L.C. Van Poucke

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.

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

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.

Aqueous Solution-Gel Synthesis of Strontium Bismuth Niobate (SrBi2Nb2O9)

The synthesis of multimetal oxides containing pentavalent elements like Nb and Ta out of an aqueous solution is very complicated due to the extremely high sensitivity of these metals towards hydrolysis. Moreover the only water-soluble starting compound is the oxalate which is not very suited for gel formation. Nevertheless, from Nb-oxalate it is possible to prepare a water-soluble Nb(V)-precursor by chemical modification. The synthesized precursor remains stable in the pH-range needed for gel formation. This Nb(V)-precursor is used for the synthesis of the ferroelectric material SrBi2Nb2O9 (SBN). An aqueous acetate-citrate solution-gel precursor for SBN was prepared. The chemical homogeneity of this precursor was investigated by means of TEM-experiments. All metal ions were found to be homogeneously dispersed in the precursor gel at least down to 10 nm. The SBN phase formation is followed by means of X-ray Diffraction. It has been shown that the perovskite phase already forms at 550°C.

Chemical Solution Deposition of ZnO Thin Films by an Aqueous Solution Gel Precursor Route

An aqueous chemical solution deposition method was used to prepare thin films of ZnO on SiO2/Si (1 1 1) substrates. Starting from an aqueous solution of Zn acetate, citric acid and ammonia, very thin films could be deposited by spin coating. Heating parameters, necessary for thin film annealing, were determined using FTIR experiments on dried gel precursors, heated up to different temperatures. The morphology and the thickness of the films were investigated by SEM. It is found that homogeneous thin films with grain sizes of about 20 nm are formed. XRD experiments show that there is an indication that the films, crystallized at 500°C, exhibit preferential grain growth along the c-axis.

Synthesis of SrBi2Ta2O9 (SBT) by means of a soluble Ta(V) precursor

Abstract Aqueous sol-gel processing of pentavalent metals like Ta and Nb is very complicated since the only water soluble compound is the oxalate, which is not very suited for gel formation. Nevertheless, starting from Ta oxalate, it is possible to prepare a water soluble Ta(V) precursor that remains stable in the pH range needed for gel formation. During synthesis the oxalate is oxidized with H 2 O 2 and subsequently complexed with citric acid and as a result the peroxo–citrato tantalum(V) complex is formed. This Ta(V) precursor is afterwards used for the synthesis of the ferroelectric SrBi 2 Ta 2 O 9 (SBT). The formation of the SBT crystalline phases was investigated by means of high temperature X-ray diffraction (HT–XRD).

The use of Hi-Res TGA, TG-FTIR, HT-DRIFT and HT-XRD in the study of the decomposition of La2(C2O4)3·10H2O

Using a combination of various techniques, more intermediate products could be detected during the thermal decomposition of La2(C2O4)3·10H2O than from conventional TGA measurements. In this way a more complete and detailed decomposition mechanism is proposed. Direct identification of the intermediate phases with HT-DRIFT revealed the presence of another oxycarbonate La2O(CO3)2, while with Hi-Res TGA a thermally unstable intermediate La2(CO3)3 was detected. All this information is consistent with the data obtained from TG-FTIR spectroscopic measurements.

The Formation of Ferroelectric Bismuth Titanate (Bi4Ti3O12) from an Aqueous Metal-Chelate Gel

Bismuth titanate (Bi4Ti3O12) was synthesized by an aqueous solution-gel process starting from solutions of bismuth acetate and a peroxocitrato-Ti(IV) complex. To gain insight into the thermal decomposition pattern of the gel several thermal analysis techniques were employed: DTA, TGA-EGA (evolved gas analysis by on-line coupling to a FTIR or mass spectrometer) and HT-DRIFT. Transmission electron micrographs showed that the gel is chemically homogeneous down to ca. 5 nm and that this homogeneity is preserved throughout the heat treatment. High-temperature X-ray diffraction measurements were used to make an in situ study of the phase formation. It has been found that single phase Bi4Ti3O12 is formed at 625°C.