Difference In Thermal Coefficients Research Articles

Effect of reinforcement particle amounts on dry sliding wear behavior of shot-peened SiC/A356 composites

Abstract Aluminum matrix composites focus primarily on improving mechanical properties such as hardness and wear, however, the softness of the aluminum matrix and thermal coefficient difference between matrix and reinforcement adversely affect these properties. In the present paper, the wear properties of aluminum matrix composites including different amounts of reinforcement were investigated before and after the shot peening process which was used for decreasing the harmful effects as mentioned. The experimental studies clearly reveal that shot peening, which is a mechanical surface treatment and generally used for improving fatigue resistance of materials, has a beneficial effect on the poor wear behavior of metal matrix composites. Obtaining results also indicate that the microhardness is improved greatly up to a certain depth by shot peening due to the higher dislocation density/microstrain which is evidenced with the X-ray diffraction tests.

Pressure-Assisted Development and Characterization of Al-Fe Interface for Bimetallic Composite Castings: An Experimental and Statistical Investigation for a Low-Pressure Regime

A review of the available literature indicates that the development of metal-reinforced castings present intriguing prospects but carry inherent challenges owing to differences in thermal coefficients, chemical affinities, diffusion issues and the varying nature of intermetallic compounds. It is supported that pressure application during solidification may favorably influence the dynamics of the aforementioned issues; nevertheless, not only certain limitations have been cited, but also some pressure and process regimes have not yet been investigated and optimized. This work employs the pressure-assisted approach for bimetallic steel-reinforced aluminum composite castings at a low-pressure regime and thoroughly investigates the role of three process parameters, namely pouring temperature (800–900 °C), pressure (10–20 bars) and holding time (10–20 s), for producing sound interfaces. The Taguchi L9 orthogonal array has been employed as the Design of the Experiment, while dominant factors have been determined via analysis of variance and the grey relational analysis multi-objective optimization technique. Supplementary analysis through optical micrographs, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) has been utilized to quantify interfacial layer thicknesses and to study microstructural and compositional aspects of the interface. Nano-indentation tests under static and dynamic loading have also been performed for mechanical strength characterization. It has been found that uniform interfaces with verifiable diffusion are obtainable, with the pouring temperature being the most influential parameter (percentage contribution 92.84%) in this pressure regime. The experiments performed at optimum conditions of pouring temperature, applied pressure and holding time produced a ~328% thicker interface layer, 19.42% better nano-hardness and a 19.10% improved cooling rate as compared to the minimum input values of the said parameters.

Investigation of parameters of Schottky diodes based on chromium silicides

In this work, the parameters of Schottky diodes obtained by sputtering chromium on silicon and annealed at 900÷1100°C. were investigated. Experimental studies of Schottky contacts based on silicides have an all-microscopic analysis of the physical and chemical properties of the layers and an analysis of the electrical behavior of the system. The Schottky model assumes that surface states are located on the border between the transition layer and silicon. Using measurements of direct and inverse volt-amperage characteristics (VAC), values of the height of the effective Schottky barrier, the rate of change in the height of the barrier, as well as the value of the density of surface states, the thickness of the transition layer were obtained and the tunneling process was considered. The temperature dependencies of electrical conductivity are given and the course of the curve is analyzed. The obtained VAC is explained by the model of thermionic emission of the Schottky theory. Height of a barrier to contact chrome-silicon silicide which is equal to 0.65 eV is determined, and the factor of ideality was close to unit. The number of steps required by the charge carriers to overcome the potential barrier is determined. It is described that the current transfer mechanism is associated with the presence of surface states at the interface, as well as the occurrence of lattice mismatch and the difference in thermal coefficients. Diode illumination by LED confirms the effect of surface states on VAC. An increase in the surface states of photogenerated holes is shown. It has been shown that the existence of surface states affects the change in the height of the potential barrier, the amount of bending of the zones, and also determines that the predominant transfer mechanism is a tunneling multistage mechanism.

Influence of welding parameters on lap joint between Al and Ti alloys using low power fiber laser

AbstractFiber laser beam welding has always been a user‐friendly and flexible method to join dissimilar materials despite differences in thermal coefficient. Many industrial applications such as automotive has replaced the conventional joining methods towards this because of the flexibility and reduction in time consumption. In the present study, dissimilar titanium alloy; Ti6Al4 V and aluminum alloy; AA2024‐0 were laser welded through a lap joint technique using a low power Yb‐fiber laser without any additional filler. The influence of welding speed on weld morphology was investigated using optical microscopy (OM) and scanning electron microscope (SEM). The cross‐section of the joints revealed that the fusion zone (FZ) and heat affected zones (HAZ) are wider when welding speed decreases with lower laser power. This result shows that the low power fiber laser has sufficient energy to melt the base materials, forming a liquid bridge to facilitate the smooth flow of molten metal between the top and bottom layer. Therefore, at lower welding speeds with constant low laser power, it was shown that there are possibilities of laser welding between two non‐ferrous metals.

Estimation of residual stresses developed in SS 304L / Zr-2.5%Nb joints

Joints between stainless steel (SS) and zirconium based alloys have various applications in nuclear technology [1]. Since mechanical integrity of these joints under reactor operating conditions for the reactor life time viz. 40-50 years is vital, the qualification criterion for these joints should be very critical. The large difference in thermal coefficient of expansion of Zr-based alloys and SS leads to generation of residual stresses at the joint interface, which can have deteriorating effect on the bond integrity and mechanical properties of the joint [2]. Therefore, a proper estimation of these stresses is essential for its qualification. One of the accepted ways of mitigating such high residual stresses is by using suitable ductile interlayers between the joining materials. In the present work, the magnitude, nature and distribution of the residual stresses have been estimated using finite element method (FEM). Simulations were performed using axisymmetric model to evaluate the residual stress field generated while cooling SS304L/Zr-2.5%Nb joint from 1273 K to ambient temperature. The stress fields were calculated assuming elasto-plastic behaviour of the joining materials. The SS part of the joint was found to experience a maximum radial tensile stress of about 260 MPa, and a maximum shear stress of 55 MPa at the SS/Zr-2.5%Nb interfacial region. The maximum compressive stress in axial direction was found to be 92 MPa. Similar simulations were also performed by introducing Ni and Ti interlayers of thickness 200 μm each between SS304L and Zr-2.5%Nb. The maximum radial tensile stress on the SS side was found to be 243 MPa and a maximum shear stress of about 74 MPa near SS/Ni interface, while the maximum compressive stress in axial direction was 55 MPa. Therefore, it can be seen that a reduction in residual stress levels can be achieved upon incorporation of Ti and Ni interlayers.

Ba- and La- Strontium Cobalt Ferrite Carbonate Composite as Cathode Materials for Low Temperature SOFC

Barium strontium cobalt ferrite (BSCF) and lanthanum strontium carbonate ferrite (LSCF)–samarium-doped ceria carbonate (SDCc) composite cathodes were developed based on various molar ratio of binary carbonate. The percentage of molar ratio for (Li/Na)2 binary carbonate in the composite cathodes were 67:33, 62:38, and 53:47. Influence of (Li/Na)2 binary carbonate addition on BSCF-SDCc and LSCF-SDCc were studied in terms of chemical, thermal, and physical properties. The composite-cathode powders were prepared using high-energy ball milling (HEBM) and followed by calcination at 750 °C for 2h. Characterizations of the composite cathode were performed through Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), and dilatometry. The FTIR result verified the existence of carbonates in all the composite cathodes. The increment in the Na2CO3 molar ratio has contributed to the growth of the BSCF-SDCc particles as observed from the FESEM micrographs and particle size. The LSCF-SDCc composite cathodes revealed a lower (1.38-6.69%) thermal coefficient difference with SDCc electrolyte. The BSCF-SDCc and LSCF-SDCc composites with 53:47 mol.% of (Li/Na)2 binary carbonate exhibit the applicable properties as SOFC cathode material.

A Mathematical Model for Electrostatic Actuators Operation with Multilayer Metallic Strip

This paper presents a mathematical model to simulate the actuation of a multilayer metallic strip. The model computes the radius of curvature of the actuator when it curls because of difference in thermal coefficient of the different metals used in the strip. The mathematical model for simulating the radius of curvature is based on the earlier work by Timoshenko [4], and Jianyu Fu[5]. The radius of curvature computed is used to calculate the voltage to uncurl the actuator. Numerical experiments were performed on the model and the trends were found to be in agreement with the experimental data.

Growth of highly textured manganese zinc ferrite films on glass substrates

Highly textured manganese zinc ferrite (Mn0.7Zn0.3Fe2O4) films have been successfully fabricated on glass substrates by pulse laser deposition at relatively low temperatures. Investigations indicated that the strain, which is induced by high deposition rate and the difference of thermal coefficient between the film and glass substrate, is attributed to the growth of textured structure. Growth of highly textured cobalt ferrite film was also achieved using the same method. This work provided a possible technique for fabricating high quality ferrite films on glass substrates.

Natural building stone composed of light-transmissive minerals: impacts on thermal gradients, weathering and microbial colonization. A preliminary study, tentative interpretations, and future directions

A number of modern buildings employing claddings of granites or marbles have experienced bowing of the rock panels together with weathering of the material. Theoretical and field-based data analysis and laboratory experimentation have assumed that heat exchange resulting from incoming solar radiation is at the material surface. However, a number of recent experiments have clearly shown that some lithologies, including both marble and granite, comprise a number of light-transmissive minerals that significantly change the thermal responses in the outer few millimetres of the rock. Further, this translucence will create mineral-to-mineral stresses where light-transmissive minerals are in contact with opaque ones. The whole is further exacerbated by differences in thermal coefficients of expansion and conductivity which themselves may depend on the mineral axis; surface modifiers such as paints further complicate the situation. The degree of light penetration, based on field measurements, can be significant and can facilitate rapid changes in temperature (ΔT/Δt > 2°C min−1) at depth within the rock thereby increasing the sub-surface stresses. The amount of light penetration for any given mineral will be dependent upon material slope, the latitude, season, and albedo. Albedo is identified as a complex variable, changing as a function of the angle of the sun to the particular surface; polished surfaces, as often with cladding, will further influence this. Data analysis suggests that, in the Northern hemisphere, south of the polar circle, the summer may not be the time of the largest heat loading on the southern aspect and that larger loadings, coupled with lower air temperatures, occur early and late in the year. This seasonal impact has great potential for thermal stresses on the southern aspect. The presence of light-transmissive minerals also allows sub-surface biotic colonization and results in weathering. Overall, where light transmissive minerals/lithologies occur the thermal responses are highly complex and in need of more rigorous consideration.

Properties of various ion-implanted sapphire substrates for GaN epilayers

The structural, electrical, and optical properties of GaN epilayers grown on various ion-implanted sapphire (0001) substrates by metal-organic chemical vapor deposition (MOCVD) were investigated. Sapphire is a typical substrate for GaN epilayers. However, there are many problems such as lattice mismatch and thermal coefficient difference between sapphire substrate and GaN. The ion-implanted substrate's surface had decreased internal free energies during the growth of the ion-implanted sapphire (0001) substrates. The crystal and optical properties of GaN epilayers grown in ion-implanted sapphire (0001) substrate were improved. Also, surfaces excessively roughened and modified by ions degraded the GaN epilayers. Not only the ionic radius but also the chemical species of implanted sapphire (0001) substrates could improve the properties of GaN epilayers grown by MOCVD. This result implies that higher quality of GaN epilayers can be obtained using ion-implanted sapphire (0001) substrate with various ions.

Investigation on defects in HPHT-grown diamond single crystals

In the diamond single crystals synthesized at high temperature and high pressure using FeNi as catalyst, there are usually supersaturated vacancies and inclusions formed during the diamond crystal growth and rapid cooling from high temperature. Some defects such as prismatic dislocation loops, stacking faults and array of dislocations are closely related to such supersaturated vacancies and inclusions. The supersaturated vacancies agglomerate into discs on the (111) close-packed planes, subsequent collapse of the discs forms the dislocation loops and stacking faults. The thermal internal stresses, which are caused by the difference of thermal contraction between the diamond and the inclusions due to the difference of thermal coefficients between them as the diamond is cooled from high temperature, may be relieved by the formation of array of dislocations. In the present paper, these defects in the diamond single crystals were directly examined by transmission electron microscopy (TEM). The characteristics and formation process of these defects were analyzed briefly.

Method for fabrication of microfluidic systems in glass

In this report we describe a new way of fabricating integrated microfluidic elements in glass. By employing a matrix of underpinning posts and a thin wall, surrounding etched flow channels, an efficient sealing of glass chips substrates to thin cover glass can be accomplished. The use of this arrangement enables the overlay sheath of glass to hermetically close the flow channel by fusion bonding while avoiding problems with void or crack formation that are due to dust particles, non-planarity and differences in thermal coefficient of expansion. As an example a structure with a thin cover glass, useful for studying phenomena in capillary electrophoresis utilizing large numerical aperture microscope lenses, was fabricated.

熱応力緩和型ＺｒＯ２／Ｎｉ系傾斜機能材料の再生冷却燃焼器への適用に関する研究 （その２） 完全ＦＧＭ型再生冷却エンジンの高空性能試験

Ceramic thermal barrier coating systems will be of growing importance for the reusable high performance engine. These are very attractive ways to achieve a high performance engine because of their potential for reducing the film-cooling requirements in the rocket chamber. However, due to the large difference in thermal coefficient of expansion between the coating materials and the metal wall and the low ductility of the ceramic coating, cracks occur in the ceramic coating layer or spalling occurs during repeated thermal cycles. One method of improving adhesion of the coating to the metal wall is to apply functionally graded materials (FGM). In this test series, high altitude performance tests (HAPT) of a regeneratively cooled 1, 200N thrust engine composed of ZrO2-8%Y2O3(PSZ)/NiCr FGM chamber were conducted with nitrogen tetroxide/monomethyl hydrazine (NTO/MMH) bipropellant. To enhance the engine performance, we employed the high performance unlike quadlet element injector. The film cooling fraction was reduced to 0% of the total fuel flow rate to obtain high performance. The combustion chamber used in HAPT was composed of perfect PSZ/NiCr FGM, i. e., the chamber inner wall was made of PSZ, the cooling wall side was made of NiCr and intermediate materials were PSZ/NiCr FGM. A total of 50 firing tests including high altitude performance tests and sea level tests were performed to evaluate the engine performance in terms of vacuum specific impulse (ISPV), and also to obtain chamber thermal data. The high performance of the engine, i. e., ISPV=318s at PC=1.4MPa, was verified.

Residual strains in epitaxial fluorides on Si(111) substrates

Growth conditions for molecular beam epitaxy of SrF2 directly on Si(111) have been optimized for excellent crystal quality. A χmin of 2.6% was demonstrated. Lattice distortion measurements were carried out by ion channeling along off-normal channeling directions in the strained SrF2 layers. The measured residual tensile strain versus the film thickness demonstrated a special thickness: When films were thinner than this thickness, their strain dropped dramatically as thickness decreased, and when films were grown thicker than it, the strain gradually decreased with increasing film thickness. This unique behavior was successfully modeled as the result of competition between the large lattice mismatch between SrF2 and Si and the large difference in thermal coefficients of expansion. Residual strain remained even in SrF2 films thicker than 600 nm, unlike CaF2 on Si(111) for which CaF2 films thicker than 300 nm were observed to have no strain.

Localized deformation of SiCAl composites

Tensile tests of SiC-6061 Al composites containing various volume fractions of whiskers or particles (20, 5 and 0 vol.%) showed that for samples containing a high volume fraction (20 vol.%) the fracture process was very localized, i.e. a very narrow neck. As the volume fraction of whiskers or particles decreased, the deformed region spread out. One might expect that the microstructure should correspond to the macroscale changes. In the highly deformed region the dislocation density is expected to be higher, in the less deformed regions the dislocation density should be lower, and if the deformation is very localized, then the high dislocation density should also be limited to a very narrow region. Overall, there is good agreement between the microstructure (dislocation density) change and the macroscale deformation of SiCAl composite tensile samples. The mechanism proposed to account for this change in deformation behavior as a function of volume fraction of SiC in aluminum is related to the expansion of the plastic zone (due to differences in thermal coefficients of expansion between SiC and aluminum) when the external stress is applied. Also, the localized deformation is related to localized clusters of SiC particles. There is a cooperative effect which leads to a region of very localized plastic deformation.

Properties and Applications of Polymer Impregnated Cementitious Materials

Basic problems related to manufacturing of polymer impregnated cementitious materials are discussed. Experimental studies on drying, impregnation and polymerization are presented. In addition ot these conditions of the treatment, mix proportion of the base material should be carefully selected so as to optimize a property aimed at. Mechanical strength of PIC is influenced by self stress generated in the material possibly due to shrinkage caused by polymerization and the difference in thermal coefficients of expansion of polymer and concrete. Annealing and addition of a plasticizer are useful methods in reducing this self stress. Improvement of strength is analyzed by Griffith theory, and uniformity of PIC is evaluated by Weibull's theory. Both of these analyses indicate that the impregnation is a reasonable method to improve mechanical properties of cementitious material, though shrinkage caused by this treatment should be deliberately taken into account when the material is further reinforced with steel or other fibers. Products manufactured by the process studied in this report have shown good performances in actual service to date. /ACI/

A new concept in microwave gas switching elements

A gas switching tube commonly known as a TR tube is an RF energy switch, the operation of which is a function of incident power level. Switch operation is achieved by gaseous ionization. The major problem in the design of gas switching devices has been that of achieving simultaneously a short recovery time and a low arc loss. This problem has been eliminated by the development of the device described in this paper. The design objective was to produce a self-contained TR window for operation at very high powers. The arc loss developed by conventional tube design at these high power levels would be sufficient to melt any known window material. The design of this device is such that the ionizable gas blanket takes the form of a thin-walled cylinder suspended in the iris in a dielectric cylinder. This configuration presents a smaller volume of gas with a reduced cross section and a much shorter diffusion length. These changes result in lower leakage power, faster recovery time, and reduced arc loss. As finally developed, the window does not involve glass-to-metal or ceramic-to-metal seals. The problem of metal sputtering or outgassing is therefore eliminated. By a unique spring pressure support, the problem of strain developed by differences of thermal coefficients of expansion is eliminated. The open-ended design of the cylinder provides excellent facilities for cooling the window. Prototype units have been successfully operated in "L" Band at power levels considerably in excess of 15 mw peak power and 30 kw average power. These units exhibited recovery times of 5 to 20 µsec with high-level attenuation of 26 to 35 db and arc loss below the level of present measuring techniques, i.e., < 0.02 db. A practical window for these high powers with a loaded Q of less than 1.5 has been fabricated.