Thermomechanical Analysis Measurements Research Articles

Thermomechanical Analysis of Coal Ash: The Influence of the Material for the Sample Assembly

Concerns over the characterization of the thermal properties of coal ash using thermomechanical analysis (TMA) in carbon assemblies (crucible and penetrating ram) has led to a study which critically examines the effect of sample assembly materials on thermomechanical analysis measurements. This study quantifies the influence of the sample assembly material on the TMA traces. For temperatures up to 1600 °C and in inert conditions, ash chemistries were identified where the slag reacts with a range of sample assembly materials [molybdenum (Mo), platinum (Pt), carbon (C), zirconia (ZrO2), alumina (Al2O3), and boron nitride (BN)]. Only ZrO2 and Pt sample assemblies were found to be unreactive to ash samples during the tests. Iron-containing ashes were found to react with C assemblies, so that previous TMA data published for such assemblies and inert gases is not characteristic of ash fusibility alone. The implications of the reactions to the interpretation of this TMA data are outlined. For ashes that contain iron, TMA results in graphite sample assemblies may be considered to have been obtained under stronger reducing conditions than those used in the standard AFT test.

Silica–PEG hybrid ormolytes: structure and properties

The effect of lithium salt doping on the structure and ionic conduction properties of silica–polyethyleneglycol composites is reported. These materials, so called ormolytes (organically modified electrolytes), were obtained by the sol–gel process. They have chemical stability due to the covalent bonds between the inorganic (silica) and organic (polymer) phase. The structure of these hybrid materials was investigated by small-angle X-ray scattering (SAXS) as a function of lithium concentration [O]/[Li] (O being the oxygens of the ether type). The spectra have a well-defined peak attributed to the existence of a liquid-like spatial correlation of silica clusters. The ionic conductivity was studied by AC impedance spectroscopy and is maximum for [O]/[Li]=15. This result is consistent with SAXS and thermo-mechanical analysis measurements and is due to the formation of cross-linking between the polymer chains for the larger lithium concentrations. These materials are solid, transparent, flexible and have an ionic conductivity up to 10 −4 S/cm.

Moisture absorption effect on adhesive filled bonded joints

AbstractThe effect of filler type and content on the performance and durability of adhesive bonded joints upon exposure to damp heat (water immersion) has been investigated using concurrently step lap shear and thermo mechanical analysis (TMA) experiments. A TMA based approach is developed in order to predict the strength of the bonded constructions. Two types of fillers were used in order to achieve the objectives: glass beads (GB) and alumina ceramic powder (ACP). It is shown that ACP filled adhesives have higher strength retention than GB filled ones. It is also demonstrated that valuable information can be extracted from TMA measurements. Generally, the increase of the coefficient of thermal expansion (CTE) of the bonded joints is found to be in agreement with the decrease of the step lap joint strengths after exposure. The increase of the CTE of the bonded joints resulting from exposure to hot water seems to increase the stresses, which in turn affect the overall strength. Similarly, bulk adhesive results show that GB filled systems are more susceptible to water than ACP filled ones, which confirms the decrease in strength of GB filled bonded constructions.

Structure and properties of an interpenetrating polymer network-like system consisting of polystyrene-polyethylene: 1. Synthesis, elastomeric and thermoanalytical characterization

A polystyrene-polyethylene (PS-PE) interpenetrating polymer network (IPN)-like system was prepared by synthesis in situ. The system differs from ‘pure’ IPN systems in that the PS network is partially grafted onto the PE network. The mean molar mass of the polymer chain between two junction points ( M ̄ c ) was calculated from the values of uniaxial compression modulus. Differential scanning calorimetry and thermomechanical analysis measurements pointed to a considerable decrease of the crystalline phase in PE with increasing PS content. The melting temperature enhancement of the crystalline phase of PE is discussed.

Characterization of solute-membrane interactions by TMA-measurements

On the basis of relationships derived by Oplatka et al. [1] correlating the derivative of tensile stress with respect to temperature for a polymer strip at constant length, (∂σ/∂T)t, with the derivative of entropy with respect to strip length, (∂S/∂l)T, relationships are derived relating the derivation of strip length with respect to temperature under constant tensile stress, (∂l/∂T)σ, with the derivative of entropy with respect to tensile load, (∂S/∂σ)T. The developed relationships are applied to evaluate TMA (thermomechanical analysis) measurements performed with asymmetric and homogeneous cellulose acetate membranes where the membrane strips were kept in deionized water. Moreover, TMA measurements were performed with homogeneous CA membranes while maintaining the membrane strips in bathing solutions of different, but in each instance, constant electrolyte concentrationcs(e.g., LiCl, LiNO3, Li2SO4, CaCl2; 0 ≤cs≤ saturation). Under constant tensile stress, σ, as-cast asymmetric CA membranes, which are kept in deionized water, first elongate with increasing temperature at temperatures up to about 50°C. However, they shrink with a further increase of temperature in the temperature range 50°C up to about 90°C, whereas creeping starts at temperatures beyond 90°C, leading again to an elongation of the strip with a further increase of temperature. The negative apparent coefficient of thermal expansion, existing between 50° to 90°C, is consistent with shrinking phenomena observed with asymmetric CA membranes [2]. In addition, it correlates well with both an entropy decrease with increasing tensile stress at constant strip length and an entropy increase with decreasing strip length at constant tensile stress. Contraction of the strip with increasing temperature at constant tensile load might be due to increasing coil formation of polymer molecules rendered possible by disintegration of joints such as H-bonds between polymer chains. Depending on the electrolyte of the bathing solution and its concentration, homogeneous CA membranes exhibit positive and negative apparent linear coefficients of thermal expansion under constant tensile stress.

Kinetics of structural relaxations in the glassy semiconductor a–Se

It has recently been suggested that the rate of structural relaxations in amorphous chalcogenide glasses is inversely proportional to the viscosity and the latter obeys the Vogel-Tammann-Fulcher relationship. In this paper, Differential Scanning Calorimetry (DSC) and Thermomechanical Analysis (TMA) experiments examining the glass transformation in a–Se are reported which have been carried out over as wide range of heating rates as practically possible on the available instrumentation covering more than three decades of heating rate variation from r = 0.05 °C/min to r = 100 °C/min. The observed glass transformation temperatures in DSC and TMA ranged from 30 °C to 65 °C. The a–Se films were vapor deposited by thermal evaporation onto A1 substrates (at ∼70 °C) under identical conditions and well aged over an equal period of time so that all the films had an identical initial structure. Analysis of the glass transition temperature, Tg, vs heating rate, r, data obtained from the DSC endotherms shows that the structural relaxation rate is non-Arrhenius and can be described reasonably well by a Vogel expression of the form exp[–A/(T – T0)] which may have a weak structure dependent factor of the form exp[–c(H – HE)] where (H – HE) is the deviation of the structural enthalpy from its equilibrium value. It was found that a phenomenological single parameter Vogel description for the retardation time over the whole temperature range accessed leads to A and T0 values comparable with those obtained from the Vogel analysis of the viscosity-temperature data of Cukiermann and Uhlmann (1972), as well as the recent thermomicrohardness data of the present authors (1989). Furthermore, it is shown that the wide differences reported for the crystallization activation energy in a–Se can be readily interpreted by a viscosity limited crystallite growth rate. The Vogel interpretation is also found to be consistent with the temperature dependence of the mean dielectric relaxation time in the Debye loss observed by Abkowitz, Pochan, and Pochan (1980). It is suggested that in the elemental chalcogenide a–Se, the relaxation rate of enthalpy, microhardness, and dielectric polarization, as well as the crystallization rate, all scale inversely with the viscosity. TMA measurements, on the other hand, although still interpretable in terms of a Vogel behavior, did not indicate a softening behavior which paralleled the viscosity.