The effect of sample mass, heating rate and partial pressure of carbon dioxide on TG, DTG and DTA curves for the decomposition
of some common carbonates has been investigated. These variables gave a marked effect, similar in magnitude for both DTG and
DTA. The effect of sample mass, or depth of undiluted sample, is shown to be due to an increase in the partial pressure of
carbon dioxide within the reacting powder. This effect is most pronounced in nitrogen but is much reduced in carbon dioxide.
Inert diluents have little effect on the curves since they do not increase the partial pressure of CO2. The first stage of the decomposition of dolomite (CaMg(CO3)2) varies with increasing partial pressure of carbon dioxide in an anomalous manner and hence the effects of these procedural
variables (except heating rate) are not similar to those observed for magnesite (MgCO3) and calcite (CaCO3). The second stage is, however, strongly dependent on these variables and behaves in a manner that would be predicted for
a sample of calcite diluted with magnesite.
A theoretical approach has been used to show that, except for certain types of reaction mechanism, the ease with which it
is possible to distinguish the form of the reaction mechanism by the reduced-time plot method depends particularly on the
rate of transfer of heat into the sample. The original reduced-time plots  were calculated from model equatioons which
assume that the sample is, from the outset, at a fixed temperature and remains under isothermal conditions throughout the
reaction. The variations produced in the appearance of reduced-time plots when the sample is programmed to rise to a given
fixed temperature through various temperature schedules have been investigated. It is shown that even relatively rapid temperature
rises can produce distortion of the reduced-time plots for various reaction equations. If the reaction mechanism is known,
however, fairly accurate values of the activation energy for the reaction can be determined, even when the furnace used has
relatively poor heat-transfer characteristics.
Polysilanes have been synthesised as potential precursors for silicon carbide fibres. One critical property of these precursors
is the ceramic yield obtained on pyrolysis. This is determined by TG and typical curves are shown. A second important property
is the suitability for spinning a fine, flexible fibre. The characterisation of ‘spinnability’ is subjective but one of the
aims of this work has been to define more precisely the temperature at which to attempt spinning. This has involved the use
of TMA and DSC.
The SHS route is based on the well-known thermite reaction, in which a strongly exothermic reaction can sustain itself and
propagate in the form of a combustion wave until the reactants have been completely consumed. The successful application of
the method to the synthesis of superconducting ceramics of stoichiometry RBa2Cu3Oy (R=Y, Er, Yb) is reported. The 123 phase was obtained when pellets of R2O3, BaO2 and Cu metal in the correct proportions were dropped into a heater held at 800°C in an oxygen atmosphere and left there for
only 10 minutes. Thermal methods (DSC and DTA) are excellent techniques with which to investigate the dependence of the reaction
on heating rate, atmosphere and starting composition.
Authors:S. J. Stevens, R. J. Hand, and J. H. Sharp
The cristobalite α-β inversion has been studied using DSC on cristobalites produced by firing high purity quartz with and without addition of a mineraliser. If no mineraliser was used, the inversion temperatures and hysteresis on heating and cooling increased with firing temperature. Firing time had little or no effect on inversion temperature. When a mineraliser was used, the same general trend was observed with increases in firing time at low temperatures leading to splitting of the inversion peak. The amount of mineraliser added had little effect. Tridymite inversions were also observed. The results are explained in terms of the degree of order of the cristobalite structure.
Authors:J. Brockman, N. Sharp, R. Ngwenyama, L. Shelnutt, and J. McElroy
Methylmercury (meHg) is a known toxin commonly found in fish. Fish is also a rich source of the trace nutrient selenium which
has been hypothesized to modify the toxicity of meHg. We analyzed 28 samples of commercially packaged albacore and light tuna
fish for selenium and mercury using standard comparator instrumental neutron activation analysis. Significant differences
in the concentration of mercury and selenium were associated with the type of fish, brand and batch. Fish consumers should
vary the brand of tuna fish to avoid routine consumption of a brand high in mercury.
Authors:J. H. Sharp, F. W. Wilburn, and R. M. McIntosh
TG, DTG and DTA curves of magnesite are dependent on procedural variables, especially sample mass, heating rate and partial pressure of carbon dioxide, in a similar manner to those of calcite , although the magnitude of the effect is less for magnesite. The first stage of the decomposition of dolomite varies with increasing partial pressure of carbon dioxide in an anomalous manner and hence the effects of these procedural variables (except heating rate) are not similar to those observed for magnesite and calcite. The second stage of the decomposition of dolomite is, however, strongly dependent on these procedural variables and behaves in a manner that would be predicted for a sample of calcite diluted with magnesia. A 1∶1 molar mixture of magnesite and calcite also behaves as would be predicted from the behaviour of the single carbonates but differently from that of dolomite.
Authors:F. W. Wilburn, J. H. Sharp, D. M. Tinsley, and R. M. McIntosh
The effect of procedural variables, including sample mass, heating rate, particle size and partial pressure of carbon dioxide, on TG, DTG and DTA curves for the decomposition of A. R. calcium carbonate and limestone has been studied. Such variables have a marked effect, similar in magnitude for both DTG and DTA. The effect of sample mass, or depth of undiluted sample, is shown to be due to an increase in the partial pressure of carbon dioxide within the reacting powder and has been called the bed-depth effect. This effect is most pronounced in nitrogen but is much reduced in carbon dioxide. Inert diluents have little effect on the TG curves but changing the composition of the inert carrier gas causes variations which are correlated with the thermal conductivity of the gas. Water vapour causes a lowering of the DTG and DTA peak temperatures.