High pressure Raman spectroscopy of ferrite MgFe2O4

An in situ Raman spectroscopic study was conducted to explore the pressure-induced phase transformation of ferrite MgFe₂O₄ to 51.6 GPa. Results indicate that MgFe₂O₄ transforms to a high pressure polymorphism at a pressure of 27.7 GPa, which was assigned to an orthorhombic structure. Upon release of pressure to ambient conditions, this high pressure polymorphism of MgFe₂O₄ remains stable. The crystallization of high pressure phase of MgFe₂O₄ is dominated by a diffusionless crystallizing mechanism.     

Raman spectroscopy: A technique for estimating extent of polymerization in PMMA

This work shows how laser-Raman spectroscopy can be applied in cases where a combination of an efficient, relatively inexpensive, simple, and non- destructive technique is desirable for the determination of the monomer content in poly(methylmethacrylate), PMMA. The method has been initially applied in order to investigate the optimum conditions for the in situ polymerization of PMMA in the pores of silica sol-gel for the purpose building a solid state dye laser. The calibration line which converts the ratio of two Raman intensities in the PMMA spectrum (at 1640 and 1725 cm^–1) into percentage of monomer content, under the current experimental conditions, has been achieved on the basis of NMR spectroscopy. In addition, it is shown in this work how Raman spectroscopy can allow for the estimation of the reaction enthalpies of partial monomer to polymer conversion after thermal treatment at different temperatures and periods of treatment. Received: 25 August 2000 / Reviewed and accepted: 28 August 2000     

Orientation of pores in microporous polyethylene films as determined by polarized absorption spectroscopy

Microporous polyethylene films with good mechanical properties can be produced on the basis of hard-elastic specimens by uniaxial deformation. The so called ”guest-host” effect [1, 2] was explored to determine the order parameter of pores in a series of PE membranes so produced by polarized absorption spectroscopy using dichroic dyes dissolved in the nematic liquid crystal ZLI 1840 (LC) as indicator. The parameter of the orientation order S_A of the pores along the draw direction was found to increase with increasing spin draw. A qualitative comparison of S_A with the thermodynamic behavior of the dye + LC systems confined in the pores was made. The results obtained are related to the mechanism of porous structure formation. Received: 9 October 2000 / Reviewed and accepted: 18 October 2000     

Structure and property changes in certain materials influenced by the external qi of qigong

 Temperature, time, pressure (or stress) are considered important factors in changing the Gibbs free energy and optimizing the structure and properties of materials during materials processing. The effects of some other variables, including the magnetic field, electrical field, electromagnetic and ultrasonic radiation, and chemical reactions have also been well characterized. These factors have been widely applied in materials processing, and their limitations have been discovered. Thus additional factors and innovative techniques are constantly being sought to overcome those limitations. This paper presents such an innovative technique called qigong. Three sets of materials-related experiments conducted by qigong doctor Yan and his collaborators are described in which for the first time the effects of qi on inanimate matter samples with no mechanical or electrical connection to the system are revealed on laboratory instruments. These experiments show that external qi of qigong produces significant structural changes in water and aqueous solutions, alters the phase behavior of dipalmitoyl phosphatidyl choline (DPPC) liposomes, and enables the growth of Fab protein crystals. These results demonstrate objective phenomena resulting from qigong and the potential of this ancient technology system, even in material processing. Important attributes of qi are summarized and the possible implications of these results from the materials perspective are discussed. Received: 26 October 1998/Reviewed and accepted: 3 November 1998     

Thermoluminescent behaviour of diamond films subjected to UV irradiation

The thermoluminescent behaviour of diamond films subjected to UV irradiation was studied by using an UV lamp of 254 nm wavelength. The UV irradiation was achieved by placing the samples 15 cm away from an UV source for different periods. The thermoluminescent signal was integrated from 0 to 350°C at a linear heating rate of 10°C/s in a N₂ atmosphere. The corresponding luminescence spectra show an excitation band centered at 450 nm while the emission band is centered around 500 nm at room temperature. The diamond films were synthesized on molybdenum substrates by the combustion flame technique and characterized by Raman spectroscopy and scanning electron microscopy. Received: 31 May 1999 / Reviewed and accepted: 22 July 1999     

Structure characterization of heat set and drawn polyamide 66 fibers by FTIR spectroscopy

Structural changes that occur during thermally induced and strain induced crystallization of polyamide 66 fibers were studied by infrared spectroscopy, density measurement and optical microscopy. Two bands at 924 and 1136 cm^–1 were shown to arise from the amorphous phase and assignment of the bands at 936 and 1200 cm^–1 to the crystalline phase were confirmed. We demonstrated that two different infrared spectroscopic methods could be used to determine the total crystallinity of polyamide 66 fibers. One is a calibration method in which the band ratio of 1200 and 1630 cm^–1 is plotted against the crystallinity measured by density measurements. The other one is an independent infrared method. Crystallinity obtained by the independent infrared spectroscopic method showed good agreement with crystallinity observed by density measurement. Infrared dichroism was used to obtain the crystalline orientation using the band at 936 cm^–1. The transition moment angle of 48° was found for the band at 936 cm^–1 with respect to chain axis. Amorphous orientation was obtained using Stein’s equation. Received: 18 September 2000 / Reviewed and accepted: 20 September 2000     

Preparation of titanium-dioxide films by heating titanium/silicon-dioxide structures on silicon in oxygen

 When titanium/silicon-dioxide (Ti/SiO₂) structures prepared by depositing titanium (Ti) on thermally oxidized silicon in vacuum were heated to temperatures of 800–1000°C in flowing oxygen gas, silicon surfaces were covered with a mixture films containing preferentially (110)-oriented Ti0₂ instead of the SiO₂ films. The thickness of the mixture films could be determined by that of the deposited Ti films. Titanium silicide grew only in the region near between the grown mixture film and the silicon substrate. The dielectric constants of the grown mixture films increased exponentially with increasing oxidation temperature and increased slowly with increasing the Ti film thickness, while the breakdown field strength increased slowly with increasing oxidation temperature and increased exponentially with increasing the Ti film thickness. The oxide films prepared at 1000°C had dielectric constants of (15–25)ɛ_o resistivities of 10¹⁰–10¹¹ Ω cm, and breakdown field strengths of about 10⁶ V/cm. Received: 10 February 1998 / Accepted: 10 March 1998     

High temperature structural evolution of a CuAlNi alloy studied by in situ X-ray diffraction and isothermal mechanical spectroscopy

The purpose of this work is to study the microstructural mechanisms associated with the eutectoid transition in a ternary Cu–12 wt.% Al–3 wt.% Ni alloy. The samples have been initially annealed at 850 °C, then slowly cooled down to room temperature. The experiments have been carried out both on cooling and on heating above 500 °C using isothermal mechanical spectroscopy and X-ray diffraction (fitted with a temperature camera). On heating, a relaxation peak with a high intensity rises up above 600 °C, then on cooling, the peak totally disappears below 580 °C, the effect being reproducible. The structural analysis, undertaken in the same temperature domain, has clearly evidenced each step of the evolution, particularly the eutectoid transformation. Consequently, the damping effect seems to be associated to the presence of the high temperature β phase.     

Microstructural characteristics of alumina-based composite prepared by in situ reaction of alumina-silicon carbide-nickel system

 In situ reaction of nickel and silicon carbide has been attempted to prepare alumina-based composites containing some kinds of dispersed phases. The composites were fabricated by reducing and sintering of Al₂O₃/NiO/SiC mixtures. Reaction products (Ni₃Si and C) and metallic Ni were found to disperse at the matrix grain boundaries, while Ni was partly trapped into Al₂O₃ grains. In addition, carbon nanoballs encapsulating Ni₃Si were produced and dispersed in the composites. The carbon cages were approximately 80–100 nm in diameter with polyhedral shape, and had lattice spacing of 0.35 nm that was typical for the graphite. Encapsulated Ni₃Si had facet planes which were parallel to the carbon layers surrounding. Production of metal encapsulated carbon nanoball within ceramic materials is the first successive result that might promote researches on such novel ceramic composites. Received: 2 January 1997 / Accepted: 15 March 1997     

Carbon coatings produced by high temperature chlorination of silicon carbide ceramics

Carbon coatings are widely used to modify surfaces of materials and improve their tribological properties. In this work, carbon layers were formed on various types of sintered and CVD silicon carbide (SiC) using a novel technique involving a reaction with chlorine and chlorine-hydrogen gas mixtures at 1000 °C. Following the work done on powders and fibers, this method successfully produced adherent coatings on SiC ceramics, which could be grown to thickness above 200 μm. Highly disordered carbon with contributions from nanocrystalline graphite was identified by Raman spectroscopy, x-ray diffraction, and energy dispersive spectroscopy. The kinetics of the chlorination reaction at 1000 °C for different gas mixtures fit to a linear reaction rate equation. Coatings produced in pure Cl₂ are graphitic and demonstrate a low hardness (1.8 GPa), Young’s modulus (18 GPa), low wear rate, and a friction coefficient of ∼0.1, which is almost constant for any testing conditions in dry or humid air. Coatings produced in Cl₂/H₂ mixtures have microhardness up to 50 GPa and Young’s modulus up to 800 GPa. Although the presence of hydrogen had little effect on the Raman spectrum of the carbon layers, its presence changed the structure and permeability of the carbon film. Received: 23 April 2001 / Reviewed/Accepted: 24 April 2001