SPECIAL REPORT/INSTRUMENTATION: Thermal Techniques for Material Characterization
Many different thermal techniques can be employed for material characterization, including thermomechanical analysis (TMA), dynamic mechanical analysis (DMA), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). These complementary techniques are used for compositional and structure elucidation, and for investigating the viscoelastic properties of materials.
During the production and processing of ceramic and glass materials, various parameters (including chemical composition and microstructure of various phases, as well as their distribution) are tightly controlled to impart the desired mechanical and chemical properties to the end product. The knowledge of thermal stability and reaction kinetics of various phases present in the ceramic and glass materials is of prime importance.
Thermal characterization and testing are employed to evaluate and optimize the chemical and physical properties of ceramic and glass materials. For example, thermal analysis could be used to investigate the pyrochemical reactions of clay-based ceramic materials. Pyrochemical reactions include events such as loss of bound (water of crystallization) and unbound (free) water, loss of organic matter, crystal structure transformation (polymorphism), and decomposition. These thermal events can easily be characterized using TGA and DSC analytical techniques.
Technique CharacteristicsSome thermal techniques are used in conjunction with fundamentally different, non-thermal spectroscopic techniques for a better understanding of the chemical properties. For example, TGA is combined with mass spectrometry (TGA-MS) and Fourier transform infrared spectroscopy (TGA-FTIR) to investigate the decomposition of products, especially evolved gases of polymer materials.
When using TMA, the test specimen is subjected to a minimal load, and changes in length or volume are measured at varying temperatures. The deformation load could be in compression, tension or bending mode. TMA is commonly used to characterize the thermal expansion and shrinkage that takes place during the processing (sintering) and use of ceramic and glass materials. Proper characterization of thermal expansion and shrinkage behavior of these materials is of paramount importance, especially when such materials are used in combination with different materials, such as polymers and metals. Any significant mismatch in the thermal expansion behavior of the two materials could potentially lead to a product failure.
With DMA, the mechanical properties of the sample are measured as a function of time, temperature and frequency. The strain is recorded as the test specimen is subjected to an oscillatory stress, which can be in compression, tension, shear or flexure mode. This technique is routinely used in the investigation of the viscoelastic behavior of polymer materials. DMA is especially useful for characterizing the a, b and g transitions of the polymers, which are difficult to detect with other techniques.
In DSC analysis, the heat flow into a sample and a reference material is measured as a function of temperature and time. This technique is routinely employed to characterize various thermal events in polymers, such as glass transition, melting, crystallization, heat capacity, phase transformation, oxidative stability and curing reaction. DSC is also commonly used to measure the specific heat capacity of ceramic and glass materials. The reaction kinetics and phase transformations that take place during the processing and use of these materials can also be characterized using this technique.
TGA is one of the simplest instruments available for thermal characterization, and the analysis results offer a wealth of information about the tested material. The technique is used primarily to investigate the thermal and oxidative stability, and the inorganic and organic composition, of the material.
During TGA analysis, the weight of the sample is measured as a function of time and temperature in an inert or oxidative atmosphere. When a sample is heated in an inert nitrogen atmosphere, the thermal decomposition of the components of the sample occurs at different temperatures based on the sample’s thermal stability. Most of the organic materials are thermally decomposed completely below 600°C; the residual noncombustible mass remaining at the end of heating to 800-900°C in an oxidizing atmosphere consists of inorganic materials. TGA is used to follow the reaction kinetics and phase transformations that take place during the processing or use of ceramic and glass materials.
A typical TGA thermogram is shown in Figure 1, which shows the decomposition profile of a reference material, calcium oxalate monohydrate (CaC2O4 · H2O). Three distinct weight-loss steps are evident and correspond to the loss of water of crystallization (25-230°C), decomposition of dehydrated calcium oxalate into calcium carbonate and carbon monoxide (230-600°C), and, finally, the decomposition of calcium carbonate into calcium oxide and carbon dioxide (600-800°C).
Calcium Carbonate AnalysisCalcium carbonate (whiting) is used in varying amounts as a flux material in the production of ceramic materials, and ceramic tile adhesives also contain a significant amount of calcium carbonate. The level of calcium carbonate in a product should be optimized to get the desired results and ensure that the primary function of the product is not compromised.
TGA is a beneficial quality control technique used to determine the amount of calcium carbonate (CaCO3) present in a product. When heated in an inert nitrogen atmosphere, calcium carbonate decomposes into calcium oxide (CaO) and carbon dioxide (CO2) at approximately 700°C:
CaCO3 → CaO + CO2
Theoretically, the decomposition of calcium carbonate results in a mass-loss of approximately 44% by weight. Since the organic/polymeric material decomposes completely at temperatures well below 600°C, any mass-loss that occurs in the temperature range of approximately 600-800°C can be attributed to the decomposition of the calcium carbonate material in the sample.
Once the mass-loss associated with the decomposition of calcium carbonate is determined, the amount of calcium carbonate present in the sample is calculated. However, care must be taken to make sure that the sample does not contain any other inorganic materials that decompose in the same temperature range.
Simethicone AnalysisTGA can be used as a quality control technique to determine the weight percent of silica in simethicones. By definition, simethicones are poly(dimethyl siloxane) (PDMS) polymers that contain 4 to 7% by weight silica. Silica is one of the basic components of clay-based ceramics.
In a typical TGA analysis of a simethicone, approximately 20 mg or more of the sample is heated from room temperature to 800°C in an inert nitrogen atmosphere (flow rate approximately 200 mL/min). The amount of residue (silica) remaining at the end of heating is determined from the TGA thermogram of the sample.
Figure 2 shows the decomposition profile of a simethicone product. As is evident, the catastrophic decomposition of the PDMS polymer begins at approximately 400°C, and the sample contains approximately 5.2% by weight inorganic material (silicon dioxide).
Carbon Black AnalysisCarbon black is used extensively in significant amounts in rubber and ink formulations, and elemental carbon is also used in the manufacture of carbon/silicon carbide/glass composite materials. The presence of an optimum level of carbon black in these formulations is critical for the desired performance of a product.
TGA analysis is routinely used as a quality control technique to determine the amount of carbon black present in polymer and ink materials. In this technique, the material containing carbon black is heated to approximately 700°C in an inert nitrogen atmosphere to burn off the organic/polymer materials. Then an oxidizing air atmosphere is introduced to oxidize the carbon black.
The mass-loss associated with the oxidation of carbon black is determined from the TGA thermogram, and the amount of carbon black present in the sample is calculated. Figure 3 shows the decomposition profile of an ink product and indicates that it contains approximately 4.1% by weight of carbon black.
Effective CharacterizationVarious thermal techniques can be employed to characterize ceramic and glass materials. Each of these thermal techniques provides unique information that can be used to optimize the thermal and mechanical properties of the end product.
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