A good judge of character?

The promise of nanotechnology is great but how do you examine the miniscule? Here Erwin Kaiserberger charts the progress in finding out if thermoanalysis can be a useful technique for particles in the sub-micrometer range  

THERMAL analysis methods find broad application for the characterisation of nanomaterials during synthesis, part preparation and control of final product properties. Calorimetric methods, like DSC and high pressure DSC are applied to study melting of nanosized materials, their temperature-induced reactions and the stability ranges. Here carbon nanotube materials and nano-filled polymers were analysed by simultaneous Thermogravimetry and DSC (TG-DSC) to show the reactivity, thermal stability, and the decomposition ranges and composition. Thermal transport properties of filled polymers were also measured to determine the orientation and distribution of carbon nanotube filler in the polymer matrix.

The aim of our studies was to demonstrate that the sensitivity of modern thermoanalytical methods is well suited for general appli¬cations on materials with a reduced particle size into the sub-micrometer range and that the results provide important parameters for the thermal characterisation of organic and inorganic nano¬materials.

Nanotechnology is likely to have a profound impact on our economy and society in the early 21st century - it is widely felt that nanotechnology may lead to the next industrial revolution. Science and engineering research in nano¬technology promises break¬throughs in areas such as materials and manufacturing, electronics, medicine, energy and the environment, biotechnology, and information technology. The thermal characterisation of nanomaterials during their processing steps and the applications was an issue from the beginning, especially for the inorganic materials. Thermoanalytical and thermophysical testing methods are successfully applied for the determination of phase transitions, thermally induced chemical reactions and decompositions, gas adsorption and desorption studies, and thermal transport properties.

Figure 1: comprison of oxidation and thermal stability of two CNT samples with weight loss (TG) and energetic effects (DSC)

Simultaneous TG-DSC is an effective tool to study the thermal behavior of powders under the influence of different atmospheres. Carbon nanotube samples show signi¬ficant difference in their thermal stability under oxidative atmosphere, depending on their preparation conditions (Figure 1).  The two samples tested reveal the same oxidizable carbon content of 92.67%, which burns out in the temperature range 400°C to 750°C, but the volatile content before start of oxi¬dation is 10 times higher in the modified sample, and the residue at 1000°C (ash) is 3 times less in the modified CNT sample. The oxidation range of this CNT samples is much lower compared with known ranges for bulk graphite and diamond samples. Though carbon atoms are involved in aromatic rings like for graphite, the C=C bond angles are no longer planar in the CNTs and the C−C bond length is actually elongated by the curvature imposed. In case single wall CNTs are ideally perfect, their chemical reactivity should therefore be highly favored at the tube tips, were there is the location of pentagonal rings.

Figure 2: thermal diffusivity, specific heat and thermal conductivity of a CVD diamond layer

Small contents of nanofillers in a polymer matrix are determined by thermogravimetry (TG). Samples are heated in an oxygen-free inert gas flow up to 600°C to decompose the polymer matrix and to quantify the polymer content; after the gas exchange to an oxidative atmosphere at 600°C, the carbon nanotube content burns off and is determined from the corresponding TG step between 600°C and 800°C. Carbon nanotubes are applied as filler for polymers to improve the thermal and electrical conductivity of the polymer. Because of the strong dependence of these properties in the axial and radial direction of carbon nanotubes, the CNT orientation inside the polymer matrix is of great interest. The light or laser-flash technology is a well developed method to study the heat transport properties of materials in a non-contact and non-destructive way.
 
Measurement of the thermal diffusivity by the laser-flash technique is a fast and accurate method for the characterisation of the thermal transport properties and of structural changes of ceramic materials, metals, polymers, and of liquids and melts.

Figure 3: Thermal diffusivity of PEEK filled with 14% (weight) carbon nanotubes, showing inhomogeneity of the structure and of distribution/orientation of CNT filler

Figure 3 shows the temperature dependence of thermal diffusivity, specific heat and thermal conductivity of a thin diamond layer produced by chemical vapor deposition. Clearly one can see the rapid change of diffusivity between room temperature and 400°C in this thin diamond layer.

In experiments with polymer-composites, filled with carbon nanotubes, the expected improvement of thermal diffusivity and thermal conductivity at room temperature was measured with the laser flash technique. The results (table 1) show the orientation effect of the CNTs in the polymer with 10 times higher in-plane thermal diffusivity (axial direction of the CNT, in-plane result) compared to thermal diffusivity of the composite with the CNT in radial orientation (through-plane result). 

It is also reported for Polymer Matrix Composites that industrial epoxy loaded with 1 wt% unpurified CVD-prepared CNTs showed an increase in thermal con¬ductivity of 70% and 125% at 40 K and at room temp¬erature, respectively1.

Figure 4: Hydrogen release from magnesium hydrides, measured iby TG-DSG and pressure DSC

The homogeneity of the CNT distribution in the polymer matrix of a PEEK-CNT composite was measured by scanning a quadratic polymer sheet (5cm by 5cm) with the flash technique in x and y direction in 100µm distance intervals. Figure 5 shows the drastic differences in thermal diffusivity over thewhole sample area, originating from the inhomogeneous distribution and orientation of the CNT filler inside the PEEK matrix. The measurement was made at room temperature.

No other non-destructive technique can provide equivalent information on the internal structure, or on structural changes in case of a sintering process, like the light/laser flash technique.

Hydrides and alanates offer a promising capacity for hydrogen storage by chemical bonds. It could be shown that the grinding of Magnesium alanate down to the nanometer range reduced the temperature for the hydrogen release from 160°C to 120°C (DTG peak-temperature)2. In figure 6 we detect the hydrogen release from a modified magnesium hydride by simultaneous TG-DSC in a helium atmosphere as a two step weight loss at 353°C and 384°C. By pressure DSC magnesium hydride looses hydrogen in an argon flow (pressure 4.6 bar, flow 100 ml/min) at 419°C (peak temperature). Also in this application, the high pressure DSC is successfully applied to study the reversibility, pressure and temperature ranges of hydrogen uptake and release from hydrides, alanates and similar chemical compounds.

Thermal analysis techniques and the determination of thermophysical properties offer manifold information on materials containing nano-sized particles. The characterisation of nanomaterials yields information on thermal pro¬perties, reactivity, oxidation behavior, thermal stability, and sintering. This could be shown for carbon nanotubes, polymers with nanofillers, and magnesium hydride. Especially the determination of thermal transport properties allows an in¬sight into orientation effects of carbon nanotubes when used to improve the thermal conductivity of polymer composites. The binder burnout and sintering of ceramic and powder metallurgical products can be optimised applying kinetic analysis based on thermoanalytical experiments3.