J. Mullens

Synthesis of ZnO nanopowder via an aqueous acetate–citrate gelation method

The synthesis of nanoparticulate ZnO via an aqueous carboxylate gelation route is presented. Starting from a solution of zinc acetate with citric acid as a complexing agent, a solid glassy gel is obtained after drying that is converted into a fine powder by calcination. It is found that a very homogeneous precursor is indispensable when preparing very fine particles with a narrow size distribution. Cryo-transmission electron microscopy (Cryo-TEM) investigation is used as a feedback tool to prevent early precipitation during gelation. Study of the thermal decomposition of the gel shows that ZnO is formed before the final decomposition step takes place. After removing the organic backbone, very small oxide particles are found. The influence of the thermal treatment parameters on the particle size is investigated and a particle growth process is found. By a proper adjustment of the final calcination temperature in dry air, the mean particle size can be controlled between ∼11 and 175 nm. It was also seen that even in inert atmosphere, ZnO is formed and that particle morphology is greatly influenced by the calcination atmosphere.

Thermal decomposition of the ammonium zinc acetate citrate precursor for aqueous chemical solution deposition of ZnO

The thermal decomposition of an aqueous chemical solution deposition Zn2+-precursor is studied by HT-DRIFT (high temperature diffuse reflectance infrared Fourier transform spectroscopy), on-line coupled TGA-EGA (thermogravimetric analysis - evolved gas analysis by Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS)), and HT-XRD (high temperature X-ray diffraction). Using these complementary techniques, it is found that the α-hydroxyl group of the citrate ligand plays a significant role in the decomposition pathway of the ammonium zinc acetate citrate precursor. TEM (transmission electron microscopy) shows that crystalline ZnO (zincite) is formed at 390°C, after dehydroxylation of the α-hydroxyl group and subsequent decarboxylation of the Zn2+-precursor complex. Before total calcination, ZnO particles are already formed and a residual organic backbone thereby remains. The results obtained by these complementary techniques clearly indicate the importance of thermal analysis in the preparation of ceramics through chemical solution deposition.

Aqueous Chemical Solution Deposition of Ferroelectric Thin Films

Thin films of various ferroelectric multimetal oxides such as (Bi 1 m x La x ) 4 Ti 3 O 12 (BLT), SrBi 2 Ta 2 O 9 (SBT) and PbZr 1 m x Ti x O 3 (PZT) have been prepared by an entirely aqueous chemical solution deposition (CSD) route. Two critical issues related with aqueous CSD have hereby been worked out: in spite of the high degree of hydrolysis of tetra- and pentavalent metal ions (Ti 4+ , Zr 4+ , Ta 5+ , ) we managed to prepare stable aqueous precursor solutions by chemical modification of these individual metals, avoiding phase segregation. Another problem related with aqueous CSD is the wetting of the substrate (both metallic and metal oxide) by the aqueous solution. The hydrophilicity of the substrates is optimized by a chemical treatment of the substrate surface. In this manner, the addition of wetting agents, hence possibly disturbing the gelation reactions, is avoided. In order to study the gelation, decomposition, crystalization and the morphology of the thin films, various characterization techniques ((cryo-)TEM, SEM, EDX, TGA-MS/FTIR, HT-DRIFT, HT-XRD, ) are used.

Low-temperature pyrolysis of CCA-treated wood: thermogravimetric analysis

Abstract A thermo-analytical study of untreated and chromated copper arsenate (CCA) treated wood samples is performed in order to obtain a better understanding of the low-temperature pyrolysis of CCA-treated wood waste in an inert atmosphere. The type of wood used in this study is Pinus sylvestris sapwood. The influence of the presence of CCA and the heating rate on the pyrolytic behaviour of wood samples is studied, as well as the release of volatile compounds and metals (Cr, Cu, As) during the pyrolysis process. This paper shows that CCA has a significant influence on the thermal behaviour of wood samples, which is more pronounced the higher the CCA concentration of the sample is. The temperature at the onset of pyrolysis, as well as the temperature where the maximum rate of decomposition occurs, are lowered by the CCA treatment. The final char yield (including the metals) is higher and the rate of weight loss is much more peaked for CCA-treated wood. It could be postulated that the CCA compounds act as promotors of the pyrolysis reactions favouring the formation of char. For higher heating rates, there is a shift of the DTG peak to higher temperatures for both untreated and CCA-treated wood samples. Within the accuracy of the evolved gas analysis (EGA) method applied, it is observed that the presence of CCA does not significantly influence the type and relative amount of measured volatiles. The volatilisation of metal compounds is shown to be strongly dependent on temperature and residence time of the wood sample at a given temperature. A critical point (10 min at 400°C) exists, below which the release of Cr and Cu is negligible and the release of As is below 10%. Above this critical point (longer times at 400°C), there is a dramatic increase in metal release for all three metals. The CCA concentration itself also has an influence in the sense that higher concentrations in the original sample give higher relative concentrations of metals in the resulting pyrolysis residue.

Use of thermogravimetric analysis-Fourier transform infrared spectroscopy in the study of the reaction mechanism of the preparation of Pb(Zr, Ti)O3 by the sol-gel method

Abstract The thermal decomposition of a sol-gel precursor of Pb(Zr, Ti)O 3 is studied by thermogravimetric analysis-Fourier transform infrared spectroscopy in argon and in air. It is found that in an inert atmosphere the presence of residual carbon inhibits the formation of the perovskite structure. In air the evolution of volatile compounds such as alcohols, esters, ethers and CO 2 at temperatures above 300°C indicate that polymerization is still going on during the thermal decomposition of the precursor.

Sulphur group analysis in solid matrices by atmospheric pressure-temperature programmed reduction

The atmospheric pressure-temperature programmed reduction (AP-TPR) has become an established and reliable method amongst the different sulphur characterisation techniques for solid materials, like coal and coal derived products, rubber and clay. The analytical method is based upon the fact that specific sulphur functional groups are hydrogenated at specific temperatures. During the last few years, several adjustments have been made to the hard- and software as well as to the experimental parameters. The changes and the reliability of the method are extensively discussed in this paper.

THE DECOMPOSITION OF COPPER OXALATE TO METALLIC COPPER IS WELL SUITED FOR CHECKING THE INERT WORKING-CONDITIONS OF THERMAL-ANALYSIS EQUIPMENT

Thermal analysis (TA) techniques require careful control of some experimental parameters. One of these parameters is the gaseous atmosphere in which the experiment is carried out. Particularly at high temperatures, even very small amounts of oxygen can drastically influence the experiment so that totally different results are obtained compared with those in a completely inert atmosphere. The conduction of inert gases such as argon or (below SOO’C) nitrogen over an oxygen absorbent before entering the TA equipment is a necessary but not sufficient condition for carrying out the experiment in a totally oxygen-free atmosphere. One has to take care that the TA equipment is completely free of the oxygen that is inevitably inside the equipment after loading. Even flushing with the inert gas for 30 min at a flow of 50 ml min-’ before starting the experiment might be insufficient to remove all the residual air if the gas inlet and gas outlet are not placed in the right positions. This is shown in the following experiment. Figure 1 shows the decomposition of 30.543 mg copper oxalate; the equipment was flushed with argon for 30 min at a rate of 50 ml mine1 before starting the experiment and the flushing was continued during the experiment at the same flow-rate. In an inert atmosphere the decomposition goes to metallic copper (theoretical remaining weight, 41.9%). As shown by the weight increase between 300 and 6OO”C, there is an oxidation of copper (theoretical remaining weight is 52.5% if 100% CuO)! The

Interaction of the organic matrix with pyrite during pyrolysis of a high-sulfur bituminous coal

Abstract High-sulfur bituminous coal containing 4.17 wt% of pyritic sulfur and the pyrite concentrate separated from this coal were used to examine the interaction between pyritic sulfur and the organic part of coal during pyrolysis. At 330–500°C, as a result of the reaction of sulfur derived from pyrite decomposition with the coal organic matrix, a significant increase in the organic sulfur in the char is observed, from 1.47 to 3.17 wt%. The enrichment in sulfur is most pronounced between 400 and 450°C, corresponding to the most intensive thermal degradation of this coal. At these temperatures, some of the pyrite is converted to pyrrhotite. The organic sulfur content is a maximum at ∼ 500°C, when all the pyrite is reduced to pyrrhotite. The pyrite in the coal undergoes conversion to troilite via pyrrhotite at lower temperatures than does pure pyrite. Compared with the thermal decomposition of pure pyrite, the pyrite present in coal starts to decompose at a lower temperature (330 vs. 400°C). The conversion to troilite also proceeds to completion at a much lower temperature. This demonstrates that the decomposition of pyrite is markedly affected by the presence of the organic coal substance.

Determination of sulfur groups in pyrolysed low-rank coal by atmospheric-pressure t.p.r.

Abstract Atmospheric-pressure temperature-programmed reduction (t.p.r.) was used to follow in pyrolysed sub-bituminous coal the changes in amount of pyrite and in organic sulfur groups as a function of temperature. At higher pyrolysis temperatures, pyrite and aliphatic and mixed aliphatic-aromatic sulfides disappeared systematically and more complex sulfur compounds such as aromatic sulfides and simple thiophenic structures were formed.

Preparation and thermal decomposition of various forms of strontium oxalate

Abstract Strontium oxalate exists in two different forms: the neutral strontium oxalate hydrate, SrC 2 O 4 · x H 2 O, and the acid salt of strontium oxalate, SrC 2 O 4 · y H 2 C 2 O 4 · x H 2 O. Depending on the concentration of oxalic acid and ammonium oxalate as precipitating agents, both forms can be obtained. At a sufficiently low pH, the stoichiometric compound SrC 2 O 4 ·1 2 H 2 C 2 O 4 · H 2 O is formed. The thermal decomposition of the different strontium oxalates is studied in different atmospheres using DSC, and TGA coupled with FTIR and MS. The TGA-EGA spectra indicate that the anhydrous acid oxalate decomposes with the release of H 2 O, CO, CO 2 and formic acid.