Homogenous materials, bulk properties

In this brief review we illustrated on selected examples how combinatorial computational chemistry based on first principles quantum theory has made tremendous impact on the development of a variety of new materials including catalysts, semiconductors, ceramics, polymers, functional materials, etc. Since the advent of modem computing resources, first principles calculations were employed to clarify the properties of homogeneous catalysts, bulk solids and surfaces, molecular, cluster or periodic models of active sites. Via dynamic mutual interplay between theory and advanced applications both areas profit and develop towards industrial innovations. Thus combinatorial chemistry and modem technology are inevitably intercoimected in the new era opened by entering 21 century and new millennium. 

Thus a measurement of the ultrasonic properties can provide valuable information about the bulk physical properties of a material. The elastic modulus and density of a material measured in an ultrasonic experiment are generally complex and frequency dependent and may have values which are significantly different from the same quantities measured in a static experiment. For materials where the attenuation is not large (i.e., a ca/c) the difference is negligible and can usually be ignored. This is true for most homogeneous materials encountered in the food industry, e.g., water, oils, solutions. 

Already a lot of scientific work has been established in processing graded bulk ceramics and in modelling and optimization of the properties of these materials. Despite these technological successes, the number of commercial successes is still limited because the production costs in most cases are still too high compared to homogeneous materials. However, new production processes are emerging on the market that allow economic production of FGM bulk ceramics. 

Apart from the surface composition the bulk properties of a particle material will affect composite deposition. Particle mass transfer and the particle-electrode interaction depend on the particle density, because of gravity acting on the particles. Since the particle density can not be varied without changing the particle material, experimental investigations on the effect of particle density have not been performed. However, it has been found that the orientation of the plated surface to the direction of gravity combined with the difference in particle and electrolyte density influences the composite composition. In practice it can be difficult to deposit composites of homogeneous composition on products where differently oriented surfaces have to be plated. 

Constant product quality requires an even feed rate, homogeneous bulk density of the material to be treated, uniform densification, and reproducible maximum pressure. This statement is true for all pressure agglomeration methods. However, while these conditions can be met relatively easily in die and roller presses with proper feed preparation and specific equipment parameters, it is rather difficult to achieve in extrusion. The reason for this is that densification and maximum pressure depend on the resistance to flow in the die channel or holes. Small variations in feed homogeneity or frictional properties can yield major differences in equipment performance and product quality. Wear or buildup in the extrusion die are among the most important parameters influencing the back-pressure which, in turn, is responsible for the amount of densification prior to extrusion. 

Semiconducting silicides epitaxially grown on silicon have gained an increased practical interest to be used in novel semiconducting devices due to their high thermal stability, homogeneous interface and smooth surface morphology [1]. Lowdimensional structures are the main object of study and application in nanoelectronics. When the structure size in one direction decreases up to several run, its properties may differ essentially from the bulk properties of a source material. Such modification of the properties looks attractive. 

Local electronic and vibrational states, created by impurities and defects near the surface, are essentially different from those in the bulk of a sample. It will be shown below that in nanomaterials (i.e. in the materials consisting of nanoparticles) all the properties (magnetic, electric, conducting etc.) are essentially different from those in ordinary bulk samples. We emphasize once more, that the physical properties, which are spatially homogeneous in bulk samples, become essentially inhomogeneous in nanomaterials due to surface influence. 

The positron affinity is a bulk property of a given homogeneous material and is not related to the interface between two materials. The more negative the positron affinity, the deeper the positron energy level in the solid. The difference between the lowest positron energies of two materials in contact can now be written as... 

While it is the bulk defect chemistry that determines the efficiency of homogeneous doping, it is the defect chemistry of boundary regions that determines charge carrier variations in the vicinity of the interface. The defect model to be described below is able to solve the first problem, the elucidation of the boundary problems deserves much more further work. If we ignore effects at very small sizes at which interfacial effects dominate the whole carrier chemistry within the particle, the material s properties are expected to be dominated by bulk phenomena. 

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