The Mahogany Tree
The Mahogany Tree
The branch of material science known as thermal analysis is the study of the change of temperature within the properties of materials. A number of different properties are studied using this method including mass, dimension, volume, stiffness, damping, heat transfer and temperature. Other concepts can be employed within the method as well, determining how temperature relates to light and sound. The main purpose of the entire discipline is to find how temperature impacts other facets of physics.
When conducting experiments with thermal analysis, researchers generally control the temperature in a standardized format. This is accomplished by either keeping, increasing, or decreasing the temperature at a constant rate or working with a variety of predetermined different temperatures. Adjustments can be made for different research techniques using oscillating temperatures.
Environment is also a major key in properly conducting thermal analysis. The atmosphere surrounding and permeating the element being researched can have drastic effects on the technical results. Some of the most common environments in which to study thermodynamics include general air or an inert gas such as nitrogen or helium. This allows the least impact on the results of heat transfer and other elements within the study.
The thermal analysis of polymers is one of the largest areas of research within the field. This comes in the form of both raw materials as well as everyday packaging and household products. Raw materials can be studied with the addition of various elements such as dyes and stabilizers to determine what may be the best use for the materials. Packaging and products are tested to identify the resistance to the environment and possible events that occur daily.
Within the field of metals, various production techniques are studied to determine the best way to use and create materials such as cast iron, aluminum alloys, copper alloys and steels. To conduct this research, a sample of liquid metal must be obtained. It is then monitored through its cooling process in a container that identifies the various aspects of thermal analysis. This allows for more efficient materials management and can help determine what processes are the best for a particular metal.
Thermal analysis is essential to the proper production, transport, storage, safety and preparation of food throughout the industry. The various techniques used by researchers are evaporation, pasteurization, freezing and cooking. Studies must be conducted on all food stock to determine the best way to preserve that food from field to the table. For example, a frozen box dinner needs to be kept at a certain temperature until it is ready to be consumed. It also needs to be cooked for a specific time period to make sure all bacterial elements are gone.
Thermogravimetry (TG) is a thermal analysis technique in which the mass of a sample is measured over time, in a controlled atmosphere, as temperature changes. The data from this procedure is examined by thermogravimetric analysis (TGA), one of several methods in which one physical property of a sample is measured relative to temperature change. Similar techniques involve the study of thermal flow, length and elasticity. Thermogravimetry is often used in the pharmaceutical industry to analyze drug stability and agriculture to regulate the dehydration process for crops.
The procedure requires a precision balance, a heat source, and a closed reaction chamber. As the sample is being heated, its weight and temperature are continuously monitored and recorded. A thermocouple, or temperature sensor, is typically placed in direct contact with the sample; thermogravimetry involves absolute changes in mass and is independent of the sample’s heating rate. The environment might be a gas or mixture of gasses at any required pressure, or a vacuum.
When plotted, the gathered data forms a curve relating the mass of a sample to its temperature. The correlation between temperature and mass change is examined by comparing points on the graph to the two axis scales. A more informative display is derived from the data by means of differential thermogravimetry, where the rate of change in mass versus temperature is plotted. Individual episodes of change not apparent in the simpler mapping can be easily distinguished, leading to a more complete and meaningful analysis.
Change in a sample’s mass, as reflected by its weight, as temperature changes may be due to the breakdown of compounds into constituent elements, the reaction of the sample with oxygen or its loss of water content. At what temperature and under which atmospheric conditions these changes occur provide important information about the sample material. Thermogravimetry is often employed to study how these factors will effect the stability and lifetime of a product. Analysis of a sample’s reaction to thermal change can also be used in forensics to help identify unknown materials.
The behavior of related materials at high temperature under a selected atmosphere is an important consideration in product design and development. Thermogravimetry can also be used to establish a material’s characteristics for use in later identification or quality control. By careful choice of temperature and atmosphere, sample materials can be selectively decomposed into constituent components. This method is often used in the study of polymers, large molecules composed of repeating parts.
Differential Thermal Analysis
Differential thermal analysis (DTA) occurs when two substances are exposed to the same temperatures and thermal changes over time. The test typically includes a reference substance, of which its behavior is known under the applied temperature conditions. Another substance is subjected to the same temperatures and rates of change as the reference. The sample can either absorb heat, generally meaning it is cooler than the reference, or emit heat when it is hotter than the first material. With the data that is plotted, researchers can determine how specific materials react at certain temperatures, as well as over time.
Reference materials typically do not melt or freeze; they need to remain at a steady state for the experiment to work. A differential thermal analysis technique is often done by placing each material in a separate container. Each container is usually in a separate cavity in the same space. Instruments called thermocouples are generally used to record temperature differences between both materials. The thermocouples can detect a change in phase, such as melting or vaporizing, as a signal.
Another method of differential thermal analysis can be the use of conductive pans, with thermocouples attached to them, inside a furnace. Both materials are more often heated equally with time when DTA is carried out like this. The results of the test are typically recorded by using a DTA curve. Differences in temperature between the materials, or a change in temperature over time, can be plotted. The graph often visualizes latent heat of transition that occurs while a substance changes from one state to another; this usually causes the sample’s temperature to temporarily be less than the reference.
Differential thermal analysis often involves testing samples at higher temperatures than other forms of thermal analysis. It can be conducted with materials such as metal, ceramic, glass, and ceramic. Reference materials that are often used include silicon carbide and aluminum. Liquid reference standards sometimes include silicon oil.
Technology in the 21st century has combined differential thermal analysis with the ability to detect loss in material mass as well as change in temperature. Software programs can automatically monitor the process and record the measurements. Before a test, instruments for differential thermal analysis generally have to be calibrated. A separate calibration procedure runs materials known to respond a particular way over given temperature ranges. Local and regional regulations can guide the process of calibration for differential thermal analysis in pharmaceutical, food, or environmental applications.
Differential Scanning Calorimeters
(DSC) measures temperatures and heat flows associated with thermal transitions in a material. Common usage includes investigation, selection, comparison and end-use performance evaluation of materials in research, quality control and production applications. Properties measured by TA Instruments’ DSC techniques include glass transitions, “cold” crystallization, phase changes, melting, crystallization, product stability, cure / cure kinetics, and oxidative stability.
Differential scanning calorimetry (DSC) measures Specific Heat Capacity, Heat of Transition, and the Temperature of Phase Changes and Melting Points. DSC measures the rate of heat flow, and compares differences between the heat flow rate of the test sample and known reference materials. The difference determines variations in material composition, crystallinity and oxidation.
Differential scanning calorimetry thermal analysis provides test data for a wide range of materials. Materials analyzed by DSC include polymers, plastics, composites, laminates, adhesives, food, coatings, pharmaceuticals, organic materials, rubber, petroleum, chemicals, explosives, biological samples and more.
A newly developed Micro-Thermal Analyzer affords images based on thermal properties such as thermal conductivity, thermal diffusivity, and permits localized thermal analyses on samples of a square micrometer area by combining the imaging ability of the atomic force microscope and the thermal characterization ability of temperature-modulated differential scanning calorimetry. Since thermal penetration depth depends on frequency, one can obtain depth profiles of thermal conductivity and thermal diffusivity by varying the modulation frequency.
Also, the analyzer can be used to characterize phase-transition temperatures, such as glass and melting transitions, of small sample regions with a precision of about ±3 K. Heating rates can be varied between 1 and 1500 K min−1. Modulation frequencies can be applied in the range from 2 to 100 kHz. We applied this new type of instrument to characterize microscopic thermal and structural properties of various polymer systems. The operation principles of the instrument are described, application examples are presented, and the future of the technique is discussed.
Micro-thermal analysis is now being used commercially to visualize the spatial distribution of phases, components and contaminants in polymers, pharmaceuticals, foods, biological materials and electronic materials. This review outlines various applications that have been described in the literature to date, the topics ranging from multi-layer packaging materials and interphase regions in composites, to the use of the technique as a means of surface treatment.
University/College: University of Arkansas System
Type of paper: Thesis/Dissertation Chapter
Date: 13 November 2016
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