Uncoated Powders Research Articles

The Microstructure and Mechanical Properties of SiCp-reinforced Copper Matrix Composites by Hot Pressing

SiCp-reinforced copper matrix composites with the reinforcement content of 30—50 vol.% were fabricated by hot pressing using Cu-coated and uncoated SiC powder. The microstructure and mechanical properties of the composites were studied. The results showed that with the increasing of SiCp volume fraction, the Brinell hardness of the composites increased first and arrived at a peak value, then decreased, while the bending strength decreased continuously. On the condition of same volume fraction of SiCp, the Brinell hardness and bending strength of the composites using coated powder were higher than that of the composites using uncoated powder. The annealing treatment lowered the Brinell hardness and bending strength of the composites. When the volume fraction of SiCp was 30 vol.%, the fracture mechanism of the composites showed a mixed manner of dimple and quasi-cleavage fracture. While the volume fraction of SiCp was 50 vol.%, the fracture behavior of the composites dominated in quasi-cleavage manner with seldom dimples.

Electrical Behavior of SiCp Reinforced Copper Matrix Composites by Hot Pressing

SiCp reinforced copper matrix composites with the reinforcement content of 30-50vol. % were fabricated by hot pressing using Cu-coated and uncoated SiC powder. And the microstructure and electrical conductivity of the composites were also studied. The results showed that with the increasing of SiCp particle size, the electrical conductivity of the composites also increased. And the oxides in the composites can decrease the electrical conductivity of the composites obviously. The electrical conducting property of the composites can be improved by the copper coating layer and suitable annealing treatment. It provided important data for the application of SiCp/Cu composites as electronic packaging materials.

STUDY OF ELECTRICAL CONDUCTIVITY FOR COPPER NANOPARTICLES WITH VAPOR-DEPOSITED SAMs

A self-assembled monolayer (SAM) of octanethiol was vapor-deposited onto the surface of copper ( Cu ) nanoparticles as a means of preventing oxidation. The presence of octanethiol on the surface of Cu nanoparticles was verified using Fourier transform infrared spectroscopy and transmission electron microscopy. The electrical resistance of copper nanoparticles with deposition of a 12-nm thickness of octanethiol on the surface was found to be 100 times greater than that of uncoated powders, indicating uniform SAM coating of the particles.

Fabrication of 8-YSZ thin-wall tubes by powder extrusion moulding for SOFC electrolytes

In this work the processing steps for producing YSZ thin tubes by means of powder extrusion moulding (PEM) technique are investigated. Different feedstocks were prepared from a commercial YSZ powder and a multicomponent thermoplastic binder system based in polypropylene and paraffin wax. The surface coating of YSZ powder with stearic acid in a high-performance dispersing instrument reduces the viscosity of the feedstock one order of magnitude respect to a feedstock with a same composition and un-coated powder. This fact allows increasing the solid loading up to 58 vol.% to obtain sintering tubes with densities higher than 97% and with wall thickness lower than 200 μm.

Optimization of the Processing of 8‐YSZ Powder by Powder Injection Molding for SOFC Electrolytes

The development of a new multicomponent thermoplastic binder system for injection molding of yttria‐stabilized zirconia (8‐YSZ) bars based in polypropylene and paraffin wax (PW) is presented in this paper. The surface coating of YSZ powder with stearic acid in a high‐performance dispersing instrument reduced the viscosity of the feedstock by one order of magnitude with respect to a feedstock with the same composition and uncoated powder. This fact allowed us to obtain a feedstock with a viscosity curve more suitable for the injection process without requiring high proportions of PW in the binder composition. The density of the samples was >98% and the conductivity of sintered parts are comparable to those of samples uniaxially compacted, around 0.10 S/cm at 900°C.

The Physicochemical and Electrochemical Properties of 100 and 500 nm Diameter Diamond Powders Coated with Boron-Doped Nanocrystalline Diamond

There is a need for advanced, corrosion-resistant electrocatalyst support materials for use in fuel cells. To this end, electrically conducting diamond powder was prepared by depositing a layer of boron-doped nanocrystalline diamond on 100 and diamond powders. The doped layer was deposited by microwave plasma-assisted chemical vapor deposition using an -rich source gas mixture. After coating, the doped diamond powder had a specific surface area of and an electrical conductivity of . The doped diamond powder had a specific surface area of and an electrical conductivity of after coating. The specific surface area of both powders decreased by ca. 50% after diamond coating due mainly to particle–particle fusion. The electrical measurements provided conclusive evidence for a doped diamond overlayer as the uncoated powders possessed no electrical conductivity. Furthermore, the fact that the electrical properties were unaltered by acid washing confirmed that the conductivity arises from the doped diamond overlayer and not any adventitious carbon impurity on the particle surface, which is removed by such chemical treatment. Scanning electron microscopy images and Raman spectroscopy yielded further evidence in support of a nanocrystalline diamond overlayer. Both powders exhibited electrochemical responses for , , and that were comparable to typical responses seen for high-quality, boron-doped nanocrystalline diamond thin-film electrodes. The electrochemical behavior of the powders was assessed using a pipette electrode that housed the packed powder with no binder. The doped diamond powder electrodes were more plagued by ohmic resistance effects than were the powder electrodes because of reduced particle contact. Importantly, it was found that the doped diamond powder electrodes are dimensionally stable and corrosion-resistant during anodic polarization at vs in at . In contrast, glassy carbon powder polarized under identical conditions underwent significant microstructural degradation and corrosion.

Surface modification of spherical NdFeB magnetic powders by a fluid-bed nickel electrodeposition

The spherical NdFeB magnetic powders were coated with Ni using a fluid-bed electrodeposition device. The oxidations of rich Nd phase or Fe phase were restrained after Ni coating. After annealing at 300 °C for 30 min, the magnetic properties of both coated and uncoated magnets were decreased. However, the Ni-coated powders showed better magnetic properties than the uncoated powders after the annealing. The compressive strength of bonded magnets improved after annealing due to enhancement of the adhesion between the adhesive and the surface of the magnetic powders.

An experimental investigation on the W–Cu composites

Fabrication of tungsten–copper net-shapes has become an important issue in recent years due to their unique properties which make them suitable for a wide variety of applications. In this investigation, W–Cu composite powders containing 20 wt.% and 30 wt.% Cu were processed by powder metallurgy technique using two types of prepared powders, namely, Cu-coated tungsten and mixtures of elemental powders. The coating method of tungsten powders was carried out using electroless coating technique. The investigated powders were cold compacted and sintered in vacuum at two sintering temperatures, 1250 °C and 1400 °C. The results show that the Cu-coated compacts have higher relative green densities than those of admixed ones for each copper content and compaction pressure, which means that the coated powders have greater compressibility than the uncoated ones. The relative green density increases with increasing compaction pressure and copper content. After sintering, the composites fabricated from uncoated powders show inhomogeneous structure due to formation of large globules of copper and tungsten agglomerates, while the structure of coated tungsten composites reveals the existence of copper within the tungsten grains, forming uniform interconnected structure. In general, composites made from Cu-coated tungsten powders exhibited higher density, hardness, compression strength, and electrical conductivity than those of composites made from admixed elemental powders. However, coefficient of thermal expansion and electrical resistivity of Cu-coated composites showed lower values.

Effect of composition on friction coefficient of Cu–graphite composites

Cu–graphite composites were prepared by hot isostatic pressing from the copper and graphite powders in the range of 0–50 vol.% of graphite. The same graphite powder was copper coated and used for the preparation of coated composites with 30 and 50 vol.% of graphite. It was confirmed that with increasing concentration of graphite the coefficient of friction and wear rate of coated and uncoated composites at first decreases. When critical concentration threshold of graphite is reached the coefficient of friction of composites becomes independent on the composition while the wear rate decreases further. This threshold is not simply 20 vol.% of graphite as stated [P.K. Rohatgi, S. Ray, Y. Liu, Tribological properties of metal matrix graphite particle composites, Int. Mater. Rev. 37 (1992) 129–149.] for metal matrix composites, but it significantly depends on the composite structure: for uncoated composites, it is 12 vol.% of graphite for fine graphite powder (16 μm), while for coarse powder (25–40 μm, [S.F. Moustafa, S.A. El-Badry, A.M. Sanad, B. Kieback, Friction and wear of copper–graphite composites made with Cu-coated and uncoated graphite powders, Wear 253 (2002) 699–710.]) it is 23 vol.% of graphite. For coated composites the concentration threshold was found above 25 vol.% of graphite. The reason is that the increased number of fine copper debris within graphite rich tribolayer occurs homogeneously also at high graphite composition.

Surface modification of stainless steel powders for microfabrication

Abstract The current trend towards miniaturization has led to the increasing use of stainless steel powders as raw material in the manufacture small parts with complex shapes, e.g. medical devices. This paper focuses on the role of coated and uncoated low carbon stainless steel powders, having d 50 = 7.3 μm, normal particle size distribution and a shape factor of 1 on additive processes used in micromanufacturing, e.g. micro powder injection moulding (μPIM). Surfaces were treated using a magnetron sputtering deposition apparatus equipped with a high frequency powder vibration and a continuous feed system. The results clearly show that the coated powders assume an “artichoke” morphology, which significantly increases the surface area. This combined with their nanocrystalline character leads to higher flowability than uncoated powders. However, no improvements have been observed concerning the critical powder volume concentration (CPVC) in feedstocks for μPIM processing. In consolidation processes, the debinding temperatures can attain values higher than 500 °C. After debinding and sintering, the coating of steel powders can show lower carbon contamination in solid solution than uncoated ones. This result is very important particularly for powder microtechnology of low carbon stainless steel.

Damaging and protective properties of inorganic components of sunscreens applied to cultured human skin cells

Titanium dioxide particles are used in sunscreens to reflect UV radiation and are chemically modified to inhibit their natural photocatalytic ability. Titanium dioxide extracted from eight randomly selected commercial sunscreens and three titanium dioxide powders obtained from their manufacturers were investigated and crystal form and modification type of each identified. Under 3.5 mW/cm 2 UVA illumination, the type of particle modification and crystal form determined whether the particles were inert or rapidly photodegraded an aqueous dispersion of methylene blue dye (MB), or whether the particles killed or protected cultured human skin cells. Mixed anatase and rutile crystal forms of titanium dioxide coated with organosilane photodegraded MB at a similar rate (11.5 min MB half-life) to Degussa P25, a standard uncoated titanium dioxide powder (11.0 min MB half-life) and generated a 2–4.9-fold increase in cell death. In contrast, pure rutile particles with an alumina coat or manganese doped protected cells from UVA irradiation. This research concludes that mixed anatase and rutile crystal forms of titanium dioxide coated with organosilane or dimethicone may not be appropriate to use in sunscreen lotions.

Fabrication, microstructure and properties of SiCp/Cu heat sink materials

Cu-coated powder was fabricated by electroless plating process, and the composition and morphology of coated powder were studied. Moreover, Cu-30, 40, 50 vol.% SiCp heat sink materials were fabricated by hot pressing using coated and uncoated powder. And the microstructure and thermophysical properties of the heat sink materials were also studied. The results show that SiCp particles distribute uniformly in heat sink materials and the interface between SiCp particles and Cu matrix is clear and well bonded. On the condition of same volume fraction of SiCp, the thermal conductivity of the material using coated powder is larger than that of the material using uncoated powder. Under experiment conditions, the thermal conductivity and coefficient of thermal expansion of Cu-30 vol.% SiCp heat sink material is 236.2 W.m −1.K −1 and 9.9 × 10 −6/K (30–200 °C) respectively. It provides important reference data for future experiments.

Development of copper coatings on ceramic powder by electroless technique

Electroless (EL) coating technique is one of the elegant ways of coating by controlling the temperature and pH of the coating bath in which there is no usage of electric current. EL nano-copper coating on ceramic particles of micron size is not reported. In this investigation, ceramic powders of ∼100 μm size have been coated with copper by EL technique in the pH and temperature ranges of 12–13.5 and 60–85 °C, respectively. The optimization of EL copper bath has been evaluated through the combination of process parameters like pH and temperature. The optimized value of pH is found to be 12.5 and temperature as 75 °C. The coated and uncoated powders have been subjected to microstructural studies by scanning electron microscope (SEM) and the phases present have been analyzed by X-ray diffraction. An attempt has been made to understand the bonding mechanism of coating. The adherence with the substrate is attributed to the chemical and mechanical bonding at the interface. A model has been suggested for the mechanical bonding effect at the interface.

Electroless coating of Permalloy powder and DC-resistivity of alloy composites

A high-resistivity coating of Permalloy (Ni–17.81Fe–1.97Mo wt.%) powders for soft magnetic composites, SMC, was developed and characterized. The coating was made using the electroless plating method employing an aqueous solution of sodium and nickel phosphates. The chemical process led to the formation of a layer up to 1 μm thick deposited on each particle surface and composed of a mixture of Ni(OH) 2, P 2O 5 and NaPO 3 compounds, as shown by X-ray photoelectron spectrometry and X-ray powder diffraction studies. The effect of coating on electrical DC resistivity was evaluated on SMC obtained mixing the metal, coated or uncoated, and thermoplastic polymer PEEK (polyetheretherketone) powders, pressing the mixture at 800 MPa and heating in air up to melting the polymer (380–410 °C). The SMC made with coated powders showed an increase in resistivity of more than two orders of magnitude compared to that obtained with uncoated powders when the polymer ranged between 0.5 and 1.5 wt.%. This difference prevails in comparison with sintered materials processed at the same pressure and sintering in hydrogen at 1350 °C. Thus, this process would restrict the eddy currents which affect the performance of soft magnetic products for AC applications.

Preparation and Characterization of Boron-Doped Diamond Powder: A Possible Dimensionally Stable Electrocatalyst Support Material

Electrically conducting diamond powder was prepared by coating insulating diamond powder ( diam, ) with a thin boron-doped layer using microwave plasma-assisted chemical vapor deposition. Deposition times from 1 to 6 h were evaluated. Scanning electron microscopy (SEM) revealed that the diamond powder particles become more faceted and more secondary growths form with increasing deposition time. Fusion of neighboring particles was also observed with increasing growth time. The first-order diamond phonon line appeared in the Raman spectrum at ca. for deposition times up to 4 h, and was downshifted to as low as for some particles after the 6-h growth. Electrical resistance measurements of the bulk powder (no binder) confirmed that a conductive diamond overlayer formed, as the conductivity increased from near zero (insulating, ) for the uncoated powder to after the 6-h growth. Ohmic behavior was seen in current-voltage curves recorded for the 4-h powder between . Cyclic voltammetric i-E curves for and were recorded to evaluate the electrochemical properties of the conductive powder when mixed with a polytetrafluoroethylene binder. At scan rates between 10 and , for both redox systems was high, ranging from 140 to 350 mV, consistent with significant ohmic resistance within the powder/binder electrode. Our results at this point suggest that the resistance is mainly due to poor particle-particle connectivity. Anodic polarization at 1.6 V vs for 1 h (25°C) was performed to evaluate the morphological and microstructural stability of the conductive diamond in comparison with graphite and glassy carbon (GC) powders. The total charge passed during polarization was largest for the GC powder and smallest for conductive diamond powder . SEM images taken of conductive diamond powder after polarization showed no evidence of microstructural degradation, while significant morphological and microstructural changes were seen for the GC powder.

Nanostructured modification of mineral particle surfaces in Ca(OH) 2–H 2O–CO 2 system

Traditionally pulverized particulates of raw minerals, such as ground calcium carbonate (GCC), wollastonite and dolomite, are usually of smooth/flat surface and pointed edges angles which have disadvantageous effects on the mechanical properties of composite polymer materials filled with the powders. On the other hand, dispersion of nanoscaled calcuim carbonate powders can not be easily achieved when filled in organic polymer, because of their huge specific surface energy and intense/strong conglomeration. Aiming at utilizing the advantages of both traditionally pulverized particles and nanosized ones and overcoming their disadvantages, a wet-coating method is developed and three kinds of raw-mineral-based composite powders have been successfully prepared by chemical precipitation in Ca(OH) 2–H 2O–CO 2 system on the pulverized particles in our group. It is shown that the composite powders of traditionally pulverized particles coated by nanosized calcium carbonate particles of 20–100 nm are of rough surface, blunt edges, larger specific surface area, compared with the uncoated powders. Some technological parameters are mainly analyzed for the coating process in the paper. Blending the coated powders to polypropylene demonstrates significant improvement in filling properties to the uncoated ones.

Synthesis, Characterization, and Processing of Cordierite-Glass Particles Modified by Coating with an Alumina Precursor

The surfaces of cordierite and glass particles were modified by coating them with an alumina precursor using a precipitation process in the presence of urea. Scanning electron microscopy (SEM), high-resolution transmission electron microscopy, X-ray diffraction, electrophoresis, and rheological measurements were used to characterize the coated powders. SEM and transmission electron microscopy morphologies of the coated powders revealed that amorphous and homogeneous coatings have been formed around the particles. The morphology of the coated powders showed a coiled wormlike surface. The coating Al2O3 layer dominated the surface properties of the coated glass and cordierite powders. The influence of the coating layer on the processing ability of cordierite-based glass-ceramics substrates by tape casting was studied in aqueous media. It could be concluded that the coating of the powders facilitates the processing and yields green and sintered tapes with denser, more homogeneous microstructures compared with the uncoated powders.

Electrical properties of conductive adhesives as affected by particle compositions, particle shapes, and oxidizing temperatures of copper powders in a polymer matrix

AbstractThe effects of particle compositions, particle shapes, and oxidation temperatures on the electrical properties of conductive adhesives have been investigated. Silver‐coated copper powders and uncoated copper powders with spherical and flake‐shaped particles are oxidized at temperatures such as 30, 175, 240, 300°C and 350°C for 2 h and dispersed in an epoxy matrix. The results of this study indicate that the electrical properties of the conductive adhesives are strongly affected by the particle compositions and oxidation temperatures and only slightly affected by the particle shapes. Silver‐coated copper powders show significantly greater oxidation resistance than uncoated copper powders. To understand how silver‐coated copper powders show such oxidation resistance, they are analyzed by the techniques of thermogravimetric analysis (TGA), X‐ray diffraction (XRD), and Auger spectroscopy to observe how metal oxides such as AgO, Cu2O, and CuO affect the electrical properties of the conductive adhesives. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 2045–2053, 2004

Improvement of the Thermal Stability of LiNi0.8Co0.2 O 2 Cathode by a SiOx Protective Coating

The surface of particles was protected by a thin layer of deposited via chemical vapor deposition (CVD). The aim was to enable lithium ions to intercalate into the cathode material through the coating layer while preventing the surface of the cathode to be directly in contact with the electrolyte media. The structure of is only slightly modified by the CVD process. The film deposited around the cathode particles was characterized. It consists of an amorphous layer of understoichiometric silicon dioxide which is 3-5 nm thick. The initial charge capacity of the starting material is not affected by the protective coating; nevertheless, the capacity fade is slightly increased upon cycling compared to the uncoated powder. The CVD process is very reproducible, and despite a small degradation of the electrochemical performance, the coating is efficient to decrease by 50% the exothermic reaction occurring at around 210°C in the charged state. © 2004 The Electrochemical Society. All rights reserved.

The electroless deposition of nickel on SiC particles for aluminum matrix composites

Nickel coatings have been deposited by an electroless method on SiC powder particles. Three different SiC powder grades, in terms of average particle size, were chosen, i.e. 8, 14 and 70 μm. The coating process was performed in few steps consisting of the SiC powder cleaning by acetone, its sensitization by HCl aqueous solution containing Sn 2+, followed by its activation by HCl aqueous solution containing Pd 2+, and finally—hydrometallurgical nickel deposition using aqueous solution containing Ni 2+, as a nickel carrier. The influence of the deposition conditions on the coating layer properties was studied. The thickness, morphology and microstructure of the layers were controlled by the growth conditions. SEM and digital image analyzing techniques were used for those purposes. Since the Ni-coated SiC powders were produced as reinforcement for Al matrix composites, their compatibility as compared with the uncoated SiC powders was also controlled by metallography.