X-ray diffraction studies on aluminum-substituted barium hexaferrite

Effect of aluminum substitution in barium hexaferrite was studied following the hydrothermal precipitation–calcination techniques. It was attempted to prepare aluminum-substituted barium hexaferrites with compositions BaAl_xFe_12−xO₁₉ having x=2,4, 6, 8 and 10. The precursors were prepared by using stoichiometric amounts of Ba, Al and Fe³⁺ nitrate solutions with urea as the precipitating agent. The hydrothermally prepared precursors were calcined at temperatures in the range of 800–1200 °C. The detailed work carried out on identification of crystalline phases through XRD revealed that, after the hydrothermal treatment, the samples showed barium carbonate, hematite and boehmite as the crystalline phases (except that boehmite was not identified for Ba/Al/Fe ratio as 1:2:10). Irrespective of the Al content, none of the 1000 and 1200 °C calcined samples showed any crystalline phase of Al. The 1200 °C calcined samples showed that Al-substituted barium hexaferrite is formed only with the Ba/Al/Fe atomic ratio of 1:2:10. With increase in the Al content, formation of hexaferrite was not observed. BaCO₃ was found be present even at 1200 °C in all the samples except for the one having Fe/Al ratio 5. The normal decomposition temperature of BaCO₃ is between 950 and 1050 °C. It is difficult to explain the increased stability of BaCO₃, which is perhaps responsible for hindering the formation of aluminum-substituted barium ferrite having Fe/Al ratio ≤2. The Al substitution in barium hexaferrite was confirmed through magnetic measurements.     

Electrical resistivity of Acacia and Eucalyptus wood chars

Electrical resistivity of Acacia and Eucalyptus wood chars, produced under different carbonization conditions, were measured using a two-probe technique. The results have shown that the final carbonization temperature is the most important factor because of its strong influence on the wood char resistivity. The electrical resistivity of these wood chars decreased drastically on increasing the carbonization temperature up to about 800 °C followed by a slight decrease with further rise of temperature up to 1200 °C. The wood species and increase in soaking time at carbonization temperatures of 800 and 1000 °C also influenced the char resistivity. It is hoped that the electrical resistivity values of wood chars can be used to determine their preparation temperature and to have a better control of the operation of electrosmelting furnaces using wood char as reductant.     

X-ray diffraction and ice nucleation studies of Agl-AgBr-Cul system

Lattice parameter calculations and ice nucleation studies for the Agl-AgBr-Cul system are reported in this study. Thirteen samples of different molar compositions were prepared by vacuum fusion. The X-ray powder diffractograms of these samples revealed the formation of cubic solid solutions of the above system. It is found that the 50:25:25 mol% composition of Agl, AgBr and Cul, has the highest nucleation temperature of –0.48° C, which is far greater than –2.53 and –2.26° C for pure Agi and Cul, respectively.     

Transmission electron microscopy and X-ray diffraction studies of quantum wells

A series of In _x Ga_1?x As (x=0·47) quantum wells with InP barrier layers have been grown on InP substrates by metalorganic vapour phase epitaxy (MOVPE) at 625°C. The nominal well widths were defined during growth at (i) 25 Å, 39 Å, 78 Å and 150 Å for one sample and (ii) 78 Å for all 4 wells in another sample. The InP barrier widths have been kept constant at 150 Å. These layers have been characterized by X-ray diffraction (XRD) which from simulation gave the nominally 78 Å well width as 84 Å and the nominally 150 Å barrier width as 150·5 Å. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) have been carried out on etched and ion-milled samples for direct measurement of well and barrier widths. The well widths found from TEM are 25 Å, 40 Å, 75 Å and 150 Å. TEM micrographs revealed that, while the InP barrier layer is of good quality and the growth is confirmed to be epitaxial, dipoles are detected at the interface and the quantum well has some small disordered regions. These thickness measurements are in good agreement with earlier photoluminescence (PL) and secondary ion mass spectrometry (SIMS) studies.     

X-ray diffraction studies of GdFe2 Mössbauer samples

X-ray diffraction techniques have been used to determine the lattice parameters and stacking fault probabilities of specimens of the cubic Laves phase compound GdFe₂ used in Mössbauer experiments. The lattice parameter for most samples was 7.400±0.005 Å. All the samples exhibited high stacking fault probabilities. The sta-cking fault probabilities lay in the range 0.12 to 0.35.     

In-situ high temparature X-ray diffraction studies of nanocrystalline iron carbides

Three samples of passivated ultrafine iron carbide particles, synthesized by laser induced pyrolysis of gaseous precursors were examined by in-situ high temperature XRD during and after high temperature (600°C) treatment under UHP He atmosphere. The results indicated that the particles sintered around 400°C, with the preservation of the carbide structure dependent on the precursor composition and the amount of retained oxygen from passivation and synthesis. Transformation of the carbide to α-Fe around 400°C was dependent on the precursor purity, temperature and type of gaseous exposure. Preservation of the carbide structure on heating was dependent on the purity of the carbide, and the concentration of oxygen and carbon in the particles. A topotactic transformation from Fe₇C₃ to Fe_0.98O was observed in the case of samples studied under UHP He, which was absent in the other two treatments and is discussed relative to the as-synthesized/passivated particles composition and structure. This topotaxy leads to the selective formation of γ-Fe₂O₃ from specific ultrafine particle carbides when treated with UHP He.     

X-ray diffraction studies of phase transformations in heavy-metal fluoride glasses

The crystallization of five ZrF₄-based glasses has been investigated using powder X-ray diffraction and differential scanning calorimetry. The crystalline phase in Zr-Ba-La-Pb fluoride glass was found to be-BaZrF₆. In other glasses the crystal phases could not be identified. Reversible polymorphic phase transformations occur in Zr-Ba-La-Li and Zr-Ba-La-Na fluoride glasses when heated to higher temperatures.     

Scanning electron microscopic study of acacia and eucalyptus wood chars

SEM studies of acacia and eucalyptus wood chars, prepared under different carbonization conditions, were undertaken to provide information on what happens in the transformation of wood to chars. The material normally lost as volatiles contributes totally to the formation of pores, cracks and pyrolytic carbon. Both woods exhibited similar devolatilization behaviour in pore structure development, crack formation and pyrolytic carbon deposition, showing a decrease in pore size with an increase in carbonization temperature and cracks/voids formation during rapid carbonization at higher temperatures (i.e. 800–1050 °C). Slow carbonization led to pyrolytic carbon deposition in resulting wood char structures and did not disturb the fibrous nature and cell structures of the wood, even at a high carbonization temperature of 1200 °C. Prolonged heating at carbonization (slow) temperatures of 800 and 1000 °C caused sintering of the adjacent fibres resulting in the formation of compacted mass.     

Structural studies of Langmuir-Blodgett multilayers by means of soft X-ray diffraction

The specularly reflected intensity from cadmium arachidate multilayers has been measured as a function of incidence angle. The films were prepared by the Langmuir-Blodgett technique and ranged from three to 21 layers in thickness. Monochromatic synchrotron radiation of 1560 eV photon energy (wavelength 7.95 Å) was used in combination with a high vacuum compatible triple-axis diffractometer. The measured diffraction profiles were fitted with a theory based on the exact solution of the Fresnel-Bragg equations for multiple reflections including absorption. The optical parameters for the layers were taken from tabulated values, and the layers were constructed using a formulation similar to that employed previously by Pomerantz and Segmüller. The individual layer spacing as well as the total thickness of the film could be derived to within an accuracy of 1% or 1 Å (whichever is larger).     

Energy-dispersive X-ray diffraction studies of the texture in cold-rolled alpha-beta brass

It is shown that energy-dispersive X-ray diffraction can be used for simultaneous measurement of several pole figures and that the accuracy is sufficient for the determination of the crystallite orientation distribution. The method is applied to the study of the texture in Cu-43 wt % Zn duplex alpha-beta brass rolled to 80% reduction. Some preliminary results of this work were presented at the Fifth International Conference on Texture of Materials, RWTH Aachen, March 29–31, 1978. On leave from Institute of Physics and Nuclear Techniques, University of Mining and Metallurgy, Cracow, Poland. Now returned.     

XPS and X-ray diffraction studies of aluminum-doped zinc oxide transparent conducting films

Aluminum-doped zinc oxide transparent conducting films are prepared by spray pyrolysis at different dopant concentrations. These films are subsequently characterized by X-ray diffractometric and X-ray photoelectron spectroscopic (XPS) techniques. The results are compared with those obtained from pure zinc oxide films prepared under identical conditions. X-ray diffraction measurements show an increase in lattice parameters (c and a) for aluminum-doped films while their ratio remains the same. This study also indicates that within the XPS detection limit the films are chemically identical to pure zinc oxide. However, a difference in the core-electron line shape of the Zn 2p_3/2 photoelectron peaks is predicted. An asymmetry in Zn 2p_3/2 photoelectron peaks has been observed for aluminium-doped films. The asymmetry parameters evaluated from core-electron line-shape analysis yield a value of the order of 0.04±0.01. The value is found to lie between those obtained for pure zinc oxide and has been attributed to the presence of excess zinc in the films.