Papers by Keyword: Precursors

Abstract: This paper compares ethene and methane precursors for homoepitaxial 4H-SiC growth in planetary reactors with regards to their impact on growth rate and defectivity of the epilayers. Therefore, a comprehensive experimental study has been performed in AIXTRON G10-SiC and G5WW C planetary reactors using a standard process based on ethene and trichlorosilane precursors with conventional 150 mm n-type 4H-SiC substrates from 3 different international suppliers. Methane substituted ethene as precursor in many experiments. It was found that methane precursor can compete with ethene in terms of growth rate, epilayer thickness, and defectivity of the epilayers. By using isotopically enriched methane, Si¹²C epilayers with a ¹²C concentration of 99.96 % have been grown which can be used for SiC-based quantum technology.

Abstract: We present the manufacturing process, by melting in a controlled environment, of specific alloys that have a benefic effect on the diffusion phenomena’s in the base material of the addition coated rods for brazing. The friable precursors obtained by melting are grinded with the purpose of integrating them in the coat of the rods, namely the addition material. The metallurgic activation activity of the precursors is enhanced by using them as nano-powders in the coat of the rods.Of major importance for deep joint capillary brazing are the fluidity and superficial tension of the melt - addition material, or, in case of non-ferrous alloys, the degree of de-oxidation of base materials. In order to improve the above mentioned characteristics, we made precursors type: 5o% Cu-45% Sn-5% P (CIF); 92% Cu-2% Si 6% P- (RAV). The batch design was done by two distinct melting processes, in high frequency currents (CIF) and electric arc (RAV) in a controlled environment. The melts were cast in graphite forms as loose briquettes. The briquettes thus obtained were analyzed structurally and sclerometrically, and after testing, they were grinded in planetary mills until we obtained nano-powders. The resulting powders will be used, as required, to prepare brazing sheaths.

Abstract: The paper presents the current state of researches carried out in order to improve homogeneity of mixtures for metallic powders; our priority is those with different participation in the mixture, by mechanical alloying. Mixtures of this type are used in the production of welding and related materials.Recently, research has been done to improve deposited metal properties by welding with coated electrodes, by depositing nanostructures, with uncertain results due to the low degree of homogeneity of blends involved in the electrode coating.The performance enhancement method for coated rods, developed by introducing in their coat activating precursors, used for brazing, is well known. The solution to improve the degree of homogeneity of alloying systems by mechanical alloying has been successfully applied for the manufacture of coated electrodes and those tubular with a composite core which are deposited by manual welding or TIG welding, type Fe25% Cr-4% W-V-Ti-La. The grouping of powder components for mechanical alloying was done in such a way that, finally, we have groups with the granulation, respectively specific weight, close and a low potential of segregation.The method of mechanical alloying of the components was used in the manufacture of coated rods in order to improve the fluidity of silver-rich buffer layers deposited in order to favor the diffusion phenomena of brazing alloys in the base materials.Testing the homogeneity of the alloying systems was performed indirectly by metallographic analysis and sclerometric tests.

Abstract: A structural-functional integrative wave-transparent material, consisted of BN coating and SiBN substrate, is synthesized through precursor infiltration pyrolysis (PIP) process. Then the technical parameters and procedures are obtained according to the key properties of the precursors. Studies on the microstructures and properties of the composite show that compact substrate and weak interface enhance the strength, and porous structure improves the dielectric performance of the composite. The composite exhibites good bending strength (~95.12 MPa), tensile strength (~34.95 MPa) and compressive strength (~80.92 MPa). Moreover the composite displays a low dielectric constant (4.17) and a low loss tangent (9.7×10^-3) at room temperature. The composite has a low changing rate of dielectric constant (~2.01% per 100°C). These results imply that the composite with excellent mechanical and dielectric properties can be an excellent wave-transparent material applied in the fields of aerospace.

Abstract: The special features of nanoparticles formation by laser ablation of volume metallic zinc targets in various solvents using sulfur precursors are studied. Semiconductor cadmium sulphide nanoparticles with average sizes 10–15 nm are synthesized by zinc ablation in the presence of hydrogen sulfide and thioacetamide, and their composition, structure, and optical properties are investigated. It is established that with the use of H₂S, only CdS particles are formed in the medium.

Abstract: Calcium phosphates biocements are biomaterials that present crystallographic and mineralogical characteristics similar to human skeletal structure. This has led to the development of new calcium phosphates biomaterials for biomedical applications, especially biomaterials for repairing defects and bone reconstruction. Calcium phosphates biocements are a promising alternative in biomedical applications, for they are easy to mold, they have good wettability, hydration and hardening capacity during its application in biological means. This work aimed at the synthesis of hydrated calcium phosphates powder, through a simple reactive method, which will be the basis for the production of calcium phosphate biocimentos with self-setting reaction. Three calcium phosphates compositions were produced via CaCO₃/phosphoric acid reactive method in the ratios Ca/P = 1,5; 1,6 e 1,67 molar. The presented results are associated to hydrated powder morphology and synthesis process control. Scanning Electron Microscopy (SEM) helped with the morphological characterization of the powders, the laser analysis method was used for determining particle size and the Fourier Transformed Infrared Spectroscopy (FTIR) gave support to the identification of H₂O e PO₄^3- grouping vibrational bands. The work showed that for the different powder compositions the hydrated calcium phosphate phase is formed by clustered fine particles. This demonstrated that the chosen synthesis method permits the obtention of hydrated calcium phosphates, precursors for later biocement production.

Abstract: Soot, a crucial component of PM_2.5 in the ambient air, is a prime product of incomplete combustion of fule. The present paper uses a series of models, including nucleation of soot precursors, surface growth of soot particles as well as particles coagulation, to do numerical simulation of soot particles formation mechanism over time in combustion flame, with the aid of MATLAB platform by C language. Simulation results indicate that the concentration distribution of soot precursors like polycyclic aromatic hydrocarbons (PAHs) have direct effect on the concentration distribution of soot nuclei. Mass addition rate on soot particles surface rises over time since the enlargement of the surface area. In coagulation process, the number of small diameter particles decreases, while that of large diameter particles increases.

Abstract: The instability failure of coal and rock mass occur frequently in deep mining. However, this destruction occurs with abnormal intensity and suddenness, with hardly any obvious macro precursors. The ability to extract this precursory information and make certain the relationship among the parameters is therefore important. During the process of instability and failure of coal samples, the evolution characteristics of deformation field were monitored by digital image correlation system. Base on the researches, the appearance of the deformation localization band could be divided into three time periods for the samples which all had splitting failure. During period I, the relative sliding component increased rapidly. During period II, the relative tension component increased rapidly. During period III, both the relative tension and relative sliding components increased rapidly.