Different Classes Of Materials Research Articles

Experimental identification of a piezoelectric material for high impulse pressure wave applications

Recently, the instruments commonly used in urology have been organized in modules which are designed specially for use with a multi-purpose “urology table”. In order to fit into this design, the size of piezoelectric lithotripters will have to be reduced. With this goal in mind, experiments have been performed to identify a piezoelectric material which can generate the greatest possible number of high pressure shots. The experimental protocol was designed to classify various materials on the basis of their capacity to fulfill these functions. Two different classes of material were investigated, namely piezoceramics (P1-60, P7-62, P1-88, P1-91, P1-94, Quartz & Silice, France) and piezocomposites (IMA021, IMA022, IMA023, Imasonic, France). Analysis of the behavior of these different types of material showed that the composites were more suitable for the application concerned; in particular, two of these materials (IMA022 and IMA023) gave surface pressures of over 4.5 MPa whereas the highest pressure obtained with a piezoceramic material was only 2.5 MPa. Pressure measurements made close to the surface of transducers showed that radial waves were generated in the ceramic transducers which could account for why they tend to break sooner than composite ones.

Aspects of Grain Size Strengthening in Polycrystals

ABSTRACTThe strengthening effect of grain boundaries is well established and observed experimentally as the Hall-Petch relationship. In this paper different mechanisms proposed in the literature to explain the observed Hall-Petch effect are reviewed critically. The fundamental implications of the different approaches are discussed with reference to experimental data for two different classes of materials;-Materials with locked dislocations, i.e. with a sharp yield point behaviour.-Materials without locked dislocations, i.e. with a smooth yielding behaviour.It is shown that a simple model (Bergström) can be used to understand the grain size strengthening in the latter class of materials while more work is needed to quantitatively understand the behaviour of materials showing a sharp yield point.

Compression Shear Test (CST) – A Convenient Apparatus for the Estimation of Apparent Shear Strength of Composite Materials

A convenient apparatus for the determination of apparent shear strength of flat as well as curved composite materials and adhesive joints, the compression shear device (CSD), is described in detail. Compared with other commonly used tests the sample preparation is very easy, inexpensive, and consumes less material. The obtained values of apparent shear strength for different classes of materials are comparable with values determined by established tests. Using an additional extensometer, the estimation of shear strain and shear modulus is also possible with the proposed equipment.

Constitutive Data for Powder Compaction Modeling

The compaction behavior of steel powders, hard metals, and ceramic powders have been investigated using a newly developed high pressure triaxial testing facility. Results from isostatic compaction, simulated closed die compaction, and compaction along different radial loading paths in stress space are presented for six commercial powders. The experimental data are compared and considerations regarding the constitutive modeling of the compaction response of the different classes of materials are presented.

Evaluation by electrochemical tests of the passive film stability of equiatomic Ni-Ti alloy also in presence of stress-induced martensite.

In this study, potentiodynamic polarization scans, potentiostatic scratch tests, and modified American Society for Testing and Materials F746 tests were carried out in simulated body fluids on commercial orthodontic wires made of different classes of materials and on titanium used as a reference. The stability of passivating film, evaluated by electrochemical techniques that abruptly damage it, e.g., potentiostatic scratch test, increased in the following order: Ni-Ti </= stainless steel < Co-Cr alloy < Ti. Because satisfactory biocompatibilty of implants relies on the presence of a stable and efficient self-repairing passive film, this ranking should be considered in view of in vivo applications. Moreover, scratch tests show that it is not possible to enhance the performance of Ni-Ti samples by modifying surface-passive film by dipping in an HF/HNO(3) mixture. Finally, straining of Ni-Ti wires under superelastic conditions and consequent presence of stress-induced martensite does not substantially modify their localized corrosion resistance.

Deposition of Ceramic Materials Using Powder and Precursor Vehicles Via Direct Write Processing

ABSTRACTDry powder and sol gel ceramic films were deposited using a Matrix Assisted Pulsed Laser Evaporation Direct Write (MAPLE-DW) technique developed at NRL for optical and electrical device applications. The MAPLE-DW technique uses a high-energy focussed photon source in combination with a “ribbon” to fabricate materials onto a range of substrates at room temperature without material degradation. Two different classes of materials were processed in this research, (1) ceramic dry powder materials and (2) sot gel precursor materials. Cathodoluminescent measurements demonstrated that the efficiencies of the transferred phosphor materials were not degraded during the laser transfer process. Scanning electron microscopy and 3-D surface profilometry of the ribbon after the MAPLE-DW process revealed a 90–95% transfer efficiency for the dry powders. Scanning electron microscopy and energy dispersive spectroscopy revealed that the sol gel materials also transferred with an efficiency in the 90-95% range. Three distinct regions were identified on the so gel ribbon after the laser transfer process. The regions suggest that the matrix absorbing layer and the sol gel materials are completely removed for the areas irradiated by the laser pulse, and in a second so-called “heat affected region,” only the sol gel material is ejected from the ribbon.

Structure and Property Characterization of Porous Low-k Dielectric Constant Thin Films using X-ray Reflectivity and Small Angle Neutron Scattering

AbstractHigh-resolution X-ray reflectivity and small angle neutron scattering measurements are used as complementary techniques to characterize the structure and properties of porous thin films for use as low-k interlevel dielectric (ILD) materials. With the addition of elemental composition information, the average pore size, porosity, pore connectivity, matrix density, average film density, film thickness, coefficient of thermal expansion, and moisture uptake of porous thin films are determined. Examples from different classes of materials and two analysis methods for small angle neutron scattering data are presented and discussed.

Magnetism and microstructure: the role of interfaces

The role of microstructure in determining the magnetic properties of materials is introduced and the role of interfaces, including intergranular phases, is presented in the context of three different classes of materials exhibiting novel magnetic behavior. This includes perpendicular interface anisotropy in metallic multilayers, origin of coercivity in hard magnets and the role of interface roughening in giant magnetoresistive materials. It is demonstrated that in these systems the microstructure of the interfaces (structural and compositional roughness, compound formation, strain and defects) and intergranular coupling and/or isolation are the appropriate length scales that determine their magnetic and magnetotransport properties. This approach not only makes it possible to atomically engineer thin films and nanostructures but also offers opportunities to elucidate the physics of magnetism and build new materials with unique properties.

Mechanics and mechanisms of fatigue damage and crack growth in advanced materials

The mechanisms of fatigue-crack propagation in ceramics and intermetallics are examined through a comparison of cyclic crack-growth behavior in ductile and brittle materials. Crack growth is considered to be a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip, which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip, which impede it. In this paper, we examine and model the widely differing nature of these mechanisms, with emphasis on behavior in ceramics at ambient and elevated temperatures, and compare their specific dependencies upon the alternating and maximum driving forces (e.g., ΔK and K max), thereby providing a useful distinction of the process of fatigue-crack propagation in these different classes of materials.

Charge fluctuations in YBa2Cu3O7-x high-temperature superconductors

Understanding the behaviour of the electrons in the high-temperature copper oxide superconductors remains a challenging problem. An important class of models1,2,3,4,5 argues that the distribution of electronic charge and spin is not homogeneous: rather, spin and charge adopt a dynamic arrangement in which the spins on the copper form antiferromagnetic stripes, separated by domain walls containing the charge1,2,3,4,5. The dynamic behaviour of the spins has been extensively studied by neutron scattering, and recent results6 have shown that the low-frequency fluctuations for different classes of materials display a universal spatial behaviour that is consistent with the stripe picture. But arguments for the existence of the stripe phases are difficult to sustain without a demonstration that charge is distributed in the domain walls. Here we report phonon measurements for the YBa2Cu3O7-x high-temperature superconductors, which reveal the presence of charge fluctuations. The inferred periodicity is that expected if the charge is located in the domain walls separating the spin stripes. Our results therefore provide strong support for the existence of a dynamic stripe phase in the high-temperature superconductors.

Selective oxidation of light alkanes: interaction between the catalyst and the gas phase on different classes of catalytic materials

Some aspects concerning the chemistry of oxidative transformation of paraffins catalyzed by different classes of materials are examined. Attention is given mainly to: (i) the role of the redox properties of transition metal oxide-based systems, and (ii) the contribution of radical-type, homogeneous and heterogeneously-initiated homogeneous reactions over nonreducible metal oxide catalysts. Technological solutions aimed at the control of oxidizing properties of the catalyst are also examined.

An investigation into the location of crack initiation sites in alumina, polycarbonate and mild steel

The effects of notch root radius on fracture toughness and crack initiation sites have been investigated in this paper using three different classes of materials. Data on alumina which represent ceramics, mild steel from the metals ffeily and polycarbonate representing plastics were obtained and analysed. The locations of crack initiation sites have been pinpointed by scanning electron microscopy. These identified sites more or less are located within the critical process zone or the theoretical plastic zone. The critical process zone size (Dc ) or the theoretical plastic zone size (RYF ) are independent of the notch root radius unlike the plain‐strain fracture toughness of notched specimens [KIC (ρ)]. The authors emphasize why the parameters Dc and RYF are useful for a quantitative evaluation of the reliability of structural materials.

Slurry erosion of metallics, polymers, and ceramics: particle size effects

One of the least well understood areas in the study of erosion by solid particles is the effect of particle size. Erosion is generally assumed to be independent of particle size above a critical value. However, there is evidence that this pattern is dependent on the process conditions. In the present study, the effect of particle size was investigated for different classes of materials, which included two pure metals, an alloy, a ceramic, and a polymer. The apparatus used was an impinging jet. Scanning electron microscopy was used to characterise the degradation following erosion. The results showed that the erosion rate peaked at intermediate particle sizes, for some of the materials studied. However, the particle size at which the peak occurred changed as a function of target and particle properties. Such observations were explained in terms of the combined effects of particle, target, and fluid flow parameters on the erosion mechanisms of the different materials.

Mechanisms of fatigue-crack propagation in ductile and brittle solids

The mechanisms of fatigue-crack propagation are examined with particular emphasis on the similarities and differences between cyclic crack growth in ductile materials, such as metals, and corresponding behavior in brittle materials, such as intermetallics and ceramics. This is achieved by considering the process of fatigue-crack growth as a mutual competition between intrinsic mechanisms of crack advance ahead of the crack tip (e.g., alternating crack-tip blunting and resharpening), which promote crack growth, and extrinsic mechanisms of crack-tip shielding behind the tip (e.g., crack closure and bridging), which impede it. The widely differing nature of these mechanisms in ductile and brittle materials and their specific dependence upon the alternating and maximum driving forces (e.g., ΔK andK max) provide a useful distinction of the process of fatigue-crack propagation in different classes of materials; moreover, it provides a rationalization for the effect of such factors as load ratio and crack size. Finally, the differing susceptibility of ductile and brittle materials to cyclic degradation has broad implications for their potential structural application; this is briefly discussed with reference to lifetime prediction.

Measures of auditory-visual integration in nonsense syllables and sentences.

For all but the most profoundly hearing-impaired (HI) individuals, auditory-visual (AV) speech has been shown consistently to afford more accurate recognition than auditory (A) or visual (V) speech. However, the amount of AV benefit achieved (i.e., the superiority of AV performance in relation to unimodal performance) can differ widely across HI individuals. To begin to explain these individual differences, several factors need to be considered. The most obvious of these are deficient A and V speech recognition skills. However, large differences in individuals' AV recognition scores persist even when unimodal skill levels are taken into account. These remaining differences might be attributable to differing efficiency in the operation of a perceptual process that integrates A and V speech information. There is at present no accepted measure of the putative integration process. In this study, several possible integration measures are compared using both congruent and discrepant AV nonsense syllable and sentence recognition tasks. Correlations were tested among the integration measures, and between each integration measure and independent measures of AV benefit for nonsense syllables and sentences in noise. Integration measures derived from tests using nonsense syllables were significantly correlated with each other; on these measures, HI subjects show generally high levels of integration ability. Integration measures derived from sentence recognition tests were also significantly correlated with each other, but were not significantly correlated with the measures derived from nonsense syllable tests. Similarly, the measures of AV benefit based on nonsense syllable recognition tests were found not to be significantly correlated with the benefit measures based on tests involving sentence materials. Finally, there were significant correlations between AV integration and benefit measures derived from the same class of speech materials, but nonsignificant correlations between integration and benefit measures derived from different classes of materials. These results suggest that the perceptual processes underlying AV benefit and the integration of A and V speech information might not operate in the same way on nonsense syllable and sentence input.

Metal film precision resistors: Resistive metal films and a new resistor concept

Metal-films for precision resistors combine a very low temperature dependence of the electrical resistance with a tolerance of the resistance of only 0.1%. Corrosion resistance and adhesion to the substrate are of major importance. There are different classes of materials being utilised. For low ohmic applications, we use Cu-Ni alloys with a composition of about 65 at. % (atomic percent) of Cu. For this special alloy, the low temperature coefficient of the resistance (TCR) is a stable, intrinsic property. For most alloys, however, annealing is essential to approach the state of zero TCR. This is the case for the NiCrAl alloys, used for the mid-range of resistances and for SiCrN for high ohmic applications. In high ohmic films, metals are often combined with non-metallic substances like oxides or nitrides. Variation of alloy composition, sputtering conditions and annealing procedures are important for obtaining optimum thin-film properties. Important tools for thin-film characterisation are electron microscopy and related techniques together with high-temperature resistance measurement.

Influence of limit stress states and yield criteria on the prediction of forming limit strains in sheet metals

Several researchers have proposed analytical methods for predicting the forming limit curve (FLC), which has been successfully used as a diagnostic tool in sheetmetal forming. However, these approaches lack ease of adaptability to various situations and also involve considerable complexity. Sing and Rao proposed a new FLC modeling approach based on limit stress states derived from yield criterion and material properties from a simple tensile test. The first aspect of this study addresses the influence of the shape of the forming limit stress curve (FLSC) upon the FLC. The FLC modeled from a singly linear FLSC exhibits good agreement with the experimental curve, unlike those modeled from an elliptical or a piecewise linear FLSC. It is, thus, established that a linearized limit stress locus describes adequately the actural localized neck condition for the materials chosen in this study. Second, the study focuses on the suitability of the different cases of Hill’s yield criterion for satisfactory prediction of FLCs. The FLCs predicted using different cases of Hill’s criterion are compared with experimental FLCs in the case of steel and copper. Different cases of Hill’s criterion provide a wider choice for FLC modeling for different classes of materials. The sensitivity of Hill’s stress exponent is also thoroughly explored for achieving a close correspondence between the predicted and experimental FLCs.

Comment on : Energetically evaluated load-indentation measurements of different classes of material by b. Rother

It is shown that the energetically evaluated load-indentation measurement procedure introduced by Rother and Dietrich is equivalent to the analysis of the force-indentation depth-curve by means of the well known Bernhardt formula. A disadvantage of the proposed “differential load feed” (DLF) procedure is that differentation enlarges the scatter of the experimental curves. Deviations of the indenter tip from its ideal shape and surface roughness also influence the experimental results in the case of DLF analysis in a manner as one observes for these methods usually used to analyse force-indentation depth-curves.

A viscoplastic constitutive equation bounded between two generalized plasticity models

Abstract The present work addresses a new rate-dependent inelastic constitutive equation, whose response is always bounded between two distinct rate-independent generalized plasticity models. Main features of the proposed model are the following: (1) it approaches the two rate-independent generalized plasticity models for the case of fast and slow loading conditions, respectively; (2) it properly describes loading-unloading-reloading conditions for any loading rate; (3) it reproduces the experimentally observed difference between the dynamic and the static yielding conditions. These are all properties to be taken into account to obtain an accurate modeling for many different classes of materials, such as metals, polymers, geomaterials, shape-memory alloys. Both the time-continuous and the time-discrete version of the model are discussed. Moreover, a solution algorithmic within a return map setting as well as the form of the discrete consistent tangent tensor are addressed. Finally, some numerical simulations illustrate the performance of the proposed constitutive model.

Exciton-to-biexciton transition in quasi-one-dimensional organics

In the previous paper we demonstrated novel multiexcitons in a neutral mixed-stack charge-transfer solid. The lowest multiexciton, the biexciton, has recently been of interest also in the context of quasi-one-dimensional organic materials that are different from the mixed-stack solids. The nature and strength of the optical transition from the exciton to the two-exciton states is of importance in understanding photoinduced absorption as well as two-photon absorption. We show that within the diverse theoretical models that describe these different classes of materials, the excited state absorption from the optical exciton to the two-exciton states changes in a fundamental way upon the formation of the biexciton. The identical nature of the exciton absorption within these models is a consequence of one dimensionality.