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DTA (Differential Thermal Analysis)

Differential Thermal Analysis (DTA) is a "fingerprinting" technique that provides information on the chemical reactions, phase transformations, and structural changes that occur in a sample during a heat-up or a cool-down cycle. The DTA measures the differences in energies released or absorbed, and the changes in heat capacity of materials as a function of temperature.

All materials behave in certain, predictable ways when exposed to certain temperatures, so the resulting DTA curve is an indication of the materials and phases present in the sample. For example, the DTA is used to indicate the relative magnitude of reactions and phase transitions of ceramic materials or batches that can be destructive so that safe drying and firing schedules can be determined. The DTA identifies the temperature regions and the magnitude of critical events during a drying or firing process such as drying, binder burnout, carbon oxidation, sulfur oxidation,structural clay collapse, Alpha to Beta quartz transition, carbonate decompositions, reggcrystalizations, melting and cristobalite transitions. A DTA of a solder or braze alloy will indicate the solidus and liquidus temperatures of that alloy.

A. Characteristics or Properties Measured
Drying, decomposition, oxidation, sintering, phase transformation, devitrification, recrystalization, melting or liquidus temperature, solidification or solidus temperature, glass transition temperature (Tg), curie point, energy of reaction, and others.

Examples of Applications
The test results are a graph of the DTA signal (microvolts) on the Y-axis plotted versus the sample temperature in °C on the X-axis. Sample graphs of enhanced output are shown below.

Ceramics-Clay Analysis

Metals-Solidus/Liquidus Determinations

Range of Test Conditions:

Room Temperature to +1,600°C
Ambient, Inert, and Reducing Atmospheres
Simple Heat-up and Cool-down, Multiple Cycles, and Programmed Thermal Cycles
Sample Size - 150 mm³



	Model DT-732	Model DT-736
Temperature Range	20°C to 1,200°C	20°C to 1,600°C
Sensitivity	1 micro-volt	1 micro-volt
Differential Thermocouple	Type "S"	Type "S"
Sample Cup	Boersma Design, High Alumina	Boersma Design, High Alumina
Sample Volume	10 to 250 mm³	10 to 250 mm³
Atmospheres	Air, Inert, Reducing, Vacuum	Air, Inert, Reducing, Vacuum
Temperature Control	Muli-segment PID Controller, Phase-angle fired SCR's	Muli-segment PID Controller, Phase-angle fired SCR's
Power Requirements	120 VAC, 15 amp, 50/60Hz (240 VAC avail.)	120 VAC, 15 amp, 50/60Hz (240 VAC avail.)
DTA Module Dimensions	18” W x 12” D x 25” T (460 x 305 x 635 mm)	18” W x 12” D x 25” T (460 x 305 x 635 mm)
Control Console Dimensions	18” W x 12” D x 5” T (460 x 305 x 130 mm)	18” W x 12” D x 5” T (460 x 305 x 130 mm)

*Descriptions and specifications are subject to change without notice.

Other sample cup designs are available. All Orton dilatometers are supplied with data acquisition and analysis software for computer display, storage and data analysis.

C. Additional Information on DTA

Principle of Operation

DTA measures the temperature difference between a reference material and the sample during a heat up or cool down. The temperature difference is an indication of the type of event that is occurring in the sample, and its magnitude.

A sample of the test material is placed into a special shape cup so the test material surrounds a thermocouple bead. The cup is made from a sintered, high purity alumina, which is relatively inert to the test sample. An identical cup (the reference cup) is filled with an inert material (powdered, high purity alumina) and is placed immediately beside the sample cup. When both cups are uniformly heated at a constant rate, the increasing temperature must pass through the materials in the cups in order to raise the temperature of the buried thermocouple beads. The "DTA signal" is the difference in temperature between these two thermocouple beads while the sample cups are heated, and is constantly saved on the computer along with the temperature inside the reference cup and the elapsed time.

The DTA curve plots the "DTA signal" in microvolts on the Y-axis against the reference material temperature on the X-axis. If nothing is occurring in the sample material and the reference material (the reference material has been carefully selected so no reactions or transitions occur throughout the test temperature range) , the heat will pass through both materials at the same rate and raise the temperature of both thermocouple beads at the same rate. The difference between the two thermocouple temperatures is zero, so a flat line is generated.

Endothermic Reaction

An endothermic reaction is a chemical reaction that must absorb a certain amount of energy in order to proceed to completion. When an endothermic reaction is encountered, the temperature of the material remains constant while the energy is absorbed. The thermocouple bead inside the material remains at the same temperature, even though the temperature outside the cup is rising. Meanwhile, no reactions or transitions are occurring in the reference material, so the temperature of the reference material continues to rise. The differential signal between the sample and reference thermocouples becomes negative, and the DTA curve drops. Once enough energy is absorbed and the endothermic reaction is complete, the temperature of the sample material quickly rises to catch up with the reference material. The resulting temperature differential reverses back to zero, and the DTA curve rises back to an equilibrium position. This endothermic reaction creates a "valley" in the DTA curve. The depth and breadth of this valley is an indication of the magnitude, temperature range, and speed of the reaction.

Exothermic Reaction

An exothermic reaction is a chemical reaction that releases a certain amount of energy upon its completion. When an exothermic reaction is encountered, the temperature of the material quickly rises above the outside temperature, and the thermocouple bead inside the material quickly rises above than the temperature outside the cup. Meanwhile, no reactions or transitions are occurring in the reference material, so the temperature of the reference material continues to rise, but not as fast as the sample material. The differential signal between the sample and reference thermocouples becomes positive, and the DTA curve rises. Once the exothermic reaction is complete, the temperature of the reference material quickly catches up. The resulting temperature differential reverses back to zero, and the DTA curve drops back to an equilibrium position. This exothermic reaction creates a "peak" in the DTA curve. The depth and breadth of this peak is an indication of the magnitude, temperature range, and speed of the reaction.

D. Frequently Asked Questions:

Temperature: Standard tests range from ambient temperature up to 1,600°C.
Atmospheres: Tests are normally performed in ambient air. Inert atmospheres can be used. If reducing or reactive atmospheres or vacuums are required, the Model DT-720 series DTA is required. See Orton for details.
Thermal Cycle: Most DTA tests are performed at a standard heating rate to the maximum temperature, then discontinued. The thermal cycle can be extended to include the cooling data. Other thermal cycles that contain multiple ramps and soaks, or multiple cycles (for solidus/liquidus determinations) are easily programmed by the user with the multi-segment controller that is supplied with the system as standard.
Heat-up Rate: Most DTA samples are normally heated from ambient to the maximum temperature at 10°C per minute. Other heat-up rates can be used. Faster heating rates are used to capture reactions that occur quickly. Fast heating rates generate deeper valleys and higher peaks, and will shift the onset temperature above the actual. Slower heating rates may miss fast and minor reactions. Slow heating rates generate shallower valleys and lower peaks, but will display the onset temperature closer to the actual. These various heating rates are are easily programmed by the user with the multi-segment controller that is supplied with the system as standard.
Sample Size: The sample cup holds approximately 150 mm³ of fine powder or liquid.
Sample Cup: The standard sample cup supplied with the system is a Boersma style cup made from a fine grained, dense, high purity alumina. A cross section sketch of the cup is shown in the "Principle of Operation" section above. The Boersma design allows the sample and reference materials to surround the thermocouple beads without contacting the beads. This protects the thermocouple beads from contamination. Orton has other cup designs if desired. The high alumina was selected due to its non-reactive nature with most materials at elevated temperaures. If the high alumina is not a desirable cup material, other materials such as platinum are available.
Sensitivity: The output of a type "S" thermocouple at 1,000°C is 9.587 milli-volts (9,587 micro-volts), and at 1,010°C is 9.703 milli-volts (9703 micro-volts). The output difference for only 1°C is 11.6 milli-volts, or 11.6 micro-volts. With a sensitivity of 1 micro-volt, the DTA with a type "S" differential thermocouple is capable of discerning a temperature difference between the sample and reference material of 0.08°C. For testing temperartures less than 1,200°C, a type "K" or type "N" differential thermocouple can be used for increased sensitivity. The output of a type "K" thermocouple at 1,000°C is 41.276 milli-volts (41,276 micro-volts), and at 1,010°C is 41.665 milli-volts (41,6653 micro-volts). The output difference for only 1°C is 389 milli-volts, or 38.9 micro-volts. With a sensitivity of 1 microvolt, the DTA with a type "K" or type "N" differential thermocouple is capable of discerning a temperature difference between the sample and reference material of 0.025°C.
DSC versus DTA: See the next section for a more complete discussion.

Comparing the DTA versus the DSC
DTA	DSC
1. Provides the same information as the DSC.	1. Provides the same information as the DTA.
2. Larger sample size - for ceramics and metals that require larger amounts to insure sample homogeneity.	2. Small sample size - ideal for polymers, petrochemicals, pharmaceuticals, etc. where sample homogeneity is not a concern. Ceramic materials require averaging the results from several runs to insure homogeneity.
3. Larger thermal mass furnaces to insure isothermal conditions for samples, slower sample turnaround.	3. Smaller thermal mass furnaces that provide rapid sample turnaround.
4. Less complicated system, and lower priced than the DSC.	4. More sophisticated, more complex, and more expensive.

DSC

DTA (Differential Thermal Analysis) and DSC (Differential Scanning Calorimetry, by the way, there are two versions) are thermoanalytical techniques that reveal the same information about materials, but use different methods to determine that information. They provide the same information on how a specimen “got from here to there” (the chemical reactions, phase and structural changes, and energies of reactions and changes) during a heatup or cool down cycle. Although the output traces of the DSC’s and DTA are visually similar, the operating principles are very different.

The DSC was developed for quickly and automatically analyzing the energies of reactions of low temperature materials such as polymers, petrochemicals, and pharmaceuticals. These types of materials are extremely uniform, so small sample sizes are homogeneous and the results are reliable. Ceramic materials are not as homogeneous, so sample sizes that are larger than DSC cups can accommodate are desired.

There is some confusion concerning DSC’s. Perkin-Elmer markets the only “power compensated” DSC, and is patented. TA, Netzsch, and others, provide a “heat flux” DSC, which is based on the DTA principle.

DSC (heat flux) is a more sophisticated version of DTA, which measures the temperature difference between a reference material and the sample during a heat up or cool down. The temperature difference is an indication of the type of event that is occurring in the sample, and its magnitude.

DSC (power compensated) measures the amount of electric energy required to maintain equal temperatures of the reference material and sample during a heatup or cool down. The electrical energy required is an indication of the type of event that is occurring in the sample, and its magnitude.

For the power compensated DSC there are separate containers for both sample and reference materials. Each container has its own heating element and temperature measuring device. The heating element and the temperature measuring device are discs with imbedded platinum coils that are separated by a thin electrically insulating wafer. Temperature is determined by a platinum resistance thermometer (RTD) which measures the resistance of the platinum as a function of temperature. The sample and reference chambers are heated equally until some reaction or change occurs within the sample material. As the temperature of the sample material infinitesimally deviates from the reference material temperature, the heat input to one container is decreased and the heat input to the other container is increased to maintain a zero temperature difference between the two containers. This is referred to as a “null balance” technique. The quantity of electrical energy per unit time that is supplied to the heating elements to maintain this null balance (over and above the normal thermal schedule) is assumed to be proportional to the heat release (exothermic reaction) or heat absorbed (endothermic reaction) by the sample. The DSC curve is expressed in Watts (energy per unit time) as the Y-axis plotted against the sample temperature as the X-axis.

This power compensated DSC has two control cycle portions. One strives to maintain the null balance between the sample and reference material, and the other strives to maintain the desired test heat up or cool down rate. These processes switch back and forth continuously to maintain both portions simultaneously.

For rapid cooling, both sample and reference cells are surrounded by a cooling jacket.

To maintain good thermal contact for rapid system response, the sample chamber and sample size must be small.

DTA

The DTA is a "passive" system, that is the material is simply heated and the differential thermocouple output signal is saved for post testing analysis. For ceramic materials, the DTA is a far simpler, and less expensive instrument that generates the same information on a larger sample in one test.