THERMOMECHANICAL ANALYSIS (TMA)
Aisyah Radhiah Binti Ahmad
INSTRUMENTATION OF TMA
1.1 The Instrumentation
Figure 1: The simple schematic diagram of TMA [1].
The sample tube that is located in the center of the furnace is used to control the temperature and atmosphere. The sample temperature is measured with a thermocouple that is located near the sample. The area around the sample can usually be both heated and cooled to prevent the heat-rise problem and the sample temperature (sensed by a thermocouple) has to be directed to programmer control.
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The sample platform and the probe are made from a material such as quartz. Quartz is chosen as it has a low, reproducible, accurately known coefficient of thermal expansion and also it has low thermal conductivity to isolate the LVDT from the temperature changes in the furnace. The probes can also be made from alumina and metals depending on the temperature range and the measurement purpose [2].
The heavy black probe measures the position of the sample surface with a linearly variable differential transformer, LVDT. The floating suspension act as a balance for the arm arrangement, combined with added weights at the top, allowing the probe weight to be counterbalanced that can be used to control the force on the sample [3].
A purge gas is used during the TMA measurements. Its purpose is to ensure continuous laminar gas flow to prevent the formation of air turbulence as the temperature is being increased and to prevent deposition of degradation products inside the various parts of the instrument. The purge gas is also used to increase heat transfer to the sample and to prevent oxidation in high temperature measurements. Helium is chosen for this purpose because of it has a high thermal conductivity [4].
Figure 2: The TMA Instrument [5].
1.2 Operating the TMA
The instrument is warmed up before putting the sample. The sample is prepared by according to the modes used. For example, the sample should be flat for compression modes to make sure the sample is in a good contact with the probe. The sample is put into the furnace and the probe touched the sample. The probe is integrated into an inductive position sensor. For temperature measurement of the sample, the thermocouple is placed near the sample.
The system is heated at a slow rate. If the specimen expands or contracts, the probe will be moved. By applying the force on the sample from the Force Generator by the probe, the sample temperature is changed in the furnace. The sample deformation such as Thermal Expansion and Softening with changing temperature is measured as the probe displacement by the Length Detector. Linear Variable Differential Transformer (LVDT) is used for Length Detection sensor. The measurement consists then of a record of force and length versus temperature [6].
1.3 Calibration of the instrument
The calibration of the instrument needs to be done before using the TMA. A standard calibration is needed and should be done at the same heating rate as is used in the sample experiment. The calibration sample that is used should be in the thickness range span of the sample. The TMA instrument must be calibrated in height force, eigen-deformation, temperature, and expansion. The calibration of the instrument need to be done to reduce the different between measures values of the parameters with their true values [4].
1.4 Modes of operation
The measurement can be carried out in various modes of sample configuration.
Figure 3: TMA probe types (left-right): compression, penetration, tension, volumetric [7].
Compression probe is used for applying low load for a wide area of sample for thermal expansion measurements (thermodilatometry). For the penetration probe, it is used for the sample that applies a high load under a small area of sample for the purpose of measuring softening temperatures. This technique can measure the transitions of a material as thin as 10 microns.
Tension probe used for the measurement of the thermal expansion and the thermal shrinkage of the sample for measuring non-self supporting specimens. It is used for the samples like thin films and fibres under tension [7].
Volumetric probe used for measuring the thermal volumetric expansion of irregularly shaped sample surrounded by an inert packing material (alumina powder or silicone oil). The glass dilatometers are used in this technique where the change in the level of a liquid is observed. By dipping the sample in the silicone oil or alumina powder, the expansion of the sample is recorded as a vertical movement by the TMA. The probe for this technique usually has a small surface area to cover the sample [8].
Figure 4: volumetric probe [8].
References
Menard, K. P. (2008). Dynamic mechanical analysis: a practical introduction. CRC press.
Menczel, J. D., & Prime, R. B. (2009). Thermal analysis of polymers, fundamentals and applications. John Wiley & Sons.
Brown, M. E., & Gallagher, P. K. (1998). Handbook of thermal analysis and calorimetry: Principles and Practice (Vol. 1). Elsevier.
Wunderlich, B. (2005). Basics of thermal analysis. Thermal Analysis of Polymeric Materials.
1.Introduction to Thermomechanical Analysis. Available from: http://www.anasys.co.uk/library/tma1.htm.
2.Brown, M.E. and P.K. Gallagher, Handbook of thermal analysis and calorimetry: Principles and Practice. Vol. 1. 1998: Elsevier.
3.Wunderlich, B., Basics of thermal analysis. Thermal Analysis of Polymeric Materials, 2005.
4.Menczel, J.D. and R.B. Prime, Thermal analysis of polymers, fundamentals and applications. 2009: John Wiley & Sons.
5.TMA analyzer / thermomechanical. Available from: http://www.directindustry.com/prod/linseis-thermal-analysis/tma-analyzers-thermomechanical-30771-406852.html.
6.Description of TMA. Available from: http://www.hitachi-hitec-science.com/en/products/thermal/tec_descriptions/tma.html.
7.Haines, P.J., Principles of thermal analysis and calorimetry. Vol. 30. 2002: Royal society of chemistry.
8.Menard, K.P., Dynamic mechanical analysis: a practical introduction. 2008: CRC press.
References
Introduction to Thermomechanical Analysis. from http://www.anasys.co.uk/library/tma1.htm
Brown, Michael E, & Gallagher, Patrick K. (1998). Handbook of thermal analysis and calorimetry: Principles and Practice (Vol. 1): Elsevier.
Wunderlich, Bernhard. (2005). Basics of thermal analysis. Thermal Analysis of Polymeric Materials.
Menczel, Joseph D, & Prime, R Bruce. (2009). Thermal analysis of polymers, fundamentals and applications: John Wiley & Sons.
TMA analyzer / thermomechanical. from http://www.directindustry.com/prod/linseis-thermal-analysis/tma-analyzers-thermomechanical-30771-406852.html
Description of TMA. from http://www.hitachi-hitec-science.com/en/products/thermal/tec_descriptions/tma.html
Haines, Peter J. (2002). Principles of thermal analysis and calorimetry (Vol. 30): Royal society of chemistry.
Menard, Kevin P. (2008). Dynamic mechanical analysis: a practical introduction: CRC press.
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