Study of the properties of functional materials by thermoanalytical and thermophysical methods

C.C.S.   T.I. Vetrova, E. Kaisersberger

Office of NETZSCH-Gerätebau GmbH (Germany) in Russia

 

Thermal analysis is a group of methods in which the change in the physical and chemical properties is recorded as a function of temperature or time during the temperature program (ICTA, ASTM 473-85). Temperature program may include heating, cooling at a constant rate, maintaining at a constant temperature (isotherm) and a combination of these modes.

Methods of thermal analysis and its application is determined by the standarts ISO, DIN, CEN and ASTM. "International Confederation for Thermal Analysis" (ICTA) defines and recommends the nomenclature, definitions, procedures and materials for the standardization of methods of thermal analysis.

 

 

The main methods of thermal analysis are:

 

  1. Thermogravimentry (TGA)
  2. Differential thermal analysis (DTA)
  3. Differential scanning calorimetry (DSC)
  4. Synchronous thermal analysis STA (DSC-TGA)
  5. Analysis of released gases (ARG)
  6. Dilatometry
  7. Thermomechanical analysis (TMA)
  8. Dynamic mechanical analysis (DMA)

 

Thermal analysis can be applied at different stages of working with materials, including functional materials:

- technological development of production processes, optimization of the synthesis conditions.

- study of the properties of the starting materials, intermediates, final materials.

- definition of the conditions of use and maintenance of created materials.

For studies of functional materials all methods of thermal analysis are widely used. Application of thermal analysis instruments to study functional materials imposes high requirements to the technical characteristics of the devices, namely, stability, accuracy, reproducibility, sensitivity, etc.

Examples of research of functional materials by means of thermal analysis

 

1. The melting of functional materials

 

DSC method allows the study the melting of very small amounts of nano-sized metal particles dispersed in a metal matrix (Fig. 1). Differential scanning calorimetry (DSC) can detect changes in the melting temperature, depending on the size of the particles.

 

Melting curves of nanodispersed lead, constituting 1 at.% in the crystalline Al matrix (17.42 mg in Al crucibles, punctured caps, N2 atmosphere, 10 K/min).

 

Figure 1. Melting curves of nanodispersed lead, constituting 1 at.% in the crystalline Al matrix (17.42 mg in Al crucibles, punctured caps, N2 atmosphere, 10 K/min).

 

 

 

 

2. Thermal stability of carbon nanotubes

 

Synchronous DSC-TG analysis is a powerful tool for studying the thermal behavior of powders under the influence of different atmospheres. Samples of carbon nanotubes show a significant difference of thermal properties in oxidizing conditions, depending on the preparation method (Figure 2). For the two tested samples it reveals the same content of oxidized carbon 92.67%, which is burned in the temperature range of 400-750оС, but the content of volatile components prior to oxidation is 10 times higher in the modified sample, and the balance in it at 1000оС (ash) is 3 times smaller. The temperature range of oxidation of the samples of CNTs is much lower than the known range for the bulk graphite and diamond samples.

 

Comparison of oxidation and thermal stability of the two samples of carbon nanotubes (thermogravimetry) and DSC

 

 

Figure 2. Comparison of oxidation and thermal stability of the two samples of carbon nanotubes (thermogravimetry) and DSC

 

 

3. Agglomeration of nanopowders

 

Three of the pelletized powders of barium titanate, crushed to different particle sizes in the nanometer range, were measured on the dilatometer at a heating rate 3 K/min (Fig. 3). Effect of particle size on the position of the range of agglomeration of binder-free ceramics can clearly be seen on the reduce of the sagglomeration temperature more than 80оС, and the change in the mechanism of sagglomeration (see the curves of TEC).

These results show a higher activity of nanomaterials during agglomeration, which can be used to optimize the conditions of the processes to conserve time and energy.

 

 

Agglomeration of barium titanate with different size particles.

 

Figure 3. Agglomeration of barium titanate with different size particles.

 

 

4. Thermogravimetry combined with gas analysis

 

Simultaneous thermogravimetric and gas analysis, determination of reaction and decomposition products is possible also for nanomaterials. This is well demonstrated on titanium dioxide powders with different functional polymeric coating. Decomposition of the coating in the ceramic powder and the identification of the type of polymer in the course of gas analysis-Fourier-IR with spectrometer is shown in Figure 4. In case of polymeric coating (methyl methacrylate), its thermal decomposition begins at 200°C, whereas the coating of polystyrene is stable up to 380°C. Start of the processes of decomposition and gas evolution processes taking place at the same temperature, as seen from the change in the integrated IR-curve (Gram Schmidt plot) and TG curve. The identification of gas components was done by searching the spectra in the library gas phase.

 

 

Expansion of the coating on nanosized TiO2.

 

 

 

 

 

 

 

Fig. 4. Expansion of the coating on nanosized TiO2.

 

 

5. Nanomaterials for hydrogen storage

 

Allanites (aluminum hydride, sodium, magnesium) may be used for hydrogen storage, due to its chemical linking. It is shown that grinding of magnesium allanite to nanoscale leads to a decrease in temperature release of hydrogen from 160°C to 120°C (DTG temperature peak). From Fig. 5 is clear that the release of hydrogen from magnesium allanite in vacuum (grain size about 30 nm) is accompanied by the loss of weight at 163°C. Weight spectrometer at this point identifies hydrogen (m/z = 2), as the gas released from the sample. Also, for those objects, and similar chemical compositions at the DSC of high pressure was carried out a research on the reversibility, and the pressure and temperature ranges of the absorption and release of hydrogen were also investigated.

 

 

 

The release of hydrogen from the nano-sized magnesium allanite during the measurements of TG-MS in a vacuum

 

 

Fig. 5. The release of hydrogen from the nano-sized magnesium allanite during the measurements of TG-MS in a vacuum

 

 

termometric and thermal conductivity

 

Measurement of termometric conductivity by laser flash method is a quick and accurate method for determining the parameters of thermal transport properties and structural changes of ceramic materials, metals, polymers, liquids and melts.

Figure 6 shows the temperature dependence of termometric conductivity, heat capacity and thermal conductivity of a thin diamond layer produced by chemical vapor deposition. A rapid decrease in diffusivity is clearly seen in the temperature range from room temperature to 400оС.

 

 

Termometric conductivity, specific heat and thermal conductivity for chemical vapor deposition diamond layer

 

 

 

 

Fig. 6. Termometric conductivity, specific heat and thermal conductivity for chemical vapor deposition diamond layer.

 

The experiments with polymer compositions filled with carbon nanotubes, using the method of laser flash reveal the expected increase in temperature and thermal conductivity at room temperature. The results show that the polymers containing carbon nanotubes, the thermal properties depend on orientation. The nanotubes are located at different angles, but parallel to one plane, so the termometric conductivity in the plane is 10 times higher (parallel to CNT) than in the plane (perpendicular to all CNT).

 

 

 

Table 1. Effect of orientation of CNTs in polymer matrix filled with carbon nanotubes, on the thermal conductivity and termometric conductivity.

 

Polimeric composite with CNT

Direction of measurement

Density

(g/m3)

Specific thermal capacity

J/(g .К)

Termometric conductivity

mm2/s

Thermal conductivity

W/(m .K)

Through the plane

1.191

1.212

0.172

0.248

In the plane

1.191

1.212

1.852

2.673

 

 

 

 

 

 

 

 

 

 

 

3. Conclusion

 

Methods of thermal analysis and determination of thermophysical properties offer a variety of information on materials containing nano-sized particles. Characterization of nanomaterials gives information on thermal properties, the nature of the oxidation, stability and agglomeration. Especially the determination of thermal transport properties allows us to understand the influence of orientation of CNTs using them to improve the thermal conductivity of polymer composites. Burning out the binder, agglomeration of ceramics and powder metallurgy products can be optimized using the kinetic analysis based on the thermoanalytical experiments [2, 3].

 

NETZSCH-Gerätebau GmbH manufactures a full range of equipment for thermal analysis and thermal conductivity:

 

DSC      Differential scanning calorimetry                            -180...1650°C

DTA      Differential thermal analysis                                   -150...2400°C

ТGA      Thermogravimetry                                                  -150...2400°C

STA       Synchronous thermal analysis                               -150...2400°C

DLA      Dilatometry                                                             -260...2800°C

TMА      Thermomechanical analysis                                    -150...1000°C

DMA     Dynamic mechanical analysis                                  -170...600°C

TCT       Thermal conductivity                                              inside...1500°C

LFA       Termometric conductivity                                       -100...2000°C

DEA      Dielectric analyses                                                  -150...400°C

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