load, temperature, and material properties. As a farther result, the high temperature creep properties of the 12Cr1MoV steel are appraised by numerical simulation. The results are in good agreement with the results from the standard test method. The results indicate that the small punch test technique is an effective method for the evaluation of the high-temperature creep properties of materials.
Authors: Guang Zhao Bai, G. Wang, Wan Jiang, Li Dong Chen, X. Shi
Abstract: Thermal conductivity of as-prepared MoSi2/SiC composites has been determined by Laser Flash method. Interfacial thermal conductance for composites with 100nm SiC and with 0.5µm has been determined by using effective medium theory. The results of interfacial thermal conductance exhibit that both the inclusion size and the clustering of the inclusions play an important role in determining composite thermal conductivity.

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