Al–O–N and Al–O–B–N thin films applied on Si–O–C fibers

Protective coatings (Al–O–N and Al–O–B–N) on Si–O–C fibers (Tyranno ZMI) were applied in order to enhance oxidation resistance under severe thermo-mechanical conditions in the 400–600 °C temperature range. The coating process consisted in three steps: (i) the transformation of the Si–O–C fiber surface into microporous carbon; (ii) the impregnation of these carbon microporous layers by an aluminium trichloride (AlCl₃) solution and then, (iii) a final heat treatment under ammonia. Processing parameters were studied in order to select the best conditions. Using these conditions, obtained results have shown that coatings were present around each fiber, with a controlled thickness, and that the mechanical properties of the fibers were preserved. Although, these coatings did not entirely stop the oxygen ingress, it has been shown that they strongly reduced the oxidation of the fiber.     

Role of precursor chemistry on synthesis of Si–O–C and Si–O–C–N ceramics by polymer pyrolysis

In this study, the role played by polymer precursor chemistry on the nature of the pyrolyzed product was examined. Variables examined included extent of conjugation of the precursor, differences between a homopolymer and co-polymer, linear and cyclic structures, siloxanes and silazanes and between vinyl and phenyl containing silanes. Results indicate increasing the vinyl content of the precursor increases the amount of free carbon in the pyrolyzed product. The introduction of an aryl group in the silane decreased yield and lastly there was no change in the yield when a silazane was used instead of a silane, however, the composition of the silazane precursor ceramic is rich in nitrogen while a silane precursor ceramic is rich in carbon.     

Evolution of crystallization and its effects on properties during pyrolysis of Si–Al–C–(O) precursor fibers

The high-temperature resistant Si–Al–C–(O) fibers were prepared through polymer-derived method using continuous polyaluminocarbosilane (PACS) fibers. Evolutions of the crystallization during the pyrolysis of the Si–Al–C–(O) precursor fibers were investigated by a series analysis. The structure of the fibers transforms from organic state to inorganic state and the crystalline phases appear during the pyrolysis. The β-SiC crystallite size increases when the temperature is higher than 1,300 °C. At the same time, the α-SiC appears. At 1,600 and 1,800 °C, the grain size of β-SiC of the fibers is 15.4 and 22.1 nm, respectively. The growth of β-SiC and the appearing of α-SiC have a great influence on the properties of the fibers. The change of the tensile strength of the pyrolysis products is divided into three stages with the growth of the crystal. The tensile strength of the Si-Al-C fibers is higher than 1.9 GPa.     

Effects of copper addition on the in-situ synthesis of SiC particles in Al–Si–C–Cu system

In this research work, SiC particles have been successfully in-situ synthesized in Al–Si–Cu matrix alloy utilizing a novel liquid–solid reaction method. The effect of copper addition on the synthesis of SiC in Al–Si–C–Cu system was investigated. The composites mainly contain spherical SiC particles and θ-Al₂Cu eutectic phases, which are embedded in the α-Al matrix. Results indicated that the temperature for forming in-situ SiC particles significantly reduced from 750 °C to 700 °C with the copper addition. The size of in-situ synthesized SiC particles can be as low as 0.2 μm. Further study found that the addition of 10 wt.% copper into Al–Si–C alloy causes its solidus temperature to decrease by about 65 °C. Additionally, the Rockwell hardness value of SiCp/Al–18Si–5Cu composites has an average of 92, which is 50% higher than that of the sample without copper addition.     

Synthesis of hexagonal plate-like Al4Si2C5 and the effect of Al4Si2C5 addition to Al2O3–C refractory

Hexagonal plate-like Al₄Si₂C₅ particles were synthesized for the first time via a carbothermal reduction process with controlled heating temperature and raw materials ratio, and their oxidation behavior was investigated. Al₄O₄C, Al₂OC, SiC and Al₄SiC₄ formed as intermediate products when the batch mixture was heated in argon atmosphere, and Al₄Si₂C₅ then formed at above 1800 °C. Possible reaction mechanisms responsible for the formation of this ternary carbide were discussed based on the reactions at the initial and later stages of the carbothermal reduction process. Al₄Si₂C₅ added to the Al₂O₃–C refractory initially reacts with CO to form Al₂O₃, SiO₂ and C. After the reaction, Al₂O₃ react with SiO₂ to form mullite on the surfaces of the refractories, which inhibit the oxidation of the refractories.     

Calculations of electrical resistivity for Al–Cu and Al–Mg–Si alloys

Abstract

A system for thermodynamical calculations (Thermo-Calc) was used to derive the solid solubility of the alloying elements in commercial Al–Cu and Al–Mg–Si alloys. The electrical resistivity was then calculated using a model developed by the authors based on the Matthiessen's rule. The calculated resistivity agreed with the observed resistivity within ±2.5 nΩ m for the Al–Mg–Si alloys and ±2 nΩ m for the Al–Cu alloys, except for Al–Mg–Si alloys containing boron or chromium and Al–Cu alloys with special compositions.     

Sintering, densification and crystallization of Ca–Al–B–Si–O glass/Al2O3 composites for LTCC application

Ca–Al–B–Si–O glass/Al₂O₃ composites were prepared based on the borosilicate glass powders (D₅₀ = 2.84) and Al₂O₃ ceramic powders (D₅₀ = 3.26), and the sintering, densification, crystallization of samples were investigated. The shrinkage of sample starts to have a sharp increase at 600 °C. The shrinkage of sample starts to have a further rapid increase after the glass softening temperature of about 713 °C. Glass/Al₂O₃ composites can be sintered at 875 °C/15 min and exhibit better properties of a relative density of 98.4 %, a λ value of 2.89 W/mK, a ε _r value of 7.82 and a tan δ value of 5.3 × 10^?4. The interface between glass and Al₂O₃ grains and the interface between anorthite and glass phase depicts a good compatibility according to transmission electron microcopy test. It is the low sintering temperature, high density and good compatibility with Ag electrodes that, guarantee borosilicate glass/Al₂O₃ composites suitable for low temperature co-fired ceramic materials.     

Synthesis and phases evolution of Si–C–Ti from polycarbosilane (PCS) and metal Ti

Si–C–Ti ceramics were synthesized by reactive pyrolysis of polycarbosilane (PCS) precursor filled with metal Ti powder. Pyrolysis of mixture with atomic ratio of Ti:Si through 3:1–3:2 was carried out in argon atmosphere at given temperature up to 1500 °C. The metal–precursor reactions, and phase evolution were studied using X-ray diffraction and scanning electron microscopy with EDX. The Ti₃SiC₂ phase was obtained firstly from reaction of PCS and Ti. Ti₃SiC₂ formation starts at 1300 °C and its amount increases significantly in a narrow temperature range between 1400 °C and 1500 °C. In addition, addition of CaF₂ can promote the formation of Ti₃SiC₂ phase.     

Facile synthesis of heterostructure NiO–SnO2 nanocomposite for selective electrochemical determination of l-cysteine

Journal of Materials Science: Materials in Electronics - In this present research, heterostructure NiO–SnO2 nanocomposite modified electrode was developed to determine l-cysteine molecule....     

Effect of cooling rate and Mg content on the Al–Si eutectic for Al–Si–Cu–Mg alloys

This study investigates and clarifies the qualitative and quantitative effects of Mg content and cooling rate (ranging from 0.5 to 4 °C/s), on the modification of the silicon eutectic structure and on the undercooling of the silicon eutectic growth temperature (ΔT_Si-eut) in the series of Al–Si–Cu–Mg alloys. The critical Mg content to produce a notable improvement in the silicon eutectic by 1.5 modification levels (regardless of the cooling rate) is 0.6 wt.% Mg. A similar increase in the modification level was also observed when the cooling rate was increased to a maximum of 4 °C/s, regardless of the Mg content. Measurements of the area and roundness of the silicon particles showed a good correlation with the modification level. The undercooling (ΔT_Si-eut) increased by up to ~ 23 °C at a relatively high Mg content and cooling rate and up to ~ 14 °C when the Mg content was increased from 0.4 to 0.6 wt.%.     

High-temperature pyrolysis of ceramic fibers derived from polycarbosilane–polymethylhydrosiloxane polymer blends with porous structures

The polymer blends of PCS (polycarbosilane) and PMHS-h (polymethylohydrosiloxane with high molecular weight) were prepared by freeze-drying process of mixed benzene solution. Melt viscosity, mass loss, and gas evolution from prepared polymer blends were analyzed. A polymer blend of HSah15 (15 mass% PMHS-h to PCS) was melt-spun to fiber form, curing by thermal oxidation and pyrolyzed at various temperatures up to 1773 K. The obtained fibers were investigated by tensile tests, FE-SEM (field emission scanning electron microscope) observation, and XRD (X-ray diffraction) analysis. After pyrolysis at 1273 K, there were no pores in the cross section of the fiber derived from pure PCS; however, there were amounts of pores in the cross sections of the fiber derived from HSah15. After pyrolysis at 1773 K, the coarse β-SiC (silicon carbide) crystals were formed on the outside surface of the fiber derived from pure PCS; however, no remarkable β-SiC crystal were formed on the outside surface of the fiber derived from HSah15.     

Low temperature TLP bonding of Al2O3–ceramics using eutectic Au–(Ge, Si) alloys

In this study, Al₂O₃–ceramics were joined via TLP bonding using interlayers of eutectic Au–12Ge (wt%) and Au–3Si (wt%) solder alloys, respectively, with a melting temperature of 361 and 363 °C and Ni wetting layers. The Ni layers were part of a metallic multilayer coating (Ti/W/Ni) applied on the ceramic surface to ensure wetting and adhesion during the joining process. For comparison, a soldering process was performed as well by changing the multilayer structure to Ti/W/Au. With respect to the different joining processes the influence of the variation in wetting layers on the interface reactions, mechanical properties and re-melting temperature was analyzed by scanning electron microscopy, shear testing, and differential scanning calorimetry. It is shown that sound joint can be produced at a joining temperature of 400 °C, achieving reasonable shear strength and re-melting temperatures more than 550 °C above the initial melting temperature of the filler metal.     

Fabrication of porous glasses by water vapor corrosion of Y–Al–Si–O–N glass

The effects of exposure conditions on the microstructural changes of the oxynitride Y–Al–Si–O–N glass system were investigated. The oxynitride glass was exposed to dry O₂ gas, or water vapor containing either O₂ or Ar, at temperatures between 1133 and 1183 K. A porous scale layer was formed by exposure to water vapor, while a non-porous deteriorated scale was obtained only by exposure to dry O₂. Formation of the porous layer was promoted by the presence of oxygen in the water vapor. The corrosion rate of the oxynitride glass in humidified O₂ near the glass transition temperature followed a linear rate law. The morphology of the porous layer was strongly dependent on the exposure temperature, which may be due to the significant decrease in the viscosity of the glass with increasing temperature. The permeability of the porous layer cut from the exposed glass behaved according to Darcy’s law; therefore, this layer was considered to be composed of three-dimensional continuous pores. The microstructure of the porous layer could be controlled by the exposure temperature, so that a graded porous glass with different morphological characteristics within the layer could be obtained by exposure to humidified gases at different temperatures.     

Investigation of joining Al–C–Ti cermets and Ti6Al4V by combustion synthesis

The paper has addressed a route for the welding of titanium alloy (Ti6Al4V) and Al–C–Ti powders by the combustion synthesis (CS) method. Al–C–Ti powders were compressed in the titanium alloy pipes with relative densities of 65%, and then the powder compact was sintered by two reaction mode at the same time as the annulus of titanium alloy and the synthesized product were joined. The paper has studied the effects of reaction mode and Al content in starting powders on the structure and property of the welded joints. And it has also discussed the microstructure of welded joints by laser-induced combustion synthesis (LCS). The mechanical properties of the welding seam have been also tested. The results show that LCS welding has realized fusion welding and the welding seam has good mechanical properties. Furthermore, SEM analysis has indicated that nano-size grains of TiC were formed in the joint layer.     

Semisolid melt squeezing procedure for production of open-cell Al–Si foams

Melt squeezing process in semisolid state was used for the first time for production of open-cell Al–Si foams with improved microstructural and mechanical characteristics. First a given amount of preheated NaCl particles was stirred into molten A356 alloy. Stirring continued during solidification of the slurry until reaching a given solid fraction of primary particles. The resulting mixture was pressed by a perforated piston to squeeze a controlled amount of the residual meltout. Open-cell foams were achieved by leaching the final Al–Si–NaCl composites in water. The suitable values of the NaCl particle size, pressurizing temperature, melt to salt ratio and piston pressure for production of uniform and high integrity foams were obtained to be 3400 μm, 605 °C, 1.5:1 and 10 MPa, respectively. The results showed that the semisolid processing employed could refine and modify the microstructure of the cell ligaments in the final foams. Mechanical properties of the foams such as energy absorption and fracture toughness were also improved by the semisolid processing.     

Modification of Al–Si alloys by metallothermic reduction using submerged SrO powders injection

Aluminum–silicon (Al–Si) casting alloys are the most frequently used alloys in the automotive industry. The modification of its microstructure is an important practice for refining the large needles of the eutectic silicon into fine fibrous or lamellar particles. In this study, the chemical and microstructural analyses of the samples taken during the submerged SrO powders injection tests in magnesium (Mg)-containing molten Al–Si alloys demonstrated the feasibility of Sr uptake in the bath, as SrO is reduced through metallothermic reactions in the molten alloys, producing a suitable silicon eutectic modification effect. The parameters that were evaluated during the experiments included temperature of the molten bath, size of SrO powders, initial Mg concentration in the molten alloy, and injection time. Based on the characterization of the reaction products of partially reacted SrO particles using the scanning electron microscope (SEM) as well as the X-ray diffraction (XRD) analysis of the samples of dross obtained during the injection tests, the sequence of the reaction was proposed. It was observed that the initial Mg content is a key parameter in accelerating the reaction rate through chemical reactions that produce MgAl₂O₄ as the main reaction product.     

Microstructure and Mechanical Properties of Rapidly Solidified Hypereutectic Al–Si and Al–Si–Fe Alloys

The microstructure and mechanical properties of rapidly solidified Al–18 wt% Si and Al–18 wt% Si–5 wt% Fe alloys were investigated by a combination of optical microscopy, scanning electron microscopy, transmission electron microscopy, x-ray diffraction, tensile testing, and wear testing. The centrifugally atomized binary alloy powder consisted of the -Al (slightly supersaturated with Si) and Si phases and the ternary alloy powder consisted of the -Al (slightly supersaturated with Si), silicon, and needle-like metastable Al–Fe–Si intermetallic phases. During extrusion the metastable -Al₄FeSi₂ phase in the as-solidified ternary alloy transformed to the equilibrium -Al₅FeSi phase. The tensile strength of both the binary and the ternary alloys decreased with a high-temperature exposure, but a significant fraction of the strength was retained up to 573 K. The specific wear gradually increased with increasing sliding speed but decreased with the addition of 5 wt% Fe to the Al–18 wt% Si alloy. The wear resistance improved with annealing due to coarsening of the silicon particles.     

Investigation of Fe2O3–Al and Cr2O3–Al reactions using a high speed video camera

Abstract

Self propagating high temperature synthesis is a simple, fast and energy efficient process with a wide range of applications, one of which is the coating of the internal surfaces of steel pipes using a centrifugal thermit process. The process involves a highly exothermic reaction between powder reactants distributed around a steel tube rotating at high speed. Although the process has been widely studied, important features, particularly how the reaction propagates, have not been completely revealed due to extremely high reaction rates and temperatures. In the present work, Fe₂O₃–Al and, to a lesser degree, Cr₂O₃–Al reactions were studied under stationary (non-rotating) and rotating conditions using a high speed video camera by which the centrifugal thermit process was, for the first time, recorded optically. Video recordings clearly demonstrate that, in contradiction to current belief, the reaction does not always propagate in a well ordered (spiral) pattern, but involves multiple, randomly distributed ignition sites.     

Dry sliding wear of eutectic Al–Si

The wear of as-cast eutectic Al–Si was studied using pin-on-disk tribotests in two different environments, air and dry argon. The counterface in all tests was yttria-stabilized zirconia. It was found that wear of the Al–Si was reduced by about 60% by the removal of oxygen from the test environment. The zirconia counterfaces showed measurable wear after tests performed in air, while there was very little wear of the zirconia for tests conducted under argon. The near-surface regions of the Al–Si pins were examined using a transmission electron microscope (TEM), using specimens produced by focussed ion beam milling. The specimens that had been worn in air were characterized by a near-surface mechanically mixed layer containing a considerable amount of both aluminum oxide and zirconium oxide—the aluminum oxide particles had evidently acted as abrasive agents to remove material from the zirconia counterface. In contrast, TEM analysis of the Al–Si tested in argon showed little zirconium oxide in the near-surface regions.