Thermal and spectroscopic data to investigate the oxamic acid, sodium oxamate and its compounds with some bivalent transition metal ions

Synthesis, characterization, and thermal behavior of transition metal oxamates, M(NH(2)C(2)O(3))(2)center dot nH(2)O (M = Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II)), as well as the thermal behavior of oxamic acid and its sodium salt (NaNH(2)C(2)O(3)) were investigated employing simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC), experimental and theoretical infrared spectroscopy, TG-DSC coupled to FTIR, elemental analysis and complexometry. The results led to information about the composition, dehydration, thermal stability, thermal decomposition, as well as of the gaseous products evolved during the thermal decomposition of these compounds in dynamic air and N(2) atmospheres.

Lubricating coating prepared by PIIID on a forming tool

In this work, the performance of a-C: H films produced by the hybrid Plasma Immersion Ion Implantation and Deposition technique as lubricating layers for a steel forming tool has been investigated. Hardened steel (AISI M2, 64 HRC) plates coated with a commercial TiN layer were used as substrates and the films were deposited in a vacuum chamber fitted with two parallel-plate electrodes. The discharges were generated in atmospheres composed of 91% C2H2 and 9% Ar by the application of radiofrequency power (13.56 MHz, 100 W) to the upper electrode while the lower one, also used as the sample holder, was biased with high voltage negative pulses (3.6 kV, 30 mu s, 300 Hz). A deposition time of 840 s was used. The effects of the gas pressure, p, on thickness, molecular structure, wettability, surface morphology and topography, hardness and friction coefficient of the films lwere investigated. Film thickness increased from 0.3 to 0.5 mu m when p was increased from 2.7 to 16.5 Pa. Generally, the films were slightly hydrophilic, with contact angles of around 84 degrees, and the deposition decreased the roughness of the steel. A polymer-like structure was detected in high pressure depositions and an amorphous carbon structure derived from the low pressure procedures. Hardness decreased from 8.2 to 7.0 GPa with increasing p. Improvement in tribological performance was indicated by the fall in the friction coefficient from 0.5 to 0.2 as the deposition pressure was reduced. Operating at the latter value (of mu) would lead to a significant reduction in wear and hence to significant economy in diverse industrial applications.

Pore size evolution during sintering of ceramic oxides

This paper reviews the influence of particle size distribution, agglomerates, rearrangement, sintering atmospheres and impurities on the pore evolution of some commonly studied oxides. These factors largely affect sintering mechanisms due to modifications of diffusion coefficients or evaporation-condensation. Very broad particle size distribution leads to grain growth and agglomerates densify first. Rearrangement of particles due to neck asymmetry mainly in the early stage of sintering is responsible for a high rate of densification in the first minutes of sintering by collapse of large pores. Sintering atmospheres play an important role in both densification and pore evolution. The chemical interaction of water molecules with several oxides like MgO, ZnO and SnO2 largely affects surface diffusion. As a consequence, there is an increase in the rates of pore growth and densification for MgO and ZnO and in the rate of pore growth for SnO2. Carbon dioxide does not affect the rate of sintering of MgO but greatly affects both rates of pore growth and densification of ZnO. Oxygen concentration in the atmosphere can especially affect semiconductor oxides but significantly affects the rate of pore growth of SnO2. Impurities like chlorine ions increase the rate of pore growth in MgO due to evaporation of HCl and Mg(OH)Cl, increasing the rate of densification and particle cuboidization. CuO promotes densification in SnO2, and is more effective in dry air. The rate of densification decrease and pore widening are promoted in argon. An inert atmosphere favors SnO2 evaporation due to reduction of CuO. © 1990.

An AM1 semiempirical study of the mechanism of sintering for ZnO in the presence of water and carbon dioxide

AM1 calculations were performed for the absorption of H2O and CO2 molecules on the surface of model ZnO crystals. The absorption of isolated molecules of each species and the co-absorption of both compounds simultaneously were considered. It was found that the absorption of H2O near a site where CO; is already absorbed favors the process of sintering, in agreement with the experimental findings. This is explained by the formation of Zn(OH)CO3H bound to the surface, a more mobile species than the ZnO unit itself. The roundening of the grains observed in atmospheres containing dry CO2 but suppressed when H2O is present, is also explained by these calculations. After absorption of CO2, the rupture of one bond - so that diffusion of the ZnCO3 species on the surface is allowed - requires much less energy than the breaking of two bonds, necessary for ZnO migration. These facts explain why the speed of surface transport does not decrease in CO2 atmospheres while sintering is indeed slowed down. © 1994.

Effect of atmosphere and dopants on sintering of SnO2

Tin oxide is an n-type semiconductor material with a high covalent behavior. Mass transport in this oxide depends on the surface state promoted by atmosphere or by the solid solution of a non-isovalent oxide doping The sintering and grain growth of this type of oxide powder is then controlled by atmosphere and by extrinsic oxygen vacancy formation. For pure SnO2 powder the surface state depends only on the interaction of atmosphere molecules with the SnO2 surface. Inert atmosphere like argon or helium promotes oxygen vacancy formation at the surface due to reduction of SnO2 to SnO at the surface and liberation of oxygen molecules forming oxygen vacancies. As consequence surface diffusion is enhanced leading to grain coarsening but no densification. Oxygen atmosphere inhibits the SnO2 reduction decreasing the surface oxygen vacancy concentration. Addition of dopants with lower valence at sintering temperature creates extrinsic charged oxygen vacancies that promote mass transport at grain boundary leading to densification and grain growth of this polycrystalline oxide.

Effect of atmosphere on the sintering and grain growth of tin oxide

High purity SnO 2 powder (>99.9%) was compacted in cylindrical pellets and sintered in atmospheres of dry argon, argon with water vapor, oxygen and CO 2 using 10 °C/min up to 1200 °C or isotherms in the range of 1000 to 1200 °C. Time, temperature and sintering atmosphere have large influence on grain growth and low influence on densification of this oxide. Surface diffusion is the dominant mechanism up to 1200 °C and evaporation-condensation is dominant above 1200 °C. The maximum linear shrinkage observed was about 2.0% and attributed to structural rearrangement of particles due to high capillary stresses developed with neighboring particles. © 1999 Trans Tech Publications.

Biomass fire consumption and carbon release rates of rainforest-clearing experiments conducted in northern Mato Grosso, Brazil

Biomass consumption and carbon release rates during the process of forest clearing by fire in five test plots are presented and discussed. The experiments were conducted at the Caiabi Farm near the town of Alta Floresta, state of Mato Grosso, Brazil, in five square plots of 1 ha each designated A, B, C, D, and E, with different locations and timing of fire. Plot A was located in the interface with a pasture, with three edges bordering on the forest, and was cut and burned in 1997. Plots B,C, D, and E were located inside the forest. Plot B was cut and burned in 1997. Plot C was inside a deforested 9-ha area, which was cut and burned in 1998. Plot D was inside a deforested 4-ha area, which was cut in 1998 and burned in 1999. Plot E was inside a deforested 4-ha area which was cut and burned in 1999. Biomass consumption was 22.7%, 19.5%, 47.5%, 61.5% and 41.8%, for A, B, C, D, and E, respectively. The effects of an extended curing period and of increasing the deforested area surrounding the plots could be clearly observed. The consumption for areas cut and burned during the same year, tended toward a value of nearly 50% when presented as a function of the total area burned. The aboveground biomass of the test site and the amount of carbon before the fire were 496 Mg ha-1 and 138 Mg ha-1, respectively. Considering that the biomass that remains unburned keeps about the same average carbon content of fresh biomass, which is supported by the fact that the unburned material consists mainly of large logs and considering the value of 50% for consumption, the amount of carbon released to the atmosphere as gases was 69 Mg ha-1. The amounts of CO2 and CO released to the atmosphere by the burning process were then estimated as 228 Mg ha-1 and 15.9 Mg ha-1, respectively. Observations on fire propagation and general features of the slash burnings in the test areas complete the paper. Copyright 2001 by the American Geophysical Union.

Influence of thermal treatments in the corrosion behaviour of HVOF Cr 3C2-NiCr coatings

Thermal spray coatings as Cr3C2-NiCr obtained by high velocity oxy-fuel spraying (HVOF) are mainly applied due to their behaviour against aggressive erosive-abrasive and corrosive atmospheres and their thermal stability at high temperatures [1]. In order to increase the corrosion protection that it offers to the substrate trying to close the interconnected pores, it is possible to apply a thermal treatment with the gun during the spraying of the coating. This treatment could be applied in different ways. One of these ways consists of spraying only a few layers of coating followed by thermal treatment and finally the spray of the rest of layers. This thermal treatment on spraying is studied related to the corrosion properties of the system. The study comprises the electrochemical characterisation of the system by open circuit potential (OC), polarisation resistance (Rp), cyclic voltammetry (CV) and impedance spectroscopy measurements (EIS). Optical and scanning electron microscopy characterisation (OM and SEM) of the top and cross-section of the system has been used in order to justify the electrochemical results.

Degradación y recuperación de varistores de ZnO

In the present work it was studied the main aspect that influences on degradation and physical-chemical properties at grain-boundary region of metal oxide varistors and its consequences on microstructure and nonohmic electric properties. Based on the comprehension of the degradation aspects it was proposed some methodologies to recover the varistors nonohmic properties after being failed with long (2000 μs) and short current pulses (8/20 μs). Our analysis shown that one of the cause of degradation process is related to the lowering of oxygen species amount at grain-boundary region. Therefore, it is possible to re-promote oxygen enrichment of such regions by specific thermal treatments in rich oxygen atmospheres (the best condition found in the present work was temperatures around 900°C for 2 h at an oxygen flux of 15 1/h). The proposed nonohmic properties recovering procedure appear to be valid for all kind metal oxide varistors studied and is very important from technological point of view.

Respuesta eléctrica de películas gruesas del sistema SnO2-TiO2, conformadas con polvos cerámicos obtenidos a través del método pechini

Tin oxide is the compound more used in the gas sensor production, mainly to reduce gases like CO and H2. Their electrical properties are strongly dependent of its surface. For this is of great importance to synthesize nanoscale tin oxide particles to use they later in the films conformation. The synthesis of SnO2-TiO2 nanoparticles by polymeric precursor method is reported in this work. The particles were characterized by DTA/TG, FT-IR, XRD and SEM. Also, the electrical response of thick films measured under oxygen and carbon monoxide atmospheres is was analyzed.

Effects of heat treatments on the electrocatalytic activity of Pt-Ru/C for methanol oxidation

The Pt-Ru/C materials of this study were prepared by a microemulsion method with fixed water to surfactant molar ratio and heat treated at low temperatures, to avoid changes in the average particle size, in different atmospheres. All samples were characterized by X-ray diffraction (XRD) and the mean crystallite size was estimated by using Scherrer's equation. Catalysts morphology was characterized by transmission electron microscopy (TEM). Average composition was obtained by energydispersive X-ray analysis (EDX). The general electrochemical behavior was evaluated by cyclic voltammetry in 0.5 M sulfuric acid and the electrocatalytic activity towards the oxidation of methanol was studied in 0.5 M methanol acid solutions by potential sweeps and chronoamperometry. Oxidation of adsorbed CO was used to estimate the electrochemical active area and to infer the surface properties. ©The Electrochemical Society.

Stacking faults in SiCf/SiC composite obtained by chemical vapor reaction process

SiC fiber-reinforced SiC matrix composite (SiCf/SiC) is one of the leading candidates in ceramic materials for engineering applications due to its unique combination of properties such as high thermal conductivity, high resistance to corrosion and working conditions. Fiber-reinforced composites are materials which exhibit a significant improvement in properties like ductility in comparison to the monolithic SiC ceramic. The SiCf/SiC composite was obtained from a C/C composite precursor using convertion reaction under high temperature and controlled atmosphere. In this work, SiC phase presented the stacking faults in the structure, being not possible to calculate the unit cell size, symmetry and bond lengths but it seem equal card number 29-1129 of JCPDS.

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Microstructural characterization of layers produced by plasma nitriding on austenitic and superaustenitic stainless steel grades

High chromium content is responsible for the formation of a protective passive surface layer on austenitic stainless steels (ASS). Due to their larger amounts of chromium, superaustenitic stainless steels (SASS) can be chosen for applications with higher corrosion resistance requirements. However, both of them present low hardness and wear resistance that has limited their use for mechanical parts fabrication. Plasma nitriding is a very effective surface treatment for producing harder and wear resistant surface layers on these steel grades, without harming their corrosion resistance if low processing temperatures are employed. In this work UNS S31600 and UNS S31254 SASS samples were plasma nitrided in temperatures from 400 °C to 500 °C for 5 h with 80% H 2-20% N2 atmosphere at 600Pa. Nitrided layers were analyzed by optical (OM) and transmission electron microscopy (TEM), x-ray diffraction (XRD), and Vickers microhardness testing. Observations made by optical microscopy showed that N-rich layers were uniform but their thicknesses increased with higher nitriding temperatures. XRD analyses showed that lower temperature layers are mainly composed by expanded austenite, a metastable nitrogen supersaturated phase with excellent corrosion and tribological properties. Samples nitrided at 400 °C produced a 5 μm thick expanded austenite layer. The nitrided layer reached 25 lm in specimens treated at 500 °C. There are indications that other phases are formed during higher temperature nitriding but XRD analysis was not able to determine that phases are iron and/or chromium nitrides, which are responsible for increasing hardness from 850 up to 1100 HV. In fact, observations made by TEM have indicated that formation of fine nitrides, virtually not identified by XRD technique, can begin at lower temperatures and their growth is affected by both thermodynamical and kinetics reasons. Copyright © 2012 by ASTM International.

Surface study of vitreous carbon obtained from glassy carbon powder

This work presents a surface study of monolithic vitreous (or glassy) carbon - MVC - obtained from vitreous carbon powder. Defective MVC pieces are crushed in a ball mill and size classified by sifting. The MVC powder is mixed with furfuryl-alcohol resin and compacted in a mould using a hydraulic press. Samples with different powder granulometries are produced in this way and carbonized in a furnace under nitrogen atmosphere. Complete carbonization of the powder is achieved in only one day and losses due to breakage of the pieces is less than 5%. These results compare very favorably with respect to traditional MVC production methods where full carbonization may require up to seven days and losses due to breakage can be as high as 70%. After carbonization, samples are sanded and polished. Surface roughness and microstructure are characterized by light microscopy. Porosity is quantified from micrographs using ImageJ software and nanometric height variations are measured by atomic force microscopy. © 2012 Materials Research Society.