Thermal decomposition of mixed metal oxalates
- Authors: Coetzee, Anita
- Date: 1993
- Subjects: Oxalates -- Research , Decomposition (Chemistry)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4388 , http://hdl.handle.net/10962/d1005053 , Oxalates -- Research , Decomposition (Chemistry)
- Description: The mixed metal oxalates, FeCu(ox)₂.3H₂0, CoCu(ox)₂.3H₂0, and NiCu(ox)₂.3.5H₂0, [ox = C₂0₄] have been prepared by coprecipitation from solution. The thermal behaviour of these compounds in nitrogen and in oxygen has been examined using thermogravimetry (TG), thermomagnetometry (TM), differential scanning calorimetry (DSC) and evolved gas analysis (EGA), and results are compared with results obtained for Cuox and Mox.yH₂0. The thermal behaviour of the mixed oxalates, MCU(OX)₂.xH₂0, differed from that of the individual metal oxalates, Cuox, Coox.2H₂0, Niox.2H₂0 and Feox.2H₂0. All three mixed oxalates on heating in N₂, first dehydrate and then decompose in at least two overlapping endothermic stages. Both CO and CO₂ were evolved in proportions which varied with the surrounding atmosphere, and from compound to compound, and with extent of reaction of a given compound. The mixed oxalates, MCU(OX)₂.xH₂0, do not show the exothermic behaviour characteristic of Cuox, and reasons for this are discussed. Thermochemical calculations were done and the enthalpies of formation of the hydrates and dehydrated oxalates were determined. It was found that the enthalpy of mixing was very small or within experimental error. X-ray powder diffraction patterns for the individual and mixed oxalates were compared. The pattern for Cuox differs from the patterns obtained for the other oxalates, confirming suggestions that Cuox has a structure different to most other transition metal oxalates. The kinetics of dehydration and decomposition of the mixed oxalates were investigated, using isothermal and programmed temperature TG and DSC experiments. The yield and composition of evolved gases during isothermal decomposition were measured and compared with the enthalpy changes. X-ray photoelectron spectroscopy studies provided some information on the electron environment of the metal atoms in the various oxalates.
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- Date Issued: 1993
Thermal decomposition of ammonium metavanadate
- Authors: Stewart, Brian Victor
- Date: 1972
- Subjects: Decomposition (Chemistry) , Solids -- Thermal properties , Ammonia
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4503 , http://hdl.handle.net/10962/d1013274
- Description: The isothermal, endothermic, stepwise decomposition of ammonium metavanadate (AMV) in inert (argon or nitrogen), oxidising (air or oxygen) and reducing (ammonia) atmospheres as well as under high vacuum (pressure < IOn bar) conditions has been investigated. The reverse reaction, the isothermal recombination of V₂ 0₅ with ammonia and water vapour has also been investigated. The decomposition and recombination reactions were followed by continuously recording the mass loss of the sample with time using a Cahn R.G. Automatic Electrobalance. This enabled small samples ( ~ lOmg) to be used and consequently any self cooling of the sample during the decomposition was minimized. The intermediates and final products formed have been characterized by chemical analysis, X-ray powder diffraction studies, infrared spectroscopy and the mass loss involved in their formation. The changes in the physical properties of the samples during decomposition and recombination have been investigated by surface area measurements (using the BET method and krypton adsorption) and eIectron microscopy. Values for the enthalpy changes involved in the decomposition have been obtained by differential scanning calorimetry. The stoichiometry of the isothermal decomposition of ammonium metavanadate, under the various conditions of surrounding atmosphere has been discussed. Except for the later stages of the decomposition in ammonia, the results correspond well to the gradual reduction of the ratio of "(NH₄)₂ 0" to "V₂0₅" units from the original 1:1 ratio in ammonium metavanadate to pure "V₂0₅" with ammonia and water being evolved throughout the decomposition in the mole ratio of 2:1. The final product of the decomposition in vacuum, argon and air is "V₂0₅" and in ammonia, below 360°, V0₂. The kinetic parameters for each of the stages of the decomposition of AMV in each of the atmospheres studied have been determined. The mechanism of the first stage of the decomposition under the different conditions of surrounding atmosphere has been discussed from both the kinetic and the thermodynamic points of view. The absolute reaction rate theory has been applied to the decomposition in inert atmospheres enabling the formulae of the activated complexes formed during each stage to be calculated. It has also been shown that the detailed atomic movements occurring during the first stage of the decomposition in ammonia can be predicted from a knowledge of the stoichiometry of the reaction and of the detailed crystal structures of the starting and product materials. The kinetics and mechanism of the recombination of "V₂0₅" with ammonia and water vapour to form AMV have also been discussed in detail.
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- Date Issued: 1972
A study of irradiation effects in solids
- Authors: Brown, Michael Ewart
- Date: 1966
- Subjects: Decomposition (Chemistry) , Crystals -- Thermal properties , Oxalates -- Thermal properties , Solids -- Effect of radiation on
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4509 , http://hdl.handle.net/10962/d1013387
- Description: One of the primary objects of this research was to determine, if possible, the nature of the radiation damage prior to thermal decomposition. The X-ray study has not wholly achieved this although more information has been derived from it than from similar work on AgMnO₄ However, the diffuse reflections obtained do indicate, quite strongly, the creation of point defects during irradiation. This is of value since such assumptions have been made in the explanation of the kinetics of decomposition of a number of irradiated solids (BaN₆,CaN₆). In addition the X-ray work has suggested future research which should produce useful information; namely, a precise study of the diffuse reflections. Another object of the research was to attempt to determine what characteristics, if any, of the kinetics of the decomposition of an unirradiated solid would predetermine a marked irradiation effect. It is obvious that the type of nuclear growth which occurs e.g. branching chain, or power law, does not characterise a substance with regard to a possible irradiation effect . The photosensitivity, or otherwise, also does not determine whether there will be an irradiation effect. However, the one property that the substances which have been studied, have in common, is a polyatomic anion, but here again ammonium dichromate does not show an acceleration of the decomposition after irradiation. Consequently it is considered that it is not possible to say, a priori, whether a solid will undergo an accelerated decomposition after irradiation. Each new solid, unless it belongs to a particular class e.g. the alkaline earth azides , must be considered afresh. Nevertheless it does appear that the irradiation effect can take two forms: - (i) the production of an unstable compound e.g. nickel oxalate, the decomposition of which affects the normal pyrolysis; and (ii) the production of point defects which determine the nature of the subsequent thermal decomposition e.g . CaN₆ . It is possible that the effect requires an interaction of the created point defects with the existing line defects.
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- Date Issued: 1966
The thermal decomposition of mercuric oxalate and inorganic azides
- Authors: Moore, D J
- Date: 1966
- Subjects: Decomposition (Chemistry) , Oxalates -- Thermal properties , Mercuric Oxide -- Azides
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:4483 , http://hdl.handle.net/10962/d1012878
- Description: The chemical reactivity of a solid is influenced to a marked degree by the presence of imperfections or defects in the solid. Bond strengths are considerably weaker at points of imperfection than elsewhere in the solid, and hence the initiation of reaction is favoured at these sites due to the relative ease of bond rupture. Line defects, such as edge or screw dislocations, jogs, Smekul cracks etc, are of prime importance in such changes. The surface of a solid or in intergranular boundaries, where a state of strain exists, are also favourable places for the initiation of a reaction, Point defects e.g. vacancies or interstitialions or atoms also play important roles in chemical change, often in conjuction with line defects.
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- Date Issued: 1966
An x-ray investigation of the thermal decomposition of unirradiated and irradiated silver permanganate.
- Authors: Woods, Geoffrey Steward
- Date: 1963
- Subjects: Decomposition (Chemistry) , Materials -- Thermal properties , Solids -- Thermal properties , Permanganates , Silver compounds
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4511 , http://hdl.handle.net/10962/d1013450
- Description: [From Introduction] The first step in the study of the thermal decompositions of solids is an examination of the kinetics, since this casts much light on the mechanism of the reaction. It must be borne in mind, however, that a theoretical expression, derived on the basis of a particular mechanism, even if it fits the observed experimental results, is not conclusive proof of the validity of the mechanism when applied to the decomposition under examination.
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- Date Issued: 1963
The thermal decomposition of silver oxide
- Authors: Herley, Patrick James
- Date: 1960
- Subjects: Silver oxide -- Thermal properties , Decomposition (Chemistry)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4525 , http://hdl.handle.net/10962/d1015017
- Description: [From Introduction]. The thermal decomposition of solids is characterized by the formation and growth of nuclei at sites on the surface of the solid or within the crystal lattice. Such nuclear formation is favoured by disorganisation of the crystal either by mechanical damage, or by the presence of impurities. Disorganisation results in positions which have a high thermodynamic instability. The nuclei are likely to be formed initially at the corners and the edges of the crystal since these are more prone to damage. Careful handling and storage in vacuo often leads to a large reduction in their number, while deliberate scratching of the surface facilitates their production. The number of potential sites for nuclear formation is also increased by pre-irradiation with ultra-violet light, though there are indications that a different type of nucleus may be produced. Nucleation can be facilitated by pre-irradiation with electrons, neutrons, X-rays, gamma-rays and atomic particles. The nature of the nuclei is not always clearly defined, but it is generally accepted that they are composed of solid reaction products e.g. in the decomposition of barium azide and silver oxalate, nuclei of metallic barium and silver, respectively, are formed.
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- Date Issued: 1960
The thermal decomposition of irradiated silver permanganate
- Authors: Sole, Michael John
- Date: 1959
- Subjects: Decomposition (Chemistry) , Irradiation , Permanganates , Silver compounds , Metals -- Thermal properties
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4502 , http://hdl.handle.net/10962/d1013265
- Description: The thermal decomposition of silver permanganate, pre-irradiated in BEPO and in a ⁶°C₀ Ϫ 'hot spot' has been investigated in the temperature range 100 - 125°C. The results are similar to those for irradiated KMn0₄ and the mechanism proposed for the latter is again suggested. The activation energy for the migration of point defects over the induction period is 1.03 ev. The decompositions of unirradiated and irradiated crystals differ in that the latter undergo physical disintegration over the acceleratory period. X-ray studies immediately prior to disintegration show strain and fragmentation in the irradiated crystal. An explanation involving the annealing of point defects at dislocation is advanced to explain the changes produced in the p/t plots with increased dosage, and fixed decomposition temperature. Summary, p. 94.
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- Date Issued: 1959