What Is a Thermogram?

The seventh conference of the International Conference on Thermal Analysis (ICTA) held in Kyoto, Japan in 1977 defined the following thermal analysis: Thermal analysis is the measurement of the physical properties of a substance between temperature and temperature under a program-controlled temperature A type of relationship technology. The most commonly used thermal analysis methods are: differential (indicative) thermal analysis (DTA), thermogravimetry (TG), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and Dynamic Thermal Mechanical Analysis (DMA), etc. Thermal analysis technology has been widely used in physics, chemistry, chemical engineering, metallurgy, geology, building materials, fuels, textiles, food, biology and other fields.

Thermal analysis (TA) refers to the method of analyzing the relationship between thermodynamic parameters or physical parameters with temperature. The International Thermal Analysis Association defined thermal analysis in 1977 as: "Thermal analysis is a type of technology that measures the physical properties and temperature dependence of a substance under a program-controlled temperature." According to the measured physical parameters, it is divided into multiple methods ^{[ 1]}

The most commonly used thermal analysis methods are: differential (indicative) thermal analysis (DTA), thermogravimetry (TG), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and Dynamic Thermal Mechanical Analysis (DMA). In addition: outgas detection (EGD), outgas analysis (EGA), twisted braid thermal analysis (TBA), jet gas thermal analysis, thermal particle analysis, thermal expansion method, thermal sound method, thermooptical method, thermoelectric method, Thermomagnetic method, temperature titration method, direct injection enthalpy method, etc. There are thermal expansion method, thermal sound method, thermal sound transmission method, thermo-optical method, thermo-electric method and thermo-magnetic method to determine the size or volume, acoustic, optical, electrical and magnetic properties ^[2]

Thermal analysis technology can quickly and accurately determine changes in the crystal form, melting, sublimation, adsorption, dehydration, and decomposition of substances. It is an important test method for the physical and chemical properties of inorganic, organic, and polymer materials. Thermal analysis technology has been widely used in physics, chemistry, chemical engineering, metallurgy, geology, building materials, fuels, textiles, food, biology and other fields.

1. The sample can be studied in a wide temperature range;

2. Can use a variety of temperature programs (different temperature rise and fall rates);

3. No special requirements for the physical state of the sample;

4. The required sample amount is very small (0.1g-10mg);

5. High sensitivity of the instrument (the accuracy of the quality change is 10 ^-5 );

6. Can be used with other technologies;

7. A variety of information is available.

DSC) Thermal analysis differential scanning calorimetry (DSC)

Differential scanning calorimetry is a technique for measuring the relationship between the power difference between the input material and the reference material and the temperature under a programmed temperature. Can be divided into power compensation DSC and heat flow DSC.

Power compensation DSC schematic diagram:

The power-compensated DSC is internally heated, and the sample and reference holders are independent components. At the bottom of the sample and reference, there is a platinum thermal resistance for heating and a platinum sensor for temperature measurement . It uses the principle of dynamic zero balance, that is, the temperature of the sample and the reference is required to maintain the dynamic zero balance state when the sample absorbs or exotherms, that is, to keep the temperature difference between the sample and the reference to zero. DSC measures the energy difference (W = dH / dt) required to maintain the sample and reference at the same temperature, which reflects the change in sample enthalpy.

Heat flow DSC schematic diagram:

1. Brass copper plate; 2. Thermocouple junction; 3. Nickel-chrome plate; 4. Nickel-aluminum plate; 5. Nickel-chromium wire; 6. Heating block.

The heat flow type DSC is an external heating type. It adopts external heating to heat the isothermal block and then transfers heat to the sample cup and the reference cup through the air and the hot pad made of constantan. The temperature of the sample cup is Nickel-chromium wire and nickel-aluminum wire are used for high-sensitivity thermocouple detection. The temperature of the reference cup is measured by a thermocouple composed of nickel-chromium wire and constantan. It can be known that what is detected is the temperature difference T, which is a reflection of the change in the heat of the sample.

This technology is widely used in a range of applications, both as a routine quality test and as a research tool. The device is easy to calibrate, uses a low melting point, and is a fast and reliable method of thermal analysis. A variety of thermodynamic and kinetic parameters can be determined, such as specific heat capacity, reaction heat, transition heat, phase diagram, reaction rate, crystallization rate, polymer crystallinity, sample purity, etc. This method has a wide temperature range (-175 ~ 725 ° C), high resolution, and low sample usage. It is suitable for the analysis of inorganic substances, organic compounds and drugs. At the same time, differential scanning calorimetry can quantitatively determine various thermodynamic parameters, with high sensitivity and low operating temperature, so it has a wide range of applications, and is particularly suitable for research in the fields of polymers, liquid crystals, food industry, medicine and biology. Work ^[3] .

DTA Thermal Analysis Differential Thermal Analysis (DTA)

Differential thermal analysis is a comparison of a stable substance (reference substance) that does not undergo any chemical reaction and physical change at a certain experimental temperature with the same amount of unknown substance under the condition of moderately rapid temperature change in the same environment. Any chemical and physical change will temporarily increase or decrease compared to the temperature of the standard in the same environment. Decreasing manifests as an endothermic reaction, and increasing manifests as an exothermic reaction. Can be divided into sealed tube DTA, high pressure DTA instrument, high temperature DTA instrument and trace DTA instrument.

Differential thermal analysis schematic diagram:

A general differential thermal analysis device is composed of a heating system, a temperature control system, a signal amplification system, a differential thermal system, and a recording system. Some models also include an atmosphere control system and a pressure control system.

When the same amount of heat is given to the test object and the reference object, due to their different thermal properties, the temperature rise must be different. The analysis purpose is achieved by measuring the temperature difference between the two. The curve obtained by taking the temperature difference between the reference and the sample as the ordinate and temperature as the abscissa is called a DTA curve.

In the differential thermal analysis, in order to reflect such a small temperature difference change, a temperature differential thermocouple is used. It is made of two different wires. Generally, an appropriate section of a nickel-chromium alloy or a platinum-rhodium alloy is used, and the two ends of the platinum wire are respectively welded with two pieces of platinum wires of equal thickness by arc welding to form a thermocouple.

In the differential thermal identification, powder samples of the same amount and granularity as the reference are divided into two crucibles, and the bottom of the crucible is in contact with two welding points of the thermocouple and the two crucibles. At equidistant and equal heights, there are temperature measuring thermocouples for measuring the temperature of the heating furnace, and their respective ends are respectively connected to the circuit of the recorder.

In the process of constant temperature heating, the temperature and time are linear, that is, the temperature change rate is relatively stable, which is convenient for accurately determining the temperature when the sample reaction changes. There is no change in the sample in a certain temperature rising area, that is, it neither absorbs heat nor emits heat. There is no temperature difference between the two welding points of the thermocouple. The thermograph is a straight line, which is called the baseline. If the sample produces a thermal effect in a certain temperature range, a temperature difference is generated at the two welding points of the temperature difference thermocouple, and a thermoelectric potential difference is generated at both ends of the temperature difference thermocouple. The signal is amplified into the recorder and the recording device is pushed away from the baseline. Move and return to baseline after the reaction is complete. The directions of the thermoelectric potentials generated by the endothermic and exothermic effects are opposite, so they are reflected on the differential thermal curve at the two sides of the baseline. The magnitude of this thermoelectric potential, in addition to being proportional to the number of samples, is also related to the material itself. About nature. The magnitude and temperature of the thermoelectric potential produced by different substances are different, so the differential thermal method can be used not only to study the properties of substances, but also to identify unknown substances based on these properties.

Its characteristics are:

1) Water-containing compounds

For substances containing adsorbed water, crystal water or structured water, when water is lost during heating, an endothermic effect occurs, and an endothermic peak is formed on the differential heat curve.

2) Substances that emit gas at high temperature

Some chemical substances, such as carbonates, sulfates, and sulfides, due to the release of gases such as CO ₂ and SO ₂ during the heating process, have endothermic effects that appear as endothermic peaks on the differential heat curve. Different types of substances emit gas at different temperatures, and the shape of the differential heat curve is different. Using this feature, different types of substances can be distinguished and identified.

3) Minerals contain variable elements

Minerals contain variable-value elements, oxidize at high temperatures, and change heat from low-value elements to high-value elements, which appear as exothermic peaks on the differential heat curve. Different valence elements, and different conditions in the lattice structure, the temperature of the exothermic effect due to oxidation is also different.

4) Recrystallization of amorphous materials

Some amorphous materials are accompanied by recrystallization during the heating process, releasing heat and forming exothermic peaks on the differential thermal curve. In addition, if the lattice structure of the substance is destroyed during the heating process and the lattice restructuring occurs after the substance becomes an amorphous substance, an exothermic peak is also formed.

5) Crystal transition

Some substances absorb heat due to crystalline transformation during heating, forming endothermic peaks on the differential heat curve. Therefore, it is suitable for analysis and identification of metals or alloys and some inorganic minerals.

DSC and DTA instruments are similar. The difference is that two sets of compensating heating wires are installed under the sample and reference container. When the sample has a temperature difference T between the reference and the reference due to thermal effects during the heating process, the difference is passed. The thermal amplifier circuit and differential heat compensation amplifier make the current flowing into the compensation heating wire change. When the sample absorbs heat, the compensation amplifier immediately increases the current on one side of the sample; otherwise, when the sample releases heat, the The current on one side of the specific object increases until the heat balance on both sides and the temperature difference T disappears. In other words, the thermal change of the sample during the thermal reaction is compensated due to the timely input of electric power, so what is actually recorded is the relationship between the difference in thermal power between the two electrothermally compensated samples below the sample and the reference over time t . If the heating rate is constant, the relationship between the difference in thermal power and the temperature T is recorded.

TGA Thermal Analysis Thermogravimetric Analysis (TGA)

Thermogravimetric analysis (TG) is a technique for measuring the relationship between the mass of a substance and temperature at a programmed temperature. Many substances are often accompanied by changes in mass during the heating process, and this change process is helpful to study the changes in crystal properties. Physical phenomena such as melting, evaporation, sublimation, and adsorption also help to study chemical phenomena such as dehydration, dissociation, oxidation, and reduction of substances.

TG analysis curve:

When the substance under test undergoes sublimation, vaporization, decomposition of gas or loss of crystal water during heating, the mass of the substance under test will change. At this time, the thermogravimetric curve is not a straight line but decreases. By analyzing the thermogravimetric curve, you can know how many degrees the measured substance changes, and based on the weight loss, you can calculate how much material is lost.

The thermogravimetric analyzer is mainly composed of a balance, a furnace, a program temperature control system, and a recording system.

There are two most commonly used measuring principles, namely the displacement method and the zero method. The so-called displacement method is based on the relationship between the tilt of the balance beam and the change in mass, using a differential transformer to detect the tilt and automatically record it. The zero-position method uses the differential transformer method and the optical method to measure the tilt of the balance beam, and then adjusts the current of the coil installed in the balance system and the magnetic field to make the coil rotate to restore the tilt of the balance beam. Because the force applied by the coil is proportional to the change in mass, and this force is proportional to the current in the coil, you only need to measure and record the change in current to get a curve of the mass change.

Through TGA experiments, it is helpful to study the changes of crystal properties, such as physical phenomena of substances such as melting, evaporation, sublimation, and adsorption; it is also helpful to study the chemical phenomena of substances such as dehydration, dissociation, oxidation, and reduction. Thermogravimetric analysis can generally be divided into two categories: dynamic (temperature rising) and static (constant temperature). The curve obtained by the thermogravimetric method is called thermogravimetric curve (TG curve). The TG curve uses mass as the ordinate, which indicates the decrease in mass from the top to the bottom; temperature (or time) as the abscissa, and the temperature from left to right ( Or time) by ^[1] .

DMA Thermal Analysis Thermomechanical Analysis (DMA)

Dynamic thermomechanical analysis is performed by applying a known amplitude and frequency of vibration to a material sample, measuring the applied displacement and the resulting force to accurately determine the viscoelasticity, Young's modulus (E *) or shear modulus of the material. (G *). It can be divided into:

1. Thermal expansion method

The thermal expansion method is a technique for measuring the relationship between the size and temperature of a substance under a negligible load under a programmed temperature control.

2.Static thermomechanical analysis method

Static thermomechanical analysis is a technique for measuring the relationship between the temperature and deformation of a substance under a non-vibrating load under a programmed temperature control.

3.Dynamic thermomechanical analysis

Dynamic thermomechanical analysis is a technique for measuring the relationship between the dynamic modulus or mechanical loss of a substance under a vibration load and temperature under a programmed temperature control.

DMA is mainly used for: glass transition and melting test, secondary transition test, frequency effect, optimization of transition process, characterization of nonlinear characteristics of elastomer, fatigue test, characterization of material aging, immersion test, long-term creep Estimate the best material characterization scheme.