DTA

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Laboratory in Physical Materials Diagnostics

Topic of the Experiment Methods of Thermal Analysis

1 Tasks 1.1.

Analyse a material sample with assistance of the simultaneous thermal analysis (STA DTA/TG).

1.2.

Determine the coefficient of linear expansion of a metal sample and compare the expansion with other materials (TMA).

2 Basics 2.1 Differential Thermal Analysis (DTA) & Thermogravimetry (TG) Thermal analysis (TA) is the generic name for methods, which allow the determination of physical and chemical properties of a material or a compound and/or  the reaction of compounds. In the dynamic thermal analysis the temperature is changed slowly (keeping the sample in the thermodynamic equilibrium) and measured together with the physical or chemical property of interest. Typically, the temperature is changed in a linear way versus temperature. In a static thermal analysis, the sample is moved rather fast to a set point temperature. temperature. Afterwards, the physical or chemical property of interest is measured versus time. The following overview shows the variety of thermal analysis methods: • Dynamic Difference Caloric Analysis (DSC) o Difference Thermal Analysis (DTA) o Dynamic Heat Flow Difference Caloric Analysis (DTSC) o Dynamic Power Difference Caloric Analysis (DPSC) • Thermo Gravimetrical Analysis (TGA) • Thermo Mechanical Analysis (TMA) Dynamic Mechanical Analysis (DMA) • • Thermo Magnetic Analysis (TM) o Optometry o Spectrometry o Luminescence (TL) o Microscopy o Refractometry o Electrometry o Sonimetry (TS)

• • • • •

Dielectric Thermal Analysis (DETA) Thermal Stimulated Current (TSC) Evolved Gas Analysis (EGA) – thermal conductivity Evolved Gas Detection (EGD) – kind and amount of created gases Emanation Thermal Analysis (ETA) - radioactive gases

This list is not complete. A number of combinations are possible. To give some examples thermogravimetry can be combined with gas chromatogaphy (TG-GC) or  infrared spectroscopy (TG-FTIR). Furthermore, different thermal analysis methods are often combined within one experimental setup, e.g. TG-DSC or TG-DTA. Often the combination of methods results in an overall sensitivity decrease. However, often the simultaneous access to different properties rules out misinterpretations. To give an example: A temperature curve measured in DSC indicating an endothermic reaction may be the result of a phase transition or a mass loss. The combination with TG allows then a decision on the real process in the sample.  A number of methods and their special application fields are standardized by the respective organizations (e.g. ASTM – U.S.A or DIN – Germany). There detailed instructions for experimental setups and guidelines for thermoanalytical experiments can be found.

2.1.1 Differential Thermal Analysis Analysis (DTA) In DTA the temperature difference of a sample and a reference material is measured versus the sample temperature. The temperature is controlled during the measurement by a suitable control unit or by a computer.  A DTA setup is shown in the figure 1 below. The difference diff erence of the temperature of the thermocouples is measured during heating of the furnace. Such difference measurement is realized simply by a series connection of the thermocouples with

Figure 1: DTA measurement system with special crucibles including thermocouples

opposite polarity. In the ideal case, the temperature difference between the sample and the reference is zero if the enthalpy of the sample does not change during heating (the reference material is not changing its enthalpy in the desired temperature range). However, in a real setup the so-called base line, i.e. the measured temperature difference is not always exactly zero. However, changes of  the baseline are rather continuous and the curve does not show peaks. Only in case of endothermic or exothermal processes in the sample, one can observe deviations or peaks to either side of the base line. The area of the peaks is proportional to the heat quantity. By measuring the temperature of the sample simultaneously, it is possible to plot the temperature difference as a function of the temperature of the sample: Formula 1: Temperature dependence ∆T  =

 f  (T )

In order to reduce the influence of systematic measurement errors the temperature is commonly changed linearly with time. A number of experimental parameters (sample material, preparation, and experimental setup) influence the shape of the base line and the position, shape and height of the peaks, respectively. Some important parameters are listed below: Heating velocity • Sample mass • Geometrical dimensions of the materials inside the furnace • Thermal conductivity of used setup parts • grain size and density of the sample material • atmosphere inside the furnace, in case of gas – flow speed • Often the atmosphere of the sample environment is used as an additional experimental parameter, e.g. oxygen influencing oxidation reactions or water vapour  influencing dewatering reactions. The combination of DTA with the thermo gravimetrical analysis (TGA) allows the simultaneous registration of temperature differences and mass changes. This technique improves remarkably the possibility for an accurate data interpretation. Therefore, modern commercial equipment allows often the simultaneous acquisition of DTA and TGA data. Both data sets – temperature difference as well as mass changes – are displayed versus the same time scale. The TGA data are plotted as derivative of the mass dm/dt, hence directly indicating the changes of the mass.

2.1.2 Equipment Setup  A typical setup of TGA T GA – DTA combination is shown in figure 2 below. The range of  the experimental parameters could be found in figure 3.

Figure 2: Device assembly of SETARAM TG-DTA 92-16

Figure 3: Experimental Parameter of SETARAM TG-DTA 92-16

2.1.3 Application Examples for DTA/TG The following examples can be reviewed also here: http://www.netzsch-thermal-analysis.com/en/products/detail/pid,6,t,4.html (i) Components of a rubber compound for tires (Fig. 4 below) The decomposition of the rubber takes place in several steps: • Plasticizer fraction desorbs (about 7% mass loss) • Rubber decomposition in fraction 1 (38% at 383°C) and fraction 2 (31% at 448°C) The carbon black portion is calculated to be 20% of mass, the ash content is 4%. The peak positions of the derivative of the TGA-curve (d∆m/dt, i.e. DTGA) allow the exact determination of the process onset temperatures. For the investigated sample, it has been concluded that the compound is a back-fiulled NR/SBR rubber blend.

Figure 4: Carbon black-filled NR/SBR rubber blend

Thermal decomposition of dolomite in a CO2-atmosphere The mass loss steps during the thermal decomposition of the mineral dolomite [CaMg(CO3)2], shown in figure 5, overlaps when the measurement is performed in a N2 atmosphere. By using CO2 as a purge gas, they can be clearly separated. The calculated DTA signal additionally yields the information that both mass loss processes are endothermic.

Figure 5: Thermal decomposition of dolomite in a CO2-atmosphere

Plasticizer content in a rubber compound In a standard measurement, the evaporation of a low-molecular plasticizer is overlapped by the decomposition of the elastomeric components (red curves in fig. 6). If the measurement is performed in vacuum, the evaporation of the plasticizer is shifter to lower temperatures (black curves in fig. 6). This behaviour is a direct consequence from the reduced pressure and the resulting reduction of the boiling temperature of the plasticizer. In this way, the plasticizer content can be determined much more precisely.

Figure 6: Plasticizer content in a rubber compound measured in air(red) or vacuum (black)

2.2 TMA TMA is the abbreviation for thermo-mechanical analysis. In this type of thermal analysis changes of sample geometry are monitored versus temperature changes. The most popular TMA is the dilatometry, i.e. the measurement of geometry changes in one dimension. In general, dilatometers are used to measure linear length variation of a sample as a function of the temperature. The application fields cover the analysis of shrinking, length changes due to recrystallization or simply the det ermination of the linear thermal expansion coefficient α(T).Figure 7 shows the principle design of a dilatometer.

Figure 7: Dilatometer with a rod shaped sample in horizontal configuration (WA - expansion measurement unit, A - supporting unit, S - push rod, O - furnace, P - sample)

In a furnace, the sample is placed on a sample holder. The sample holder should allow the movement of the sample with minimum friction losses. Attached to the sample the push rod is found. The push rod transfers the sample movement to the expansion measurement unit. Inside of this unit, the movement is detected based on electrical induction. The determination of the correct thermal expansion of the sample requires the measurement of the length change (rod + sample) but also the knowledge of the thermal expansion coefficient of the transfer rod. Further requirements for a dilatometric measurement are: • No forces or torques introduced by the setup or sample geometry • Sample holder must have same temperature as the sample throughout the experiment

2.2.1 TMA Equipment  A schematic drawing of the available experimental experimental setup is shown in figure 8 below. below.

Figure 8: Overview NETZSCH TMA 402

2.2.2 Application Examples for TMA The following examples you can look up at this hyperlink: http://www.netzsch-thermal-analysis.com/en/products/detail/pid,15,t,4.html Polycrystalline Alumina (Al2O3) Figure 9 presents comparison of three test runs (lines) of a polycrystalline alumina with the corresponding literature data (crosses) between room temperature and 1575°C. No visible deviations exist between the individual curves. Evaluation of the thermal expansion values at 500°C, 1000°C and 1500°C clearly shows that the measurement results are within 1% of the corresponding literature data. The test demonstrates the outstanding reproducibility and accuracy of the used dilatometer.

Figure 9: Polycrystalline Alumina

Glass Coefficients of thermal expansion (CTE), glass transition temperatures and softening points are crucial parameters for the characterization of glass materials. Presented in figure 10 there are three tests on same types of glass but from different batches. One can see that the coefficients of thermal expansion are in good agreement within the instrument’s uncertainty boundaries. The glass transition temperature and the softening point of sample 3 (blue curve) show slightly lower values, indicating a slightly different composition.

Figure 10: Glass

Sintering of Zircon During the production of high-tech ceramics, a ceramic powder is mixed with a binder  and pressed to a so-called green body. By thermal treatment, the binder is removed (burned out) and the ceramic is sintered to the final part. In order to determine the quality of the final part, the binder burnout and sintering temperatures as well as the shrinkage during sintering have to be known. These properties can be measured quickly and easily using push rod dilatometry. Presented in the figure 11 are tests on an yttrium-stabilized zircon green body and on the sintered ceramic.

Figure 11: Sintering of Zircon

Production of Cordierite Ceramic Cordierite is a popular magnesia-alumina-silica ceramic used in various kinds of  industrial applications. It is used, for example, as a carrier for catalysts in the automotive industry. During the production of this ceramic, various raw materials are ground and mixed to form a green body. During firing under oxidizing atmospheres, the organic additives are burned out and the cordierite phase is formed at high temperatures.

Figure 12: Heating in the production process of cordierite ceramic

3 Preparation of the Experiment You should inform yourself about: Definition of thermal analysis • This definition included analysis procedures • Understanding of the working principle of DTA, TG and TMA • Examples of use for this methods • Possibilities of temperature measurement with different temperature regions • Please also inform yourself about typical values of the thermal expansion coefficients of metals, glasses and synthetics. Solve the problems listed below in advance of the experiment.

3.1 Problems 3.1.1 Which of the following physical changes could NOT be detected by the thermogravimetry? a) b) c) d)

Loss of moisture Sublimation Melting Gas adsorption

3.1.2 Which of the following physical changes could NOT be detected by the thermogravimetry? a) CaCO3 + SO2  CaSO4 + CO2 b) CaCO3 + SiO2  CaSiO3 + CO2 c) CaCO3 + Na2SO4  CaSO4 + Na2CO3

3.1.3 Which thermal effects can be visualised by DTA? a) b) c) d)

Loss of moisture Sublimation Desorption of vapour  4FeO + O2  2Fe2O3

3.1.4 A sample of polymer was analysed by simultaneous TG-DTA. Which of the following thermal changes, that might occur, would be detected by DTA and which by TG? a) b) c) d) e) f)

Glass transition Plasticizer loss Residual curing Crystallisation Melting Oxidation and degradation

3.1.5 The decomposition of zinc oxalate dehydrate (ZnC2O4*2H2O) was shown by TG and DTA to occur in two stages with a loss of mass of about 19% at 200°C and a total loss of 57% at 400°C. If we suggest the reactions as described below, how could we identify both gaseous and solid products? a) ZnC2O4*2H2O  ZnC2O4 + 2H2O b) ZnC2O4 ZnO +CO +CO2

3.1.6 How much water and carbon dioxide would result from the decomposition of 25mg ZnC2O4*2H2O?

4 Useful hints General: Have a data storage device with you (e.g. USB - stick) to pick t he electronic data after the experiment.

4.1 DTA/TG • • • • •

•

Weight the sample (powder) inside the crucible (depending on the density in the range of 20 - 50mg) Prepare the reference material (Al2O3-powder) inside the reference crucible Definition of the measurement conditions (temperature region, heating rate, temperature profile) Check, if a tare measurement (without sample) for the temperature profile already exists Check the device parameters: o Cooling water on o Inert gas supply Argon on (inside of the furnace) Gas carrier (air, argon, nitrogen) for the sample on o PC input of the measurement parameters and start of the measurement

4.2 TMA • • • •

Estimate the chosen sample regarding to size, shape and homogeneity Definition of the measurement conditions (temperature region, heating rate, temperature profile) Check cooling water on PC input of the measurement parameters and start of measurement

5 Hints for a Correct Analysis During the discussion of the equipment setup, make notes on important constructive details, which may guide you to the best measuring results. Which conditions have to be fulfilled?

5.1 DTA/TG Plot the data of the DTA- and TGA-curve (transfer the data per mail or storage media)! Describe the observed effects between the single temperature regions and interpret them qualitatively! If it is possible, then analyse the TGA curve quantitatively and compare it with the results of a theoretical t heoretical model or other references!

5.2 TMA Plot the length variation ∆l/l0 versus temperature (transfer the data per mail or  storage media)! Evaluate the shape of the curve qualitatively! Consider particularly any sudden changes of the slope. Indicate the linear thermal expansion coefficient in selected temperature regions!

6 Literature Device producer: • • • • • • • •

http://www.setaram.com/ (SETARAM Instrumentation) http://www.netzsch-thermal-analysis.com/en/home/ http://www.netzsch-thermal-analysis.com/en/ home/ (Netzsch Instruments) http://glo.mt.com/home (Mettler Toledo TA) http://www.perkinelmer.com/ (PerkinElmer Instruments) http://www.linseis.de/html_en/thermal/thermal_start.php (Linseis) http://www.tainstruments.com/ (TA Instruments) http://www.shimadzu.com/products/lab/thermal/index.html http://www.shimadzu.com/products/lab /thermal/index.html (SHIMADZU) http://www.baehr-thermo.de/uk/index_uk.html http://www.baehr-thermo.de/uk/index_uk .html (Bähr Thermoanalyse GmbH)

 Available at the FH library: • • • • • • •

Methoden der thermischen Analyse / Wolfgang F. Hemminger. - Berlin [u.a.] : Springer, 1989 Grundlagen der Kalorimetrie : mit 6 Tab. / Wolfgang W olfgang Hemminger. - Berlin :  Akad.-Verl, 1980 Theory of calorimetry / Wojciech Zielenkiewicz. - Dordrecht [u.a.] : Kluwer, 2002 Differential scanning calorimetry : an introduction for practitioners ; with 13 tables / Günther Walther Heinrich Höhne. - Berlin [u.a.] : Springer, 1996 Modulation calorimetry : theory and applications ; with 27 tables / Yaakov Kraftmakher. - Berlin [u.a.] : Springer, 2004 Thermal analysis : a revision 5.0 tutorial / Swanson Analysis Systems. Systems. Houston, PA : Swanson Analysis Systems, c1992 Low thermal expansion glass ceramics : with 18 tables / Hans Bach. - Berlin [u.a.] : Springer, 1995