Browsing by Author "Campbell, S.J."

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A Mössbauer effect study of barium ferrite ball-milled in air
(Hyperfine Interactions, 1994) Jayasuriya, K.D.; Kaczmarek, W.A.; Wu, E.; Campbell, S.J.
The effects of dry-milling BaFe12O19 in air for periods of 190, 360, 590, 690 and 1000 h have been investigated by X-ray diffraction and M�ssbauer effect measurements. The sizes of the BaFe12O19 particles decrease on milling, as expected, although a partial decomposition of BaFe12O19 to ?-Fe2O3 is found to take place on extended milling (1000 h). The room temperature M�ssbauer spectra are consistent with superparamagnetic relaxation associated with the fine BaFe12O19 and ?-Fe2O3 particles. The X-ray diffraction patterns of all the milled samples exhibit features indicative of a disordered structural state, consistent with the nanoscale particles and a nanostructured state.
A Mössbauer study of ball-milled Co?Fe?Si?B: I: dry milling
(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1993) Jayasuriya, K.D.; Campbell, S.J.; Calka, A.; Jing, J.
X-ray diffraction and M�ssbauer effect measurements of ball-milled Co?Fe?Si?B material which had first been re-crystallized from amorphous melt-spun ribbon are presented. Fully amorphous powders have been obtained after only 50 h milling with a ball-to-powder ratio of BPR ? 84. The amorphous powders exhibit chemical and structural inhomogeneities compared with the starting melt-spun amorphous ribbons.
A Mössbauer study of ball-milled Co?Fe?Si?B: II: with surfactants
(Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1993) Jayasuriya, K.D.; Campbell, S.J.; Calka, A.; Jing, J.
The use of a range of organic surface active substances in the ball milling of Co?Fe?Si?B has been studied by X-ray diffraction and M�ssbauer effect measurements. Effects characteristic of superparamagnetic relaxation of the magnetic moments are observed in the M�ssbauer spectra of samples milled with either an anionic or a cationic surfactant. This behaviour is linked with the decreased particle size and high shape homogeneity of surfactant-aided milled samples compared with the amorphous powders resulting from standard dry milling. Impurity iron-oxide phases are also found to occur in samples milled in the presence of surfactants.
A Mössbauer study of the oxidation state of Fe in silicate melts
(American Mineralogist, 2004) Jayasuriya, K.D.; O-Neill, H.S.C.; Berry, A.J.; Campbell, S.J.
Fe3+/?Fe ratios were determined from M�ssbauer spectra recorded for a series of 17 anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities from fO2~ 105 bars (Fe3+/?Fe = 1) to 10?13 bars (Fe3+/?Fe = 0) at 1682 K. Fe3+/Fe2+ was found to be proportional to fO2 to the power of 0.245 � 0.004, in excellent agreement with the theoretical value of 0.25 expected from the stoichiometry of the reaction Fe2+O + 0.25 O2 = Fe3+O1.5. The uncertainty in the Fe3+/?Fe ratios determined by M�ssbauer spectroscopy was estimated as � 0.01 (1?) from the fit of the data to the theoretical expression, which is significantly less than that quoted for previous measurements on silicate glasses; this results from fitting the spectra of a large number of systematically varying samples, which allows many of the ambiguities associated with the fitting procedure to be minimized. Fe3+/?Fe ratios were then determined for samples of the anorthite-diopside eutectic composition equilibrated at selected values of fO2, to which up to 30 wt% Fe2O3 had been added. Fe3+/?Fe was found to vary with ?Fe (or FeOT), but both the 1 wt% and high FeOT data could be satisfactorily fit assuming the ideal stoichiometry (i.e., Fe3+/Fe2+ ?fO2 1/4) by the inclusion of a Margules term describing Fe2+-Fe3+ interactions. The large negative value of this term indicates a tendency toward the formation of Fe2+-Fe3+ complexes in the melt. The resulting expression, using the ideal exponent of 0.25, gave a fit to 289 Fe3+/?Fe values, compiled from various literature sources, of similar quality as previous empirical models which found an exponent of ~0.20. Although the empirical models reproduce Fe3+/?Fe values of glasses with high FeOT reasonably well, they describe the data for 1 wt% FeOT poorly. The non-ideal values of the exponent describing the dependence of Fe3+/?Fe on fO2 at high FeOT are an artifact of models that did not include a term explicitly to describe the Fe2+-Fe3+ interactions. An alternative model in which Fe in the silicate melt is described in terms of three species, Fe2+O, Fe3+O1.5, and the non-integral valence species Fe2.6+O1.3, was also tested with promising results. However, at present there is no model that fits the data within the assessed accuracy of the experimental measurements.
A specific heat study of natural haematite around the Morin transition and the effects of entrapped water
(Journal of Physics and Chemistry of Solids, 1985) Jayasuriya, K.D.; Stewart, A.M.; Campbell, S.J.
The Morintransition in samples of natural haemitite has been examined by specificheat and M�ssbauer measurements. Specificheat measurements on as-received samples reveal an anomaly at ? 273.2 K and irregular behaviour at higher temperatures. However, heat treatment of samples demonstrated that these effects were due to water trapped inside the natural specimens. No anomaly due to the Morintransition was observed in the specificheat although the expected changes in the M�ssbauer hyperfine parameters were observed for the different magnetic phases below and above the Morintransition. The results are consistent with a coexistence of the two phases and a spread of temperatures ? 40 K over which the transitions take place throughout the material of purity 99.6 wt%.
An experimental determination of the effect of pressure on the Fe3+/?Fe ratio of an anhydrous silicate melt to 3.0 GPa
(American Mineralogist, 2006) Jayasuriya, K.D.; O-Neill, H.S.C.; Berry, A.J.; McCammon, C.C.; Campbell, S.J.; Foran, G.
The effect of pressure on the Fe3+/?Fe ratio of an anhydrous andesitic melt was determined from 0.4 to 3.0 GPa at 1400 �C with oxygen fugacity controlled internally by the Ru + RuO2 buffer. Values of Fe3+/?Fe were determined by M�ssbauer spectroscopy on quenched glasses with a precision of �0.01, one standard deviation. This precision was verified independently by XANES spectroscopy of the same samples. The XANES spectra show a systematic increase in energy and decrease in intensity of the 1s ? 3d transition with increasing pressure. The results to 2.0 GPa are in good agreement with predictions from density and compressibility measurements fitted to a Murnaghan equation of state, but the datum at 3.0 GPa has higher Fe3+/?Fe than predicted from the trend established by the lower-pressure data. This might be due to a coordination change in Fe3+ at high pressure; although there is no evidence for this in the M�ssbauer spectra, such a change could account for the change in intensity of the 1s ? 3d transition in the XANES spectra with pressure.
Crystallization and preliminary x-ray analysis of shikimate dehydrogenase from Escherichia coli
(Wiley-Blackwell, 2000) Maclean, J.; Campbell, S.J.; Pollock, K.; Chackrewarthy, S.; Coggins, J.R.; Lapthorn, A.J.
Abstract
Magnetic transitions in dysprosium: A specific-heat study
(Physical Review B, 1985) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.
The magnetic transitions of the heavy rare-earth metal dysprosium have been examined by specific-heat measurements using a small single crystal (0.6782 g) and a large polycrystalline sample (43.25 g). A value for the latent heat of 39.1�1.5 J mol-1 is obtained at the discontinuous transition from ferromagnetism to helical antiferromagnetism, and a temperature hysteresis of 1.2�0.4 K is observed at this transition. The data around the continuous transition from helical antiferromagnetism to paramagnetism at the N�el temperature TN=179.90�0.18 K have been analyzed in terms of the equation & &)+B�+D�t, where t=(T-TN)/TN. Agreement to within two standard errors was obtained for the critical parameters of the single-crystal and polycrystalline data. The value of the critical exponent ? obtained from the data of the single crystal is 0.24�0.02, whereas that obtained from the data of the polycrystalline sample is 0.14�0.05. We find that the scaling law ?+=?- is valid for the dysprosium critical data. In order for the condition B+=B-, predicted by theory, to be satisfied we found it necessary to include the confluent singular term in E in our fit. The values of ? that we obtain are not close to the predictions of renormalization-group theory for magnetic systems with short-range exchange interactions.
Specific heat study of a holmium single crystal
(Journal of Physics F: Metal Physics, 1985) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.
The magnetic transition at the Neel temperature, TN, of the heavy rare-earth metal holmium has been examined by specific heat measurements. The transition at TN=(132.245+or-0.080)K is found to be continuous and the data around TN have been analysed in terms of the equation C+or-=(A+or-/ alpha +or-) mod t mod - alpha +or-(1+E+or- mod t mod x+or-)+B+or-+D+or-t where t=(T-TN)/TN. The value of the critical exponent alpha is 0.27+or-0.02 for the data in the critical regions -3.8
Specific heat study of the transition from ferromagnetism to antiferromagnetism in terbium
(Journal of Magnetism and Magnetic Materials, 1984) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.; Gopal, E.S.R.
The specificheat of single crystal terbium has been measured over the range 200?400 K. A value for the latent heat of (13.6 � 0.6) J mol?1 is obtained at the first order antiferromagnetic to ferromagnetic transition temperature Tc = (221.45 � 0.03) K. A temperature hysteresis of (0.24 � 0.03) K is observed in the transition.
Surfactant assisted ball-milling of barium ferrite
(IEEE Transactions on Magnetics, 1994) Jayasuriya, K.D.; Wu, E.; Kaczmarek, W.A.; Campbell, S.J.
The effects of wet-milling barium ferrite in the presence of cationic or anionic surface active substances have been investigated by X-ray diffraction and Mossbauer spectroscopy measurements. Evidence for superparamagnetic relaxation of fine particles, both in the surfactant assisted ball-milled samples and in samples dry-milled in air is obtained from the room temperature and 4.2 K Mossbauer spectra. The similarity between samples milled at pH 3 with the cationic or anionic surfactants is due to electrostatic interactions of the surfactants' hydrocarbon groups with the positively charged barium ferrite surface
The critical specific heat of terbium
(Journal of Physics F: Metal Physics, 1983) Jayasuriya, K.D.; Gopal, E.S.R.; Campbell, S.J.; Stewart, A.M.
Specific heat measurements have been carried out on a high-purity single crystal of terbium of mass 3.734 g between temperatures of 200 and 400K. The data around the Neel temperature TN, have been analysed in terms of the equation C+or-=(A+or-/ alpha +or-) mod t mod - alpha +or-(1+E+or- mod t mod x+or-)+B+or-+D+or-t where t=(T-TN+or-)/TN+or-. The parameters obtained from our best fit are alpha += alpha =-0.20+or-0.03, A+/A-=0.58+or-0.34, B+=B-=21.68+or-13.93 J mol-1 K-1, E+/E-=-2.06+or-4.12, D+=D-=4.99 J mol-1 K-1 and TN+=TN-=229.9503+or-0.0035K. This fit is valid in the critical regions -3.4
The specific heat capacity of GE varnish (200-400K)
(Journal of Physics E: Scientific Instruments, 1982) Jayasuriya, K.D.; Campbell, S.J.; Stewart, A.M.
The specific heat capacity of GE varnish, a commonly used adhesive and insulator, is found to vary linearly with temperature over the range 200-330K. The deviation of the heat capacity above this temperature is associated with the approach to the curing temperature of this resin.
The specific heat of samarium metal (80-400 K)
(Materials Chemistry and Physics, 1986) Jayasuriya, K.D.; Stewart, A.M.; Campbell, S.J.
We have measured the specific heat of several high purity polycrystalline specimens of samarium metal over the temperature range 80 K to 400 K. A broad peak is observed at TNh ~106K, the temperature at which antiferromagnetic ordering first sets in with ordering of the moments on hexagonal sites. This behaviour contrasts with the sharp peak observed by previous workers on samarium samples of lower purity.
XANES calibrations for the oxidation state of iron in a silicate glass
(American Mineralogist, 2003) Jayasuriya, K.D.; Berry, A.J.; O?Neill, H.S.C.; Campbell, S.J.; Foran, G.J.
Fe K-edge X-ray absorption near edge structure (XANES) spectra were recorded for a series of anorthite-diopside eutectic glasses containing 1 wt% 57Fe2O3 quenched from melts equilibrated over a range of oxygen fugacities at 1409 �C. The Fe3+/?Fe ratios were determined previously by 57Fe M�ssbauer spectroscopy and vary between 0 (fully reduced) and 1 (fully oxidized). Using the M�ssbauer results as a reference, various methods for extracting Fe3+/?Fe ratios from XANES spectra were investigated. The energy of the 1s ? 3d pre-edge transition centroid was found to correlate linearly with the oxidation state. Correlations also exist with the energy of the K absorption edge and the area of peaks in the derivative spectrum associated with the 1s ? 4s and crest (1s ? 4p) transitions. The Fe3+/? Fe ratios determined from linear combinations of end-member spectra (Fe3+/? Fe ~0 and ~1) were found to deviate significantly from the M�ssbauer values. This may indicate the susceptibility of this method either to errors arising from the treatment of the background or to changes in Fe2+ or Fe3+ coordination with the Fe3+/? Fe ratio. The general applicability of any XANES calibration for determining oxidation states is limited by variations in the Fe coordination environment, which affects both the intensity and energy of spectral features. Thus previous calibrations based on mineral spectra are not applicable to silicate glasses. Nevertheless, systematic trends in spectral features suggest that Fe3+/? Fe values may be obtained from XANES spectra, with an accuracy comparable to M�ssbauer spectroscopy, by reference to empirical calibration curves derived from compositionally similar standards.