Volume Fraction Of Silver Research Articles

Simulation of a carbon graphite brush with distributed metal particles

The electric and thermal fields within a metal graphite brush in sliding electrical contact with a fast moving rotor are simulated. The composite nature of the brush, a carbon graphite matrix with randomly distributed metal (silver) particles, is treated via finite elements in an inner zone about the brush-rotor contact, and composite mixture theory in an outer zone away from the contact. In the inner zone, individual elements are randomly assigned metal or carbon properties in a manner consistent with the volume fraction. Computations are three dimensional. Electrical and frictional heatings are considered. Simulations of thermal and electric fields are performed over ten different specific distributions of metal particles; averages are then taken over this ten-member ensemble. Simulation results indicate and verify the existence of a critical volume fraction of silver for good brush performance. At this critical volume fraction, individual silver particles link into networks and encourage efficient transfer of electrically and frictionally generated heat away from the contact region. Temperatures remain moderate with this critical volume fraction (even at extreme heatings), and a hot zone does not form.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Toughening zirconia with bimodally distributed silver inclusions

Ceramics have excellent hardness, chemical stability and refractory properties. However, the application of ceramics as engineering components is often handicapped by their brittleness. A major research objective has therefore been to improve their toughness. Recent studies reveal that ceramics can be toughened through the addition of metallic inclusions [1-4]. For example, the toughness of A1203/ 20vo1% A1 composite is 2.8 times that of matrix alone [3] and the toughness of A1203/14 vol % Ag composite is 1.5 times that of matrix alone [4]. A crack bridging mechanism is proposed to be responsible for the toughening effect. In the study reported here, silver inclusions were added to zirconia, and the mechanical properties of the ZrO2/Ag composites at room temperature were investigated. Zirconia (TZ-3YA, TOSOH Co., Japan) and various quantities of silver oxide (Ag20, Johnson Matthey Co., UK) were tumble milled together in ethyl alcohol for 4 h. The grinding media used was zirconia balls. The slurry of the powder mixtures was dried using a rotary evaporator, then the dried lumps were crushed and passed through a plastic sieve. Powder compacts were formed by uniaxially pressing at 44 MPa. Sintering was performed in air at 1300-1500 °C for 1 h. The heating and cooling rates were 5 °C rain -1. The silver inclusions resulted from the decomposition of silver oxide during the heating stage. The sintered specimens were machined longitudinally using 325 grit resin-bonded diamond wheel at a cutting depth of 5 ~m per pass. The specimens were not chamferred. The final dimensions of the specimens were 3 m m x 4 mm x 36 mm. The strength of the specimens was determined by the four-point bending technique at ambient conditions. The upper and lower spans were 10 mm and 30 ram, respectively. The four loading pins were kept in position by rubber bands to avoid undesirable friction constraints. The rate of loading was 0.5 mmmin -I. The fracture toughness was determined by the single-edge-notched beam (SNEB) technique. The notch was generated by cutting with a diamond saw. The width of the notch was around 0.45 ram. The partial pressure of liquid silver at elevated temperature was rather high; for example, the vapour pressure of silver at 1500 °C was 880 Pa [5]. Therefore, the volume fraction of silver decreased as sintering proceeded. The volume fraction of silver after sintering was determined by measuring the weight loss of the composites. The final density was determined by the water displacement method. The

The Effect of the Volume Fraction of Silver on the Thermal Resistance of Die Attach Adhesives

A theory for the thermal resistance in adhesive joints between semiconductor dice and substrates has been developed. The theory takes into account the thermomechanical shear stress in the bond layer caused by differences in thermal expansion. The theoretical result indicates that only small reductions in thermal resistance can be obtained by increasing the bulk thermal conductivity of the adhesive by filling it with silver particles. Experiments were carried out in order to verify this effect. An epoxy resin was loaded with various amounts of silver particles and used to bond thermal test chips to copper substrates. There was a surprisingly small difference in thermal resistance between the various adhesives. The difference was considerably less than the difference in bulk thermal conductivity indicated, thus confirming the basic result of the theory.

Characterisation of Silver Filled Adhesives for Attachment of Microelectronic Chips to Substrates

Polymer based adhesives filled with silver particles are widely used in microelectronics for bonding semiconductor chips to substrates. This paper presents results from thermal conductivity and thermal resistance measurements in such adhesives as function of the volume fraction of silver and the thickness of the bond layer. This is an increasingly important issue because the power dissipation in typical semiconductor chips is increasing rapidly due to technological developments. The thermomechanical stress caused by different thermal expansions in the chip and the substrate was also measured. Thermomechanical stress may cause cracking of the chips and subsequent failure of the microelectronic device. Crack growth in the bond layer during temperature cycling was investigated by measuring the thermal resistance in the layer. The cracks are created because of the differences in thermal expansion which causes a cyclic mechanical boding of the adhesive during temperature cycling.

Characterisation of Silver Filled Adhesives for Attachment of Microelectronic Chips to Substrates

Polymer based adhesives filled with silver particles are widely used in microelectronics for bonding semiconductor chips to substrates. This paper presents results from thermal conductivity and thermal resistance measurements in such adhesives as function of the volume fraction of silver and the thickness of the bond layer. This is an increasingly important issue because the power dissipation in typical semiconductor chips is increasing rapidly due to technological developments. The thermomechanical stress caused by different thermal expansions in the chip and the substrate was also measured. Thermomechanical stress may cause cracking of the chips and subsequent failure of the microelectronic device. Crack growth in the bond layer during temperature cycling was investigated by measuring the thermal resistance in the layer. The cracks are created because of the differences in thermal expansion which causes a cyclic mechanical boding of the adhesive during temperature cycling.

Sputter deposition of yttria-stabilized zirconia and silver cermet electrodes for SOFC applications

Yttria-stabilized zirconia (YSZ) thin films were deposited by reactive magnetron sputter deposition from a composite Zr-Y target in Ar-O 2 mixtures. YSZ deposition rates of 2.7 μm h -1 were obtained at a sputter source power of 150 W. Deposition with total pressures from 3 to 20 mTorr yielded continuous, crack-free films in a compressive state of stres. X-ray diffraction and electron microscopy results showed that the films were polycrystalline cubic YSZ with a columnar structure and an average grain diameter of 15nm. Fully dense films were obtained at a deposition temperature of 350°C. Temperature-dependent impedance spectroscopy analysis of YSZ films with silver electrodes showed that the oxygen-ion conductivity was as expected for YSZ. Ag 1- x [(Y 2O 3) 0.1(ZrO 2) 0.9] x cermet thin films havew been deposited by reactive magnetron co-sputtering silver and Zr/Y targets in Ar-O 2 mixtures. The resistivity ϱ of as-deposited and annealed films as a function od silver volume fraction ⨍ Ag varied over several orders of magnitude. Evaporation of silver from cermet films. observed during long-term annealing at 750°C, was eliminated by depositing a 1 μm thick Sr-doped LaCoO 3 cap layer on the cermet. The cermet with ⨍ Ag≈0.5 is a promising air electrode material for solid oxide devices operating at temperatures <-705°C, exhibiting both low resistivity ( 1×10 -4Ω cm ) and high thermal stability.

Effect of silver addition on superconductivity in the Bi1.6Pb0.4Sr1.6Ca2Cu3O10?y system

Silver/superconductor composites containing 0 to 80 vol% silver have been prepared and their properties determined. Optimum heat treatment at sintering temperatures (∼ 800° C) under low oxygen pressures produces material with high critical current density and improves physical properties. Magnetic susceptibility measurements have been found to be consistent with resistivity results. In order to retain a single high-Tc phase with increasing silver content, decreased oxygen partial pressures are required. Using the normal-state resistivity of these composites, a percolation threshold at a silver volume fraction of ∼43% was observed, while zero resistivity measurements show that a continuous superconducting network can be obtained with up to ∼80 vol% silver. The critical current density of 21 vol% silver-doped samples was found to be 1520 A cm−2 at 77.3 K, compared to 260 A cm−2 for an undoped sample.