D. Noël

Thermoanalytical characterization (DTA and TG) of RBa2Cu3O7−x (R≡Y, Gd, Nd) superconductors

Differential thermal analysis (DTA) and thermogravimetric (TG) analysis were used to characterize RBa2Cu3O7-x (R  Y, Gd, Nd) superconductors. Even if the resistivity-temperature curves are similar for the three compounds, DTA shows that the behavior of these materials during synthesis and sintering is different. Of the three systems, the Nd-Ba-Cu-O system is the most favorable for the formation of the 123 superconductor because secondary phases are present in smaller quantities. TG curves suggest the presence of a transition from the orthorhombic phase to the tetragonal phase in a nitrogen atmosphere at about 550–600°C for the three perovskites. The energy required to insert oxygen atoms in the crystal structure is smaller for the neodymium and gadolinium compounds than for the yttrium compound (Gd < Nd < Y). There is also a proportional relation between the ionic radius of the rare earth element and the onset melting temperature.

Quantitative analysis of resins used in fiber-reinforced composites by reversed-phase liquid chromatography

Abstract An improved reversed-phase liquid chromatographic method has been developed to analyze the epoxy resin present in a typical fiber-reinforced composite system, Narmco Rigidite 5208/WC3000. After solvent extraction of the resin from the pre-preg (continuous-fiber reinforcement preimpregnated with partially cured resin) using tetrahydrofuran, acetophenone is added as an internal standard in order to obtain reproducible quantitative results. The separation is achieved with a linear water-acetonitrile gradient (50% to 100% acetonitrile in 15 min). The peak area or peak height is measured and normalized with respect to the acetophenone peak and the weight of the extracted epoxy resin. A reproducibility of less than 3% relative standard deviation was obtained for quantitative measurements of the main peaks. A smaller standard deviation was obtained with peak height than with peak area measurements but the latter is recommended for definitive quantitative analysis since the results are less column-dependent and permit easier interlaboratory comparison. The main peaks in the Narmco 5208 chromatogram have been identified by comparison of their retention times with those of individual components and by infrared spectroscopy of collected fractions. Several batches of Narmco 5208 prepreg have been analyzed and their compositions estimated by comparing the results with those obtained for the individual components of the system.

Quantitative thermal characterization of carbon-epoxy composites using differential scanning calorimetry and thermogravimetric analysis

Abstract A differential scanning calorimetry (DSC) method has been developed to characterize carbon-epoxy prepregs. DSC was done with nitrogen as the atmosphere in order to avoid the oxidation reactions which degrade the epoxy resin during heating. Since the resin distribution in a given batch of prepreg is not entirely uniform, and since the sample used for DSC is relatively small, the percentage of resin may vary significantly from sample to sample, even though they are taken from the same sheet of prepreg. Hence, the exact resin content must be determined for each sample in order to obtain reproducible values for the curing enthalpy. A thermogravimetric (TG) method using an oxidizing atmosphere has been developed for this purpose. After the DSC experiment, a TG analysis is done on the same sample. The exact mass of resin is found on the TG curve at the inflexion point corresponding to the end of oxidation of the epoxy resin and to the beginning of oxidation of the carbon fibres. For Narmco 5208/WC3000 prepreg, this inflexion point occurs at about 580 °C. This procedure gives enthalpy values with a relative standard deviation smaller than 1%. The aging of the prepreg when exposed for 66 days at room temperature and 50% relative humidity was also studied using these thermal methods.

High-performance size-exclusion chromatography of fiber-reinforced organic-matrix composites

Abstract Different resins used in fiber-reinforced organic-matrix composites have been characterized by means of high-performance size-exclusion chromatography (HPSEC). By using a single column packed with 5-μm particles of highly cross-linked styrene-divinylbenzene copolymer, together with tetrahydrofuran as eluent, efficient separation is achieved in less than 8 min. Of the different systems analysed, Narmco Rigidite 5208 was chosen for a more complete study involving optimization of the separation and determination of the peaks which are most suitable for characterizing the epoxy resin. HPSEC was also used to check the batch-to-batch reproducibility in this system and to evaluate the aging which occurs when it is exposed at 22°C and 50% relative humidity. The peak corresponding to the amine hardener was found to be the most appropriate for following the advancement of the curing reaction. An index obtained by dividing the total area of peaks corresponding to molecules of higher molecular weight by the area of peaks corresponding mainly to monomers was used as an indicator of the degree of polymerization. In spite of the lower resolution achieved in HPSEC compared with high-performance liquid chromatography, a rapid separation based on molecular size can be useful for controlling the quality of an epoxy system and for determining the degree of polymerization.

Thermoanalytical characterization (DTA and TG) of RBa1.5Sr0.5Cu3O7−x and RBaSrCu3O7−x (R = Y, Gd, Sm OR Nd) superconductors

Abstract Differential thermal analysis and thermogravimetric analysis were used to characterize RBa 1.5 Sr 0.5 Cu 3 O 7− x and RBaSrCu 3 O 7− x superconductors (R=Y, Gd, Sm or Nd). It was found that the replacement of Ba by Sr increases the onset melting temperature of the perovskites. The effect is larger for compounds which contain an R atom with a small ionic radius. TG curves show that the temperature of the orthorhombic-tetragonal transition is modified by the presence of Sr.

Performance of sulfation and nitration plates used to monitor atmospheric pollutant deposition in a real environment

Abstract Sulfation and nitration plates were exposed outdoors for various periods of time to evaluate their performance in a real environment. These passive monitors are used to estimate the deposition of pollutants on metallic surfaces, and thus to evaluate the influence of the atmosphere on the corrosion. Single-column ion chromatography was used to determine the quantity of anions absorbed on the plates. This technique is better than other analytical procedures such as turbidimetry or colorimetry because passive monitors exposed in an atmosphere with a low degree of pollution can be analyzed without preconcentration. However, the pH of the sample to be injected on the Chromatographic column must be adjusted to between 6.0 and 12.0 in order to obtain reproducible sulfate values. For sulfation plates, the additivity of the deposition process is excellent for a period of exposure up to 3 months, with a reproducibility of about 2%. For nitration plates, the deposition process is not cumulative due to a physical change of the monitor during exposure. The correlation between the amounts of sulfate found on sulfation and nitration plates was also examined.

Characterization Of Epoxy-Graphite Composites By Diffuse Reflectance FTIR

Diffuse reflectance Fourier transform infrared spectroscopy has been used to obtain IR spectra of epoxy-graphite prepregs and cured panels. The spectra are influenced by fibre orientation and front-surface reflection effects which limit the usefulness of the method for quantitative analysis. However, for qualitative work, excellent spectra can be obtained with practically no sample preparation.

Production of SiC in a Thermal Plasma

ABSTRACT The needs of high temperature structural ceramics have rekindled great interest in developing methods for producing high-purity, ultrafine powders. Among different ways of fine powder preparation, gas phase synthesis in a thermal plasma is one of the most important techniques. A continuous thermal plasma process has been developed to produce ultrafine SiC powders by gas phase synthesis from silicon tetrachloride and methane in an argon and argon + hydrogen plasma jet. The reaction conditions to control the physical and chemical properties of the powders have been investigated. Bulk properties of SiC powders were determined by X-ray diffraction and Fourier transform infrared spectroscopy. Surface characteristics and particle size were examined by scanning electron microscopy. It is found that the reaction conditions have significant effects on the size and composition of particles. The particles, 0.1–0.5 μm in size, can be either completely amorphous or crystalline β-SiC. The presence of Si-O bonds is detected in all powders and the amount of oxygen strongly depends on the operating conditions used. Gas phase synthesis in a thermal plasma can be an efficient means for producing, on a continuous basis, ultrafine particles of different compositions and morphologies.

Nondestructive determination of crystallinity by diffuse reflectance FT-IR in composites made from polyphenylene sulphide and carbon fibres

Diffuse reflection Fourier transform infrared spectroscopy can be used for nondestructive determination of the degree of crystallinity at the surface of composite materials made from polyphenylene sulphide reinforced with carbon fibres. In particular, the ratio of the heights of the peaks at 1075 and 1093 cm−1 can be used as a quantitative indicator of the crystallinity. This has been confirmed by annealing highly amorphous prepreg to give samples of varying degrees of crystallinity, in which case a good linear relationship is observed between the peak ratio and the enthalpy of crystallization as determined by differential scanning calorimetry.