C. P. de Souza

Synthesis of tungsten carbide through gas–solid reaction at low temperatures

Abstract Conventionally, WC powders are produced heating a mixture of W powder and carbon black. This method is based on solid state reaction controlled by diffusion and phase transformations. Thus high temperatures (>1400°C) and long times are necessary. This paper presents a method able to synthesize WC at low temperatures (∼850°C) in 2 h. It is based on a gas–solid reaction between a W source and a C-containing gas. The gas phase is a mixture of H2 and CH4. Ammonium paratungstate and tungsten blue oxide are used as the tungsten source. The higher reactivity of the precursor material and the better contact between the gas–solid reactants explain the lower temperatures and higher reaction rates of this method compared to the conventional one. Very fine WC crystallites (

A low temperature synthesized NbC as grain growth inhibitor for WC–Co composites

Abstract Niobium carbide can be used to inhibit WC grain growth in hardmetal. The performance of a NbC powder produced at low temperature by solid–gas reaction (an experimental powder) as WC grain growth inhibitor is compared with that of a commercial NbC powder. It is verified that NbC effectively inhibits heterogeneous WC coarsening. This results in an increase in hardness. The commercial and experimental NbC powders exhibit a comparable performance in inhibiting the WC grain coarsening, in spite of a significant difference in particle size and shape. The commercial NbC powder is very fine while the experimental one is coarse and porous, but its crystallites are finer than those of the commercial product. The milling procedure used to prepare the alloys is able to reduce the particle size of the experimental NbC, and thus guarantee a dispersion of the particles with a quality level comparable to that found for the alloy prepared with the commercial NbC.

Synthesis of niobium carbide at low temperature and its use in hardmetal

Abstract A new method of synthesizing niobium monocarbide at a low temperature (950 °C) and in a short time (2 h) is described. Conventionally, niobium monocarbide (NbC) can only be produced at 1600–1800 °C for longer periods. The method consists of the carburization of a niobium complex by a gaseous (H2+CH4) mixture. The method of preparation of the precursor and its carburization are presented. Precursor and carbide are characterized by XRD, SEM, TG/DTG and granulometry. Shape, mean size and size distribution of the NbC particles are similar to those of the niobium precursor. Reactions during carburization do not alter these characteristics. NbC particles are very large and porous. NbC powder is used in a hardmetal alloy as a WC grain growth inhibitor. Its performance is compared to that of a commercial NbC powder. NbC synthesized by the new technique exhibited a slightly higher efficiency in inhibiting the WC coarsening, evidenced by the SEM of the hardmetal structures and hardness measurements.

Síntese do cerato de bário pelo método de complexação combinando EDTA-citrato e avaliação catalítica na oxidação do monóxido de carbono testada em reator de leito fixo

The barium cerate perovskite was synthesized using a methodology based on the EDTA-citrate complexing method and its catalytic properties were tested in fixed-bed reactor at different stream feeding gas flow rates (50 and 100 mL.min-1) and temperatures (100 to 550 °C). The intended crystalline phase was obtained with crystallite size of 133 nm, together with a small amount of unwanted BaCO3 phase (3%). When synthesized by the EDTA-citrate complexing method, the barium cerate presents an irregular spherical shape with clusters of different sizes, and the presence of pores of different size distributed randomly; its specific surface area was 2.61 m2.g-1. At flow rate of 50 mL.min-1, higher conversion then at 100 mL.min-1 was observed, becoming more apparent between 300 and 400 °C. Moreover, at 50 mL.min-1 flow rate total conversion of CO to CO2 occurred at 400 °C, while the conversion was close to 60% at 100 mL.min-1. Using the perovskite as catalyst the oxidation process is completed at 400 °C, while in reaction without catalyst the complete conversion is achieved only at 550 °C. Therefore, the BaCeO3 had strong activity in carbon monoxide oxidation, although its specific area was small.

Synthesis and characterization of Mo2 C based materials over activated carbon derived from sewage sludge pyrolysis for catalytic applications

Transition metal carbides have been successfully used as substitute materials for conventional noble metal catalyst in several important industrial reactions due to their interesting physicochemical properties. Surface structure, chemical composition and metal-support interactions, as well as processing conditions, are of utmost importance in the use of such materials in catalysis. The present study aimed to synthesize and evaluate pure molybdenum carbide with and without support, and bimetallic Mo-Ni carbide over a carbon active support derived from sewage sludge pyrolysis. The support was chemically (KOH) and physically (thermal treatment) activated before use. TG/DTG, XRD, XRF, SEM, BET and particle size evaluation were performed, together with adsorption/desorption isotherms. Results indicated that the applied synthesis method was adequate for the obtainment of pure materials. The increase in surface area of the support was significant, from 13 to 141 m2.g-1 after the thermal and chemical treatment; also, supporting Mo2C over carbon provided an increase from 45 to 73 m2.g -1 in surface area, which indicated its potential as a catalytic material as well as the effectiveness of the applied methodology.

Élaboration et carac′erisation de céramiques de type PHT substituées au tantale Pb1−x/2□x/2(HfyTi1−y)1−xTaxO3 (PTHT)

Abstract Preparation and characterization of tantalum-substituted PHT ceramics Pb 1−x/2 □ x/2 (Hf y Ti 1−y ) 1−x Ta x O 3 (PTHT) . This paper deals with the preparation of five oxalic precursors containing lead, hafnium, titanium and tantalum with Pb 1−x/2 ) [(NH 4 ) 2(1−x)(1−y)+3x H 2y(1−x) ] (2x/(2+x)) [(Hf y Ti (1−y) ) 1−x Ta x O(C 2 O 4 ) (x+2) ], dH 2 O general formula [for each precursor y = 0.52 but x takes different values (0.01 ; 0.025 ; 0.05 ; 0.075 ; 0.1)]. The pyrolysis of the complex was performed with a slow heating rate. The thermolysis led to the oxides which were characterized from the microstructural point of view (XRD) and grain size distribution. After the sintering, the electrical properties of the ceramic were studied using Complex Impedance Spectroscopy.