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An X-ray diffraction study of the crystalline to amorphous phase change in cellulose during high-energy dry ball milling

In methods used widely for determining crystallinity of cellulose substrates by X-ray wide angle diffraction the scattering characteristics of truly amorphous cellulose is needed. So far, such scattering curves have not been available. It was the aim of this study to arrive at an experimentally determined scattering curve of amorphous cellulose. For this purpose, two cellulose samples (a pine wood pulp and a microcrystalline powder) were ball milled in a high-energy equipment in order to destroy the crystalline structure. The structural changes in the cellulose samples and the kinetics of amorphization were followed by taking X-ray diffractograms. After 30min of milling, no more change in the structure could be observed and the diffraction curve of amorphous cellulose with two diffuse maxima at spacings of 0.427nm (strong) and 0.247nm (weak) was obtained. The amorphization was found to follow a first order kinetics, the rate constant of k=(0.0033±0.0004)s -1 holding for both samples. However, while the amorphization of the microcrystalline cellulose started immediately, with the wood pulp sample an induction period of about 300 seconds was observed, which is probably due to residual morphological structures present in this sample.

Crystal growth of lead fluoride using the constant composition method II. The effect of Pb/F activity ratio on the kinetics of crystal growth

Abstract A potentiometric pF-stat method has been used for the investigation the kinetics of lead fluoride crystal growth on seeds (α-PbF 2 ) at 25°C. The kinetics was studied as a function of ionic strength, pH and lead-to-fluoride ions activity ratio, which were changed only by changing Pb(NO 3 ) 2 and KF concentrations. Depending on the excess of lead(II) nitrate or potassium fluoride in supersaturated solution two different crystal phases grow: (a) In solutions with large excess of Pb(NO 3 ) 2 and corresponding pH =4.35−5.00, a large needle-like crystals with some sponge-like precipitate are grown. The effective order of crystallization of this mixture is 1.55, indicating preferentially one-dimensional growth of needles. (b) In solutions with KF in excess, and consequently with pH =5.0–7.0, however, voluminous bipyramids of orthorhomic α-PbF 2 crystals are grown. The effective order of crystallization of this known phase is 4.0, i.e. presumably an exponential rate law is valid.

Microstructure evolution of an equimolar powder mixture of ZrO2-TiO2 during high-energy ball-milling and post-annealing

Abstract High-energy dry ball-mill and post-anneal processing were applied to synthesise ZrTiO 4 oxide powder from an equimolar TiO 2 –ZrO 2 powder mixture. X-ray diffraction was used to identify structural changes in the milled and the subsequently post-annealed samples. Applied milling assemblies, vials and balls made of WC–Co hard-metal and also of agate materials, were successful in such way that a fully amorphous structure appeared in the samples if milling lasted long enough. The milling time required for complete disordering was almost five times longer with the agate assembly (about 50 h) in comparison with the WC–Co one (about 10 h). Also, the hard-metal assembly incorporated in the milled samples some amount of a WC impurity due to contamination, whereas contamination of samples by agate material was not observed by XRD. The preparation of the crystalline ZrTiO 4 powder phase from the amorphous one, was possible if the samples were exposed to a subsequent heat treatment. The structural transformations occurring during heating the samples went in two different ways depending upon the previously used milling assembly. In the first case annealing of samples performed for 1 h at 650°C caused a structural transformation of the WC phase into the WO 3 phase, whereas this temperature was not high enough to allow the crystallisation of the amorphous ZrTiO 4 . Annealing of the samples at 900°C for 1 h caused complete crystallisation of the amorphous powder into the crystalline ZrTiO 4 phase, without any evidence of the presence of the former W-base phases. Apparently this temperature favoured the formation of the solid solution of the WO 3 phase in the ZrTiO 4 crystalline phase. The crystallisation of the amorphous ZrTiO 4 phase in samples milled in an agate assembly was possible even at 650°C, although at this temperature it was a very slow process, and prolonged annealing time was necessary. If the samples, milled in an agate assembly up to 50 h, were heated at 900°C for 1 h, they can fully transform into the crystalline ZrTiO 4 phase.

Crystal Growth of Lead Fluoride Phases Using the Constant Composition Method. III. Effect of pH and Ionic Strength

Abstract The kinetics of crystal growth of the lead fluoride phases was studied at 25°C using the so-called constant composition method (pF-stat method). A linear growth law was obeyed for αPbF 2 growth from equivalent lead nitrate and potassium fluoride solutions with low supersaturation and pH between 5.2 and 5.6 in pure water and in 0.1 mol dm −3 KNO 3 solution. The rate is about 5 times greater at high ionic strength. It decreases linearly with changing pH of the supersaturated solution both to higher and to lower values than mentioned (addition of KOH and HNO 3 , respectively), i.e. reaching the pHs at which two other lead fluoride phases grow: (a) with large excess of Pb(NO 3 ) 2 over KF and KOH to pH = 5.2 phase(s) of so far unresolved structure, which undergoes the recrystallization in water to pure αPbF 2 , and after annealing to 450°C decomposes into β-PbF 2 , and an unidentified phase (probably α(K N Pb N−1 )F 2−N phase with N varied); (b) with large excess of KF over Pb(NO 3 ) 2 and HNO 3 to pH = 4.3, the βPbF 2 phase was identified. The characterization of the grown phases was done on the basis of polarizing microscopy, Fourier transform infrared (FTIR) spectra and X-ray powder diffraction analysis.

Microstructural and microhardness study of the La2−xSrxCuO4 high-temperature ceramic superconducting system

Abstract Microstructural behaviour and microhardness of the La 2− x Sr x CuO 4 alloy system was studied using microhardness testing and observations with light and scanning-electron microscopes. Two possible processes for the grain growth in the course of sintering were observed. Primary recrystallization is operative for strontium content x x > 0.10. Consequently, the threshold composition is about x = 0.10. For the latter case ( x > 0.10) unusually large grains appear, with a frequency distribution similar to the expected log-normal distribution. The microhardness shows a sharp minimum near x = 0.06. The influence of the microstructure on the measured transport properties is also discussed, particularly the treatment-induced superconductivity of the parent compound.

Microhardness of the YbxY1-xInCu4 alloy system: the influence of electronic structure on hardness

We show that the Vickers microhardness, measured on flux grown single crystals of the YbxY1?xInCu4 alloy system, although sample dependent, exhibits clear concentration dependence; it increases with decreasing x. Such a dependence is not expected because the cubic lattice parameter increases with decreasing x and one expects then a decrease of hardness with decreasing x. Also, such a concentration dependence is in accordance with neither the Mott?Nabarro theory nor other known experimental results. We ascribe the observed dependence to the change of the electronic structure of the YbxY1?xInCu4 alloy system with x.

Non-ionic surfactant adsorption on lead fluoride under dynamic steady-state conditions

Abstract The adsorption of non-ionic surfactant Triton X-100 on the surface of lead fluoride microcrystals has been examined as a function of the mutual mass ratio of surfactant and α-PbF2 crystals during their growth under dynamic steady-state conditions, i.e. using the potentiometric pF-stat method, known as the constant composition method, which means: constant supersaturation (constant activity of lattice ions during the crystal growth process); constant pHeq = 5.30; constant relatively low ionic strength I = 0.018 mol dm−3; temperature 25°C; the same method of preparation of the stable supersaturated solution; the same seed crystals and speed of mixing; constant Triton X-100 concentration above the cmc. The concentration of unadsorbed Triton X-100 in supersaturated solution, i.e. the adsorption isotherms, were determined by UV-visible spectrophotometry at different growth times. A very broad concentration region was examined from the cmc up to 33 × cmc. Two-step adsorption isotherms, i.e. LS-type, were obtained for all systems under dynamic steady-state conditions, which then led to the L-type isotherm in the same concentration region of non-ionic surfactant, when systems were allowed to stay for 1 day after running the growth experiment (batch thermodynamic equilibrium adsorption isotherm). Strong interactions between the terminal OH group of the Triton X-100 end molecule and F− ions at the lead fluoride crystal surface, in the region between cmc and about 10 × cmc (c1), led to the first step of the isotherms, and also to the one-step, L-type equilibrium isotherm afterwards. The second step of the LS-type isotherms, which appears in the region above c1, is a consequence of the cooperative process between the surfactants hydrophilic chains and/or of the hydrophobic parts of their molecules. The experimental data for the LS-type isotherms were fitted by a recently proposed equation [B.-Y. Zhu, T. Gu, Adv. Colloid Interface Sci. 37 (1990) 1] and the following parameters were estimted: maximum adsorbed amounts; hemimicelle concentrations; aggregation number of hemimicelles; adsorption constants k1 and k2 for both processes, taking place at first and second steps of the LS isotherms; the Gibbs free energy of hemimicellization at 25°C. The aggregation number of the hemimicelles was found to be around six and the free energy of hemimicellization was about −10 kJ mol−1.