Thermal Analysis of Materials in Research

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Information about Thermal Analysis of Materials in Research
Engineering

Published on March 28, 2014

Author: mahendraprabhuk7

Source: slideshare.net

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A common topic in Materials and Metallurgical engineering

Fundamentals of Thermal Engineering. Page-1- COMMON TECHNIQUES IN THERMAL ANALYSIS OF MATERIALS IN RESEARCH Mahendra Prabhu Kuvettu 4so08me057, 5th Semester, Bachelor of Engineering Course Department of Mechanical Engineering, St. Joseph Engineering College, Vamanjoor, Mangalore Contents: 1. Introduction 2. Thermal Gravimetric Analysis (TGA) 3. Differential Thermal Analysis (DTA) 4. Differential Scanning Calorimetric (DSC) 5. Thermo mechanical Analysis (TMA) 6. Dynamic Mechanical Analysis (DMA) 7. Conclusion 8. References 1. Introduction ‘Science’ is a vast premise with plenty of divisions and subdivisions like thermal, mechanical, fluid dynamics, kinematics , aerospace, geology, marine, seismic ……. to list just a few ! Every field of science has various techniques of analysis of materials or matter. Even thermal engineering involves a number of such techniques, measurement machines/apparatus, hypothesis, notations, laws, processes, units and hundreds of formula e. This paper presentation consists of various prominent analysis methods involved in thermal sciences. Or better know as applied thermodynamics in our daily curriculum. All the techniques of analysis anyone will be coming across in this presentation may probably be unfamiliar for students of present circumstances. Keen and dedicated efforts have helped me in gathering this information on very lucrative topic which is inevitable for mechanical engineer in the present scenario. My sole aim in these pages is to share my knowledge. Share creative ideas and allied talent in writing skills. If I tell it well—and I will do my best—I hope you will come away surprised, enthused, moved, or at the least with your interest piqued. Prior to the systematic details of this essay, I feel it’s my obligation to express my heart felt gratitude to all the lecturers who provided me an inspirational motivation. I’ve always been a perfectionist and a novel appreciator of fine English literature with emphasis on hope and encouragement. Savouring international award winning essays by students is one of my hobbies lately. Pure and genuine thanks to my parents and my sister for allowing me to spare my prominent

Fundamentals of Thermal Engineering. Page2 time towards the accomplishment of my heartfelt task. A awe-inspiring but seldom travelled journey begins here……….. 2. Thermal Gravimetric Analysis (TGA) This is used to measure the change in weight of a substance during heating or cooling. This mass difference can be measured as a function of temperature or function of time (at constant temperature). Sample can be liquid or solid. The graph of difference in mass versus temperature is initially plotted. In the graph we can observe the behaviour of a characteristic curve. Onset and offset points can be plotted on the curves and different levels of weight difference can obtain for different temperatures. Fig 1: detection of mass change in TGA 2.1 Common applications of TGA a) Study of material properties and decomposition pattern of substances. b) Study degradation mechanism in chemical reactions and kinetics. c) Determination of organic content in a sample. d) Determination of inorganic content in a sample. Fig 2: TGA curves for various polymers 3. Differential Thermal analysis (DTA) DTA involves heating or cooling a test sample and an inert reference under identical conditions, while recording any temperature difference between the sample and reference. This differential temperature is then plotted against time, or against temperature. Changes in the sample which lead to the absorption or evolution of heat can be detected relative to the inert reference. DTA can therefore be used to study thermal properties and phase changes which do not lead to a change in enthalpy. The baseline of the DTA curve should then exhibit discontinuities at the transition temperatures and the slope of the curve at any point will depend on the micro structural constitution at that temperature. A DTA curve can be used as a finger print for identification purposes. The area under a DTA peak can be to the enthalpy change and is not affected by the heat capacity of the sample. Fig 2: A sample DTA analysis of Mg-9Zn has been depicted.

Fundamentals of Thermal Engineering. Page3 3.1 Applications of DTA a) Helps to differentiate between exothermic and endothermic reactions. b) Helps in the study of reaction/ c) Decomposition, melting, crystallization, change in crystal structure, glass transition. 3.2 Sharing common graph of DTA with TGA. Fig 4: Analysis of DTA can be performed with TGA in the graph as they share the common baseline of temperature. 4. Differential Scanning Calorimetric (DSC) Differential scanning calorimeter or DSC is a thermo analytical. Technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. 4.1 Methods involved in DSC a) Both the sample and reference are maintained at nearly the same temperature throughout the experiment. b) The sample holder temperature increases linearly as a function of time. c) The reference sample should have a well- defined heat capacity over the range of temperatures to be scanned. d) The basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transition, more (or less) heat will need to flow to it than the reference to maintain both at the same temperature. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic. e) Temperature of the sample is measured relative to a reference for the heat transferred. The concept is similar as DTA (compare and reference) Measure “heat flow” required to keep both specimens at same temperature. Heat capacity Equations for heat flow a) dq /dt = heat flow b) dT/dt = heating rate c) (dq /dt) / (dT/dt) = dq/dT = cp Where Cp is the coefficient of temperature at constant pressure 4.2 Glass Transition a) It is the step of sudden deviation on the demographer. b) This is the transition stage from disordered solid to liquid state c) Observed in glassy solids like polymers, rubber (natural /synthetic) Etc. d) The temperature at which this phenomenon occurs is known as glass transition temperature. 4.3 Crystallization a) Stage of disordered to ordered transition (just like liquid water forming a crystallized solid ice upon cooling) b) Material can crystallize! c) Observed in glassy solids, e.g., polymers d) Where Tc is crystallization temperature 4.4 Melting a) It is the Negative peak on Thermo gram colleagues b) This is the Ordered to disordered transition c) Melting temperature denoted as Tm

Fundamentals of Thermal Engineering. Page4 d) Melting happens to crystalline polymers; glassing happens to amorphous polymers Fig 5: A sample DSC graph is depicted below combining all three phases Fig 5 is a schematic DSC curve demonstrating the appearance of several common features temperatures. A curve of heat flux versus temperature or time is plotted. Two different conventions: exothermic reactions in the sample shown with a positive or negative peak; it depends by the different kind of technology used by the instrumentation to make the experiment. This curve can be used to calculate enthalpies of transitions this is done by integrating the peak corresponding to a given transition. The enthalpy of transition delta H = KA, where K is the calorimetric constant; A is the area under the curve. The calorimetric constant K will vary from instrument to instrument, and can be determined by analysing a well characterized sample with known enthalpies of transition. 4.6 Applications of DSC a) To observe fusion and crystallization events as well as glass transition b) Glass transitions may occur as the temperature of amorphous solid is increased. These transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a change in heat capacity; As the temperature increases, an amorphous solid will become less viscous. At some point the molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the crystallization temperature (Tc). This transition from amorphous solid to crystalline solid is an exothermic process, and results in a peak in the DSC As the temperature increases the sample eventually reaches its melting temperature (Tm). The melting process results in an endothermic peak in the The ability to determine transition temperatures and enthalpies makes DSC an invaluable tool in producing phase diagrams for various chemical systems. 5. Thermo Mechanical Analysis (TMA) This is the technique of measure change in dimensions during heating or cooling. Also involves measurement of dimensional changes under constant load as a function of temperature. The probe of TMA can be changed for different information just like universal testing machine. Fig 6: Displacement versus temperature TMA graph 5.1 Applications of DTA a) Determination of linear expansion coefficient. b) Evaluation of material anisotropy. c) Determination of phase transition. d) Determination of melting point. 5.2 Advantages

Fundamentals of Thermal Engineering. Page5 a) Not limited to bulk specimen (can be applied to thin films and fibres). b) Sensitive method to detect glass transition temperature. c) Can defect other properties beyond thermal properties like creep behaviour, softening, modulus, viscosity etc. 6. Dynamic Mechanical Analysis (DMA) DMA is the measurement of mechanical properties of materials under oscillatory load as a function of temperature. 6.1 Properties a) Mostly used with polymers. b) Shows viscous elastic response. c) DMA considers only elastic region. d) Some DMA properties are storage modulus, loss modulus, damping factor etc. e) Various analysis modes include dual cantilever, compression, shear, single cantilever, tension. f) Very valuable for proper analysis. 6.2 Applications a) Determination of glass transition temperature (sensitive method). b) Determine upper use temperature of structural composite. c) Selection of proper materials for specialized application (acoustic, auto mobile etc.) d) Study of phase homogeneity of blends. 7. Conclusion The assorted methods of material analysis in thermal sciences are explained in this paper. The techniques may look simple but are practically complicated to perform. People cannot merely judge a branch just by vague knowledge of text books in our syllabus. Prior to reading this material only few might know that thermal sciences is really a innovative and complex stream in itself . These various methods of analysis that one comes across shows how vast and endless this vivid and flamboyant stream can be……….and increasing its dimensions to new horizons day by day. 8. References 1. Please log on to the website: www.thermalscience.com 2. Incorporation of ideas from my routine habit of reading newspapers like the ‘The Hindu’ and the ‘Deccan Herald’. 3. The ‘Malayama Manorama’ year book - 2010 and the Oxford thesaurus. 4. Various reliable sources on the Internet like ‘Microsoft Encarta’ and ‘Wikipedia On-line’ encyclopaedias.

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