Materials with Functionally Graded Properties

Materials with the gradient in functional properties could serve as an example of intermediate material class on the boundary between homogeneous and non-homogeneous materials. Recently developed technique allows for the manufacturing of unimorph-type devices, namely Reduced And INtemally Biased Oxide Wafer (RAINBOW) or THin layer UNimorph DrivER and sensor (THUNDER) (Almajid et al. 2001 Elissalde and Cross 1995 Elissalde et al. 1996 Wu et al. 1996 Wise 1998). 

THUNDER actuators are produced by bonding of thin piezoelectric ceramic plate to metal sheet at elevated temperature by using a high-temperature adhesive. Internal stresses are induced in such actuator due to the ceramic-metal thermal expansion mismatch. This process leads to curved stress-biased actuator capable of high displacements. 

Both types of structures do not include any pre-stressed metal to ceramic stmc-tural bonds (like for example conventional bimorphs), which could lead to lifetime limitations. 

ANSI/IEEE Standard 176-1978 (1984) Standard on piezoelectricity reproduced in IEEE Trans Sonics Ultrason SU-31 1-55 

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N. Noda and T. Tsuji, Steady Thermal Stresses in a Plate of Functionally Graded Material with Temperature-Dependent Properties, Transactions of the Japan Society of Mechanical Engineers, Vol. 57A(535), 625-631, 1991. 

The synthesis of advanced materials requires a strict control in stoichiometries with controlled particle size and shape to their design novel nanoarchitecture with functional gradient properties in nanomaterials. To tailor the properties in precursor, by soft chemical approach for the synthesis of technological-grade nanoparticles with favourable properties, is a challenge to material scientist. The structural-functional relationship plays an important factor in designing the cost-effective synthesis with the ability to retain well-defined stoichiometries and properties in nanodevices, from the molecular precursor at molecular scale to nanomaterials at nanoscale [1-7],... 

Liquid-phase infiltration of preforms has emerged as an extremely useful method for the processing of composite materials. This process involves the use of low-viscosity liquids such as sols, metal- or polymer-melts. Using this infiltration process, it is possible to design new materials with unique microstructures (e.g. graded, multiphase, microporous) and unique thermomechanical properties (graded functions, designed residual strains and thermal shock). 

The concept of functionally graded materials (FGMs) is to tailor nonuniform distribution of components and phases in materials, and hence combine mechanical, thermal, electrical, chemical, and other properties that cannot be realized in uniform materials. For example, the material structure may have a smooth transition from a metal phase with good mechanical strength on one side, to a ceramic phase with high thermal resistance on the other side (see Fig. 13). With a gradual variation in composition, FGMs do not have the intermaterial boundaries found in multilayer materials, and hence they exhibit better resistance to thermal stress. 

The functionally graded materials (FGMs) are characterised by a non-linear 3D-distribution of phases and corresponding properties [1,2]. They are distinguished from isotropic materials by gradients of composition, phase distribution, porosity, texture, and related properties (hardness, density, resistance, thermal conductivity. Young s modulus, etc.) [3-5]. The FGM is characterised not only by the presence and appearance of compositional or other gradients but also by the sophisticated behaviour of FGM component in comparison with conventional (macroscopically uniform) materials. 

Functionally graded materials (FGMs) distribute the material functions throughout the material body to achieve the maximum heat resistance and mechanical properties ideal for spacecraft where one side may be exposed to extremely high temperatures and the other side may be exposed to extremely low temperatures. Because FGMs characteristically have continuously varying material properties, many analytical methods developed for conventional composites with distinct phases may not be directly applicable to FGMs. 

Carbon-carbon composites made with the functionally graded fiber arrangement technique present the opportunity to tailor thermo-physical properties into carbon materials. In this paper, the changing of the fiber architecture is the method for FGM. Fibers or matrices are other options for FGM. This functionally graded fiber arrangement technique can be applied to a wide range of materials processing. 

It was experimentally demonstrated that some thick Korean kaohn-ferric oxide Functionally Graded Materials could be produced by a newly developed filtration method, and surface treating of the FGMs could be processed by a KrF excimer laser and cutting of the FGMs could be processed by a Nd-YAG laser. It was concluded that the characteristic properties of the FGMs surface when processed with the lasers depends directly on the functionally graded characteristics. 

Plasma spraying is a consolidation process for powders with the additional capability of a composition control of the spray formed structures. The paper reports on the first steps to adapt this method to the production of functionally graded thermoelectric materials with a locally maximized figure of merit. Iron disilicide (FeSi2) was used to test the performance of the technique on thermoelectric material. It was found that plasma spray forming is applicable to produce dense materials with thermoelectric properties comparable to hot pressed ones. Problems were however found with the thermal stability of the microstructure. 

An epoxy infiltration process has been used to fabricate zirconium phosphate with much improved physical and mechanical properties for robust applications in various electrochemical devices. Results show that phase composition, microstructure and properties vary gradually along the depth profile, confirming the functionally-graded character of these materials. 

Functionally-graded ZrP materials with improved physical and mechanical properties of ZrP have been produced by epoxy infiltration. These materials show classical functionlly-graded characteristics with continuous change in composition, physical and mechanical properties. 

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