Characterization of silver-palladium submicronic powders

Various properties of submicronic silver-palladium powders (70/30 and 75/25 Ag/Pd % wt/wt), recently obtained by a new process of precipitation in aqueous solution, are described. These powders are intended to be used in the multilayer ceramic capacitor industry. The morphology of the particles has been analysed through various measurements, such as SEM images, tap density, BET surface area, and the size distribution by a sedimentation technique. The powders were found to be made of spherical shaped particles of mean diameter close to 0.3 m with a narrow size distribution. The individual grains were agglomerated only to a small extent. After being conditioned as a paste by blending with an organic binder, screen-printed and fired, the particles sintered into a conductive layer of low resistivity (15–30 Wcm for 2 m thickness) with only a few remaining voids, as observed by SEM. Thermal measurements indicated no melting of a pure silver phase at 960 °C, thus indicating that alloying was achieved before this temperature.     

Synthesis of nanocrystalline AgPd alloys by chemical reduction method

Chemical reduction was used to produce nanocrystalline Ag-Pd alloy particles for applications in electronic component fabrication. The alloy particles were prepared at room temperature from formaldehyde-sodium hydroxide solutions and silver and palladium nitrates. Characterization of these particles by X-ray powder diffraction (XRD) reveals bi-metal and nanocrystalline silver-palladium alloys. Energy dispersive and XRD analysis of these products shows a complete range of solid solution. Characterization of the morphology, and the oxidation behavior of the Ag-Pd alloy powders are reported.     

The oxidation kinetics of spherically shaped palladium powder

The oxidation kinetics of spherically shaped palladium (Pd) powders was investigated over the temperature range between 350 and 800°C. Two types of Pd powders differing from each other in particle sizes were used in this study, 0.30 and 0.85 m in diameter. The fraction of total oxide formed was measured in situ by means of a thermobalance method. In this system, the volume of product palladium oxide (PdO) increases significantly vs. the volume of reactant Pd, and consequently the experimental data were explained well by Valensi-Carter equation. Apparent activation energies of oxidation for the two types of Pd powders were both estimated to be 140 kJ/mol.     

Silver-palladium submicronic powders

The structure of new submicronic spherical Ag-Pd powders (75/25 and 70/30 wt/wt% as spheres of mean diameter ca. 300 nm) has been investigated by various techniques. X-ray diffraction, carried out on both powder and single particles, showed two distinct metal phases, the first one being pure silver, the second one slightly alloyed palladium. X-ray microanalysis performed under SEM indicated that each grain displays intermetallic character, both metals being present in proportion close to the mean composition. Space-resolved information has been obtained by three different techniques: X-ray microanalysis carried out under transmission electron microscopy, X-ray photoelectron spectroscopy and secondary ion mass spectrometry, the three of them leading to converging results. Each grain appears as a core made of pure silver surrounded by a 10–15 nm thick layer of slightly alloyed palladium with a mean Pd/Ag ratio 89%/11% which possibly increases from inside to outside.     

A study of alternative metal particle structures and mixtures for dental amalgams based on mercury additions

The perception that mercury in dental amalgam is toxic to the human organism has prompted worldwide efforts by the scientific community to develop alternative amalgam-like materials that utilize little or no mercury. In this investigation, an attempt is made to develop a new dental alloy system by adding liquid mercury to silver-coated Ag₄Sn intermetallic particles in lesser amounts than are used in conventional amalgam alloys. An effort to precipitate the important eta-prime (Cu₆Sn₅) phase was made by adding pure Cu and Sn powders to the alloy formulation during trituration. Tytin^® a popular Ag-Sn-Cu single-composition, spray-atomized conventional dental alloy was used as the control to obtain baseline data for comparisons of microstructures and mechanical properties. Amalgamation of the coated particles with mercury, with or without the addition of Cu and Sn powders, mostly produced specimens with chemically non-coherent microstructures that were relatively weak in compression. These results were due, in part, to mercurys inability to chemically wet the Ag-coated particles and Cu and Sn powders because of naturally occurring surface oxide films. The strongest specimens tested had silver dendritic coatings, resulting in compression strength values up to 40% of the controls. Their higher strength is attributed to mechanical interlocking at the particle/matrix interfaces.     

Production of cast aluminium-graphite particle composites using a pellet method

Copper- and nickel-coated graphite particles can be successfully introduced into aluminium-base alloy melts as pellets to produce cast aluminium-graphite particle composites. The pellets were made by pressing mixtures of nickel- or copper-coated graphite particles and aluminium powders together at pressures varying between 2 and 20 kg mm^–2. These pellets were dispersed in aluminium alloy melts by plunging and holding them in the melts using a refractory coated mild steel cone, until the pellets disintegrated and the powders were dispersed. The optimum pressure for the preparation of pellets was 2 to 5 kg mm^–2 and the optimum size and percentage of aluminium powder were 400 to 1000m and 35 wt% respectively. Under optimum conditions the recovery of the graphite particles in the castings was as high as 96%, these particles being pushed into the last freezing interdendritic regions. The tensile strength and the hardness of the graphite aluminium alloys made using the pellet method are comparable to those of similar composites made using gas injection or the vortex method. The pellet method however has the advantage of greater reproducibility and flexibility. Dispersion of graphite particles in the matrix of cast aluminium alloys using the pellet method increases their resistance to wear.Formerly with Indian Institute of Science, Bangalore, India.     

Physicochemical and catalytic properties of palladium supported on poly(o-methoxyaniline)

Palladium was introduced into a conjugated polymer poly(o-methoxyaniline) (POM) by reacting the powdered polymer with aqueous solution of PdCl₂ of low acidity (PdCl₂: 2.3 × 10⁻³ mol/dm³, HCl: 0.66 × 10⁻³ mol/dm³). Various Pd²⁺ complexes with Cl⁻, H₂O, OH⁻ ligands coexisted in this solution but predominated [PdCl₂(H₂O)₂] ones. Several techniques like X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron and Raman spectroscopy, extended X-ray absorption fine structure have been used to characterize the poly(o-methoxyaniline)-Pd systems. In particular, the state of Pd species in the Pd/POM of various content of palladium (2-8 wt.% Pd) and chemical changes in the polymer matrix induced by insertion of palladium were studied. The protonation and redox reactions involved on palladium incorporation resulted in palladium ions and Pd metal in the final samples. Metallic Pd produced due to spontaneous reduction of palladium ions by the polymer formed large crystalline particles 200-1000 nm in size. The Pd²⁺ species in the form of anionic complexes like [PdCl₄]²⁻ acted as the counterions at low content of palladium (2-4 wt.% Pd). At high palladium content (8 wt.% Pd), several atoms like Cl, N and/or O were identified by extended X-ray absorption fine structure (EXAFS) technique in the nearest environment of Pd atoms. The structural groups of POM (like N groups and/or OCH₃) as well as H₂O, OH⁻ molecules are, therefore, considered as probable species in the coordination sphere of palladium. The catalytic properties were studied for the as-prepared Pd/POM and the samples additionally reduced with aqueous solution of NaH₂PO₂. They were used in the hydrogenation of CC bonds in maleic acid (MAC) and CO groups in 2-ethylanthraquinone (eAQ) at 60 °C and atmospheric pressure of hydrogen using xylene-octanol-2 or water medium. The correlation between Pd/POM activities and the content of Pd metal was found. Activation of the as-prepared Pd/POM with NaH₂PO₂ improved their catalytic properties. Much higher and much stable activities were then obtained in both MAC and eAQ hydrogenation reactions.     

Dissolution kinetics and diffusivity of silver in glassy layers for hybrid microelectronics

Silver and palladium/silver compositions are widely used in hybrid microelectronics, as electrodes for dielectric layers and multilayers, terminations of thick film resistors and interconnections. Interactions between Ag and the adjacent films are known to affect the microcircuit performances. The present study is aimed at collecting data on the behavior of Ag-based films in contact with glassy layers. Most experiments were performed with a glass with composition 68.2 PbO : 30.5 SiO₂ : 1.3 Al₂O₃ wt %. Two different systems were analyzed. The first system consists of thick films prepared from a paste containing glass and either 3 or 15 wt % silver particles; both fine (spherical grains, 0.5–1 m diameter) and coarse (flakes, 2–5 m, <1 m thick) Ag powders were used for these pastes. The distribution of Ag in the film was studied with X-ray diffraction, scanning electron microscopy and fluorescence analysis. The results show that Ag floats on the glassy layer. Diffraction of X-rays generated by a synchrotron radiation source allowed us to study the kinetics of silver dissolution in the glass; this phenomenon is consistent with the Avrami theory, with an apparent activation energy E _dis=0.69±0.04 eV. The second system analyzed, Ag-based terminations of glass layers fired at various peak temperatures, enabled us to obtain quantitative values for both Ag solid solubility (about 2.5 wt %) and Ag diffusion coefficients D _Ag(T ). Typical values of D _Ag(850 °C) are 30.3±11.9 10^–8 cm²/s; an apparent activation energy of the diffusion process is E _a=0.6±0.1 eV.     

Electroless silver coating on fine copper powder and its effects on oxidation resistance

Electroless silver coating on fine copper powder (3.4 μm) and its effects on oxidation resistance were investigated by varying the silver contents. As-coated copper powders were characterized using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was found that the uniformity of silver coating was improved with silver content. When the silver content reached 20 wt.%, silver was homogeneously distributed around the copper particles and few free silver particles were detected. As a result, the sheet resistances of metal films reduced with the silver content (presenting the improvement of oxidation resistance), and at the level of 20 wt.% silver content, it had a minimum value and hardly increased with the increasing oxidation time.     

Potential for integration of electrophoretic deposition into electronic device manufacture; demonstrations using silver/palladium

A silver palladium (Ag/Pd) alloy powder is used as an example material to illustrate potential applications of electrophoretic deposition (EPD) in electronics manufacturing, in particular the forming of multilayer devices. The dispersion and deposition of the Ag/Pd powder in acetic acid is characterized. It is found that deposition can be explained by the direct action of electrostatic force on individual particles. Depositions of this Ag/Pd powder are then used to demonstrate: forming of a continuous layer on a rigid substrate; forming of continuous layers in laminated and sintered BaTiO₃ multilayers; and incorporation of patterned depositions into a multilayer by either overcasting the patterned deposition with a particulate slip to form a multicomponent tape, or direct lamination of the patterned deposition to a low temperature co-fired ceramic (LTCC) tape. It is shown that electrically conducting layers can be formed at an average thickness of only two times the diameter of the starting powder. Continuous conductor lines thirty times the average diameter of the starting powder were formed.     

All-optical hydrogen-sensing materials based on tailored palladium alloy thin films

Optical reflectance measurements were performed to determine the hydrogen response characteristics of 20-nm-thick Pd-Au (Ag) films. The response characteristics displayed a strong dependence on alpha, mixed alpha/beta, and beta Pd-hydride phases formed in the films. The response time peaks in the alpha --> beta phase transition region (1625 s at 0.4% H(2) for Pd(0.94)Ag(0.06) and 405 s at 1% H(2) for Pd(0.94)Au(0.06)), consistent with critical slowing down phenomena. The alpha --> beta phase transition region was shifted and inhibited by changing the alloy element to Au and increasing its corresponding content to 40 atom %, respectively. Initial hydrogen uptake rate measurements determined that, due to the adsorption of ambient background gases, the rate-limiting step for alpha or beta phase PdH formation is dissociative chemisorption of hydrogen for each palladium alloy film. By tuning the alloy content and composition of the palladium films, the surface properties of the film become more receptive toward the rapid detection of hydrogen and a novel hydrogen-sensing material using Pd alloyed with 40 atom % Au is presented.     

Experiments on Dynamic Behavior of Molten Metal and Solid Particles Under Direct Current Passing

At electrodischarge sintering of powder mixtures by electric direct current, some essential displacements of molten metal and solid particles were revealed. Such displacements at conventional sintering were not known to us. The objects were powders (mixtures): Sn–Cu (dispersivity 100–200 µm); Sn–high-temperature alloy on nickel base or electrocorundum; copper alloy-electrocorundum (dispersivity 63–500 µm); Ni (dispersivity 10 µm). The electric direct current was passed in vertical direction parallel with the axis of cylindrical mould or through the series circuit steel-powder nickel layer-hard alloy. If the cylindrical container (mould) did not have the upper punch (a load on top was absent), the sample after solidification had the form of a bullet. This sample became convex on top and concave on bottom. At the application of prepressing (start pressing) on top, the movement of suspension (molten matrix and solid particles) occurred in a similar way. Under the action of electromagnetic forces, a redistribution of solid particles in volume of suspension occurs. Particles with more high conductivity than liquid phase begin to cluster at the center. For the case of lower conductivity, the motion of the particles to the outer side surface of mould prevails. As the current passed through the series of circuit steel-nickel powder layer-hard alloy, three named parts of the circuit were sintered into a single block. After destruction at testing, the interlayer had the form of a ring. This shows the nonuniformity of the passing of electric current through the interlayer. The cause of displacements of solid (nonmolten) particles may be the simultaneous influence of thermic convection and squeezing out by the electromagnetic Lorentz-forces, which are directed radially. The phenomena described may be applied in the production of objects with a gradient (variable in volume) concentration of particles added specially. This refers to products of their interaction with molten matrix as well.     

Sensor Properties of Pd-Doped SnO2 Films Deposited by Laser Ablation

Polycrystalline SnO₂ and SnO₂ films were surface-doped with palladium using laser ablation. The effect of the energy density of pulsed KrF laser radiation on the plasma generation process and Pd deposition rate was studied, and the depth profiles of Pd in the films were determined. The gas response of the Pd/SnO₂, SnO₂, and Pd/SnO₂ films was studied between 200 and 380°C using a mixture of 1 vol % H₂ with N₂. Surface doping with Pd was found to enhance the hydrogen sensitivity of SnO₂ by two orders of magnitude.     

Nano-structured intermetallic compound TiAl obtained by crystallization of mechanically alloyed amorphous TiAl, and its subsequent grain growth

The amorphization process during mechanical alloying (MA) was investigated for the Al-50at%Ti and Al-50at%Ti-10vol%TiB₂ powder mixtures. Pure metallic powders of Al and Ti were finely mixed and transformed to the amorphous phase after being milled for about 2880 ks. In the case of Al-50at%Ti-10vol%TiB₂ powder, the amorphous alloys with a fine dispersion of TiB₂ particles could be obtained for a shorter milling times than that required for the powders without TiB₂ ceramics. As a result of heat treatment for the mechanically alloyed amorphous powders, a nanocrystalline intermetallic compound of TiAl () could be produced. Subsequent grain growth of the phase during heat treatment was investigated by estimating the grain-growth exponent and the activation energy for grain growth. It was found from this estimation that the grain growth was further suppressed as the powders were mechanically alloyed for longer times. Furthermore, the addition of the TiB₂ particles that could be dispersed during MA finely and homogeneously in the amorphous matrix was found to be effective for suppression of the grain growth especially at elevated temperatures as well as for a long annealing.     

Sinterability of magnesium-oxide powder containing spherical agglomerates

The magnesium-oxide (MgO) powders were prepared by calcining basic magnesium carbonate (4MgCO₃·Mg(OH)₂·4H₂O; BMC) powder at a temperature between 600°C and 1200°C for 1 to 5 h. The resulting MgO powders contained spherical agglomerates with diameters of 10–50 m; the external shapes of these BMC agglomerates remained unchanged even after the calcination. With increasing compaction pressure, the densification of MgO powder compacts proceeded by (i) the rearrangement of agglomerates (50 MPa), (ii) the collapse of agglomerates (50–100 MPa), and (iii) the closer packing of primary particles (100 MPa). The MgO compact was fired at 1400 °C for 5 h. The relative density of the sintered MgO compact whose starting powder was prepared by calcining the BMC at 1000°C for 3 h attained 98.0%. The bending strength of this sintered MgO compact attained 214 MPa.     

Amorphous formation processes of Al-20 mol% Cr alloys by mechanical grinding of equilibrium intermetallic compounds

The amorphization of Al-20 mol % Cr alloys were investigated when different starting powders were mechanically ground using a vibration-mill. Mechanical grinding (MG) of two types of starting powders were examined: (1) single (Al-20 mol % Cr) phase powders and (2) a mixture of (Al-15 mol % Cr) and (Al-30 mol % Cr) phases powders. Amorphous Al-Cr alloys could not be obtained by MG of single intermetallic compound ( phase) as the starting powders. But in the case of MG using the mixture of intermetallic compounds ( and phases), some parts of the MG powders changed phase into an amorphous phase with a Cr content of about 30 mol % at the initial stage of the milling. On further milling, the formed amorphous phase reacted with residual crystalline phase (Al-15 mol % Cr) to produce an amorphous phase with a Cr content of 20 mol %. The former amorphous phase crystallized into the and phases, and the latter crystallized into an equilibrium phase by subsequent heat treatment.     

Electrical properties of silver paste prepared from nanoparticles and lead-free frit

Recently, PbO containing glass systems in commercial silver paste have been used due to their low glass transition temperature, good thermal and electrical properties. However, PbO is a hazardous material to both health and the environment. In this study, Pb-free silver paste was prepared by mixing commercial silver powder and silver nanoparticles. The commercial powder has an average particle size of 1.6 microm. The silver nanoparticles with particles size of 20-50 nm were synthesized by a chemical reduction method using surfactant. Pb-free frit was added into the mixed silver powder as the amounts of 3, 6 and 9 wt%. Using the obtained paste, thick films were fabricated by a screen printing on alumina substrate and the films were fired at temperature from 400 to 550 degrees C. The films had thickness of 6-11 microm and sheet resistivity of about 4-11 microomega cm.     

Sintering of magnesium oxide powder prepared by vapour-phase oxidation process — Relationship between particle size and mechanical properties of consolidated specimens

Relationship between powder properties and bending strengths of the sintered magnesium oxide (MgO) specimens was examined using seven kinds of MgO powders prepared by a vapour-phase oxidation process; the average primary particle sizes were 11, 25, 32, 44, 57, 107 and 261 nm. These compressed powders (specimens) were fired at 1600 or 1700 °C for 1 to 15 h. Although the densification behaviours of the specimens varied with the primary particle size of the starting powders, the relative densities of the specimens fired at 1700 °C for 5 h were all in the range of 97–98%. The relationships between bending strengths and grain sizes of these sintered specimens could be classified into two categories, according to the primary particle size of the starting powder: (i) at and below 32 nm and (ii) 44–261 nm. In range (i), the bending strengths of the sintered specimens were as low as 120 MPa; the grain size was reduced from 50.7 to 35.8 m as the primary particle size decreased from 32 to 11 nm. In range (ii), as the primary particle size increased from 44 to 261 nm, the bending strength of the sintered specimen was enhanced from 162 to 183 MPa, while the grain size was reduced from 28.3 to 13.7 m.     

316不锈钢滤板上钯银合金的脉冲电镀

研究了氨性溶液中在３１６不锈钢滤板上脉冲电沉积钯银合金时，电镀条件对镀层组成，表观形貌和金相结构的影响。镀层由钯银合金和少量游离银组成，合金相具有面心立方结构，择优取向为（１１１）面，镀层银含量随平均电流密度或导通时间的增大而减少，提高镀液中银的浓度有利于获得高银含量镀层。银含量小于２６ａｔ．％的镀层外观光亮。研究证明钯银合金和银的沉积速率受控于银氨络离子的传质过程。确定了钯银合金选择渗氢膜的电镀