| dc.description.abstract | The introductory chapter begins with a brief account of the thermal decomposition of simple and complex oxalates. General theory of kinetics of decomposition of solids is also presented. The conventional ceramic method of preparation of zirconates is examined and the advantages of the chemical methods over the ceramic methods are highlighted. Since the zirconates of bivalent metals are either piezoelectric, ferroelectric or antiferroelectric, a brief account of these properties is given. The salient features of the perovskite structure responsible for the peculiar dielectric properties of the metazirconates are discussed. Previous work on the zirconyl oxalates is reviewed and the scope of the present work is discussed.
Chapter II deals with an account of the methods of preparation of zirconyl oxalates of bivalent metals, the procedures for their chemical analysis and the instrumental techniques employed in the present investigation. It is difficult to prepare stoichiometric zirconyl oxalates of bivalent metals, due to easy hydrolysis of zirconium compounds in aqueous solution. Considerable effort was necessary to standardise the methods of preparation of these compounds. When the preparations are carried out following the various reported procedures, the resulting materials showed considerable variation in M:Zr:C?O? ratios. The appropriate ammoniation of the acid zirconyl oxalate, and the metathesis of the resulting ammonium salt under controlled pH conditions and appropriate mole ratio of bivalent metal ions yields stoichiometric compounds.
Careful chemical analyses of these preparations show very good agreement with the calculated values. Here again, methods are standardised for the determination of various constituents in the starting materials and in the products of thermal decomposition at different stages. Due to improper stoichiometry most of the earlier reports on the thermal decomposition of zirconyl oxalates have to be questioned. For example, barium zirconyl oxalate prepared under low pH conditions (less than 5) has Ba:Zr ratio more than one and hence results in BaCO? and BaZrO? as the final products of decomposition.
The thermal decomposition of acid zirconyl oxalate, ammonium zirconyl oxalate, zirconyl oxalate, and a mixture of barium and zirconyl oxalates are presented in Chapter III. The oxalate decomposition is a two-step process in the case of acid and ammonium zirconyl oxalates whereas it is a one-step process in the case of zirconyl oxalate. This variation in thermal decomposition characteristics is attributed to the difference in the structure of the two types of compounds. The probable structures for the zirconyl oxalate, the acid and ammonium zirconyl oxalates and the zirconyl oxalates of bivalent metals are proposed. A mixture of barium oxalate and zirconyl oxalate behaves like a mechanical mixture only. DTA shows all the peaks attributable to individual oxalates. The results indicate that the thermal behaviours of zirconyl oxalates of bivalent metals are in no way comparable with a mixture of MCO? (where M = Ba, Sr, Ca, Pb etc.) and ZrO(C?O?)?. Besides, these experiments also demonstrate that barium carbonate and zirconium dioxide are not the intermediate products formed during the thermal decomposition of barium zirconyl oxalate contrary to the earlier report.
Chapter IV comprises a detailed study on the thermal decomposition of barium zirconyl oxalate heptahydrate. As mentioned before, the previous reports describing the decomposition of BZO are to be doubted due to improper stoichiometry. Thus the scheme proposed is questionable. Based on the combined results of thermal analyses, identification of residues at various stages of decomposition both by chemical analysis and physical methods, and the analysis of the evolved gases, a new scheme has been proposed for the thermal decomposition of BZO. The dehydration of heptahydrate takes place in two steps which are distinct on both the DTA and DTG curves. The oxalate decomposition is also a two-step process. During the first endothermic process, an oxalato?carbonate of apparent composition Ba?Zr?O?(C?O?)?(CO?)? is formed. This is unstable and cannot be isolated with a definite stoichiometry. The second step of oxalate decomposition is a low enthalpy process. Both carbon monoxide and carbon dioxide are simultaneously evolved during this stage. An ionic carbonate of definite stoichiometry is formed at this stage. This ultimately gives rise to cubic barium zirconate. Barium carbonate and/or zirconium dioxide are the invariable products of decomposition at this temperature when the starting material has Ba:Zr ratio different from 1:1. The products of decomposition of BZO at any stage are X-ray crystalline. The final product is the cubic form of BaZrO?.
The thermal decomposition of strontium zirconyl oxalate tetrahydrate and calcium zirconyl oxalate heptahydrate forms the subject matter of Chapter V. Decomposition of SZO resembles that of BZO. Dehydration and oxalate decompositions are two-step processes. Here also an oxalato-carbonate intermediate of apparent composition is formed, which further decomposes to Sr?Zr?O?CO?. This is also an ionic carbonate as indicated by its IR spectrum. But CZO decomposes in a different fashion. Though the dehydration is a two-step process, the oxalate groups decompose in a single step to produce a carbonate containing unidentate and bidentate carbonate groups. This further decomposes to the ionic carbonate Ca?Zr?O?CO?. These intermediate ionic carbonates of SZO and CZO form the respective metazirconates at a higher temperature. The strontium zirconate thus formed is the orthorhombic phase while calcium zirconate is the pseudo?cubic phase.
Kinetics of decomposition of anhydrous BZO and the carbonates Ba?Zr?O?CO? (B) and Sr?Zr?O?CO? (S) are presented in Chapter VI. Anhydrous BZO decomposes in the temperature range 400–420 °C at a measurable rate. Both (?–t) plots and the reduced time data indicate that the reaction is deceleratory throughout. Of the various deceleratory equations tried, only the contracting volume equation is found to give a satisfactory fit. However, the activation energies 35.46 and 36.07 kcal mol?¹ obtained from Arrhenius plots are much lower than the value 75.53 kcal mol?¹ obtained from log(1??)¹?² plots. The kinetics of decomposition of the carbonates B and S are found to depend strongly on their thermal history. They are prepared in three different ways: (i) Ba?Zr?O?CO? and Sr?Zr?O?CO? prepared in air (B? and S?); (ii) a portion of B? and S? are allowed to age for six months at ambient temperatures around 25 °C (B? and S?); and (iii) Ba?Zr?O?CO? and Sr?Zr?O?CO? prepared in a vacuum (B? and S?).
All the six preparations show sigmoidal (?–t) plots. B? decomposes in the temperature range 600–635 °C obeying a simple square law up to 0.2?, and a first?order decay law thereafter. The activation energies are 49.44 and 54.62 kcal mol?¹. B? also obeys the same law with activation energies 47.01 and 46.70 kcal mol?¹. Due to ageing effect, the temperature is brought down to 550–570 °C range. B? obeys a cube law till 0.5? and the first?order decay law beyond 0.5?. The activation energies are 172.6 and 153.7 kcal mol?¹. The temperature range is also shifted to 705–730 °C for vacuum?prepared samples B?. The higher values are explained by carbon contamination possibly poisoning the active sites of decomposition.
Samples S? and S? show temperature ranges 550–575 °C and 530–550 °C, obeying cube law and first?order decay law respectively. For S? the acceleratory period is more pronounced, the inflection point being 0.5? whereas for S? it is 0.3?. The negative intercepts on the time axis imply the absence of induction periods. Activation energies for S? are 54.93 and 47.25 kcal mol?¹ while for S? they are 44.19 and 40.09 kcal mol?¹. S? follows cube law only in 0 < ? < 0.2, then square law until 0.5?, with the decay period following first?order law. Temperature range 665–685 °C and activation energies 201.4, 178.0 and 150.2 kcal mol?¹ are higher than S? and S?. Prout–Tompkins equation is tried but does not give better fits. This study highlights importance of sample pre?history on kinetics.
Chapter VII deals with the thermal decomposition of lead zirconyl oxalate hexahydrate. The decomposition is complicated because of the ease of reduction of Pb(II) to metallic lead. In oxygen or air, the decomposition is simple and straightforward. The hexahydrate forms the anhydrous oxalate in a single step. The anhydrous oxalate decomposes in two steps to form the carbonate Pb?Zr?O?CO?. Though indicated by splitting of DTA and DTG peaks, no oxalato?carbonate intermediate of definite composition could be isolated. The carbonate decomposes to tetragonal lead zirconate. In non?oxidising atmospheres or vacuum a portion of Pb(II) is reduced to Pb(0) and a lead?deficient carbonate is produced. Since lead zirconate is the only stable compound in the PbO–ZrO? phase diagram, the carbonate decomposes to tetragonal lead zirconate and monoclinic zirconia. In different atmospheres amorphous PbZrO? is formed, crystallising at a higher temperature. This behaviour is different when compared with alkaline earth zirconyl oxalates where decomposition is accompanied by simultaneous particle size growth.
The present study reveals many similarities as well as differences between the titanyl oxalates and zirconyl oxalates of bivalent metals. The temperature ranges of decomposition are almost identical. Both yield stable intermediate ionic carbonates of general formula M?Ti?O?CO? or M?Zr?O?CO?. The foremost difference lies in preparation: titanyl oxalates can be prepared under highly acidic conditions, whereas zirconyl oxalates cannot. This may be due to existence of zirconium as free Zr?? ion in strong acid and its higher coordination number, producing zirconium?deficient oxalates. Another difference is that intermediates from zirconyl oxalates show no tendency to retain CO? in solid matrix unlike titanyl oxalates. This is explained by the crystalline nature of zirconyl decomposition products. For example, barium titanyl oxalate becomes X?ray amorphous during decomposition until barium titanate forms. BZO intermediates remain crystalline throughout, with final product barium zirconate showing strong X?ray patterns.
Results based exclusively on TGA and DTA are limited in scope. Because of the dynamic nature of thermoanalytical experiments, incomplete reaction and overlap of successive reactions cause variable weight losses and complex DTA peaks. Without proper attention these lead to wrong conclusions. In the present study, a combination of techniques helped in elucidating the complex decomposition behaviour of zirconyl oxalates of bivalent metals.
Preparation of zirconates from zirconyl oxalates is technologically promising, since they can be obtained at as low a temperature as 500–700 °C. Moreover, high purity and controlled particle size can also be achieved. | |
