**3.1. Kinetic analysis**

154 Heat Treatment – Conventional and Novel Applications

these requirements is difficult.

transport.

possible

In the manufacturing of nanomaterials by sol-gel method the second, high temperature, i.e. at temperatures above 1400 K, stage is essential. The first stage of this method is the sol-gel technique. This stage is carried out at lower temperature. The intermediate product of pyrolytic decomposition of PAN-DMF-TiCl3 is formed - the powder containing nanocrystalline TiCx in carbon matrix [6]. Nanocrystallites of titanium carbide are characterized by high fraction of lattice defects, i.e., vacant carbon sites, presence of oxygen and/or nitrogen. In the second stage carbonisation and purification of TiCx/C composite in argon takes place. It is essential to obtain the materials with high values of the C/Ti ratio, while maintaining the proper, nanometric grain size, in order to obtain the most favourable properties such as hardness and oxidation resistance. The selection of parameters meeting

There was assumed that the good basis are kinetic studies. They allow to determine the intermediate and final products, distinguish the stages of the process, determine the

The kinetic measurements were carried out using TG-DSC-MS technique. Nanoparticles size, lattice parameter, chemical and phase composition before and after heat-treatment were determined with the following techniques: XRD (Philips PW3040/x0 X'Pert Pro), HRTEM (JEM 3010), SEM (JEOL JSM 6100), EDX (Oxford Instruments, ISIS 300), XPS (SIA 100 Cameca), total carbon measurement (MULTI EA2000, AnalyticJena), and the presence of free carbon was estimated by Raman Spectroscopy. The measurements were carried out under non-isothermal and isothermal conditions. The advantage of this method is the possibility of continuous registration of measured values and use of small weighed amounts of samples during measurements. The procedure is illustrated at the example of heat treatment in argon of the nanocomposite powder containing nc-TiCx (x≤0.7) in carbon matrix obtained by sol-gel method. The method ensures maintaining the small size of crystallites, by physically limiting the volume available for their growth, and the matrix prevents agglomeration of particles and oxidation during storage and

At a certain temperature range, and under certain conditions, the reaction of carbonisation is

Heating of the samples in an argon atmosphere can lead to the growth of crystallites. The aim of this study was to develop conditions for annealing of the composite, under which, as a result of carbonisation, TiCx reaches a high stoichiometric composition, i.e. x> 0.8, at the minimal growth of crystallites. Obtaining such optimal material properties can be controlled

The thermogravimetric measurements were carried out at sample heating rates of 10, 20, and 50 Kmin-1. The mass of the samples were ca. 40 mg. During the measurements the samples were heated up to 1473, 1573, 1673 and 1773 K. These temperature values correspond to the isothermal conditions. The samples were heated under isothermal

by selecting the appropriate temperature and rate of the process.

<sup>x</sup> x y TiC yC TiC <sup>+</sup> + → (38)

temperature ranges of their courses and obtain a quantitative description.

The kinetic description of the process was based of thermogravimetric measurements. The results of the measurements are presented on the plots of sample temperature, TG, DTG and HF function dependencies on time (Fig. 11).

Initially the measurements were carried out under non-isothermal conditions at a linear change in sample temperature, then at the transitional regime, and finally under isothermal conditions (Fig.11a). It should be added that these results were the basis of the description of the process. The theory of kinetics under non-isothermal conditions require that this function depends only on the sample heating rate and temperature. The results were evaluated by neural networks method. Neural networks, used to analyze the non-isothermal measurements, were previously described in [20-23]. The TGu function was the described (dependent) variable, and the sample heating rate and temperature were the describing (indepedent) variables. All the measurement series were examined simultaneously. The received network was GRNN 2/11310. Statistical analysis of this network is given in Table 6.


**Table 6.** Statistical estimation of GRNN 2/11310 network

In columns 2, 3, 4 the statistical evaluation of training (Tr), verification (Ve) and testing (Te) subset is listed.

A high accuracy was obtained. The TG dependencies determined experimentally could have been used in kinetic calculations.

The essential operation is the division of the process into stages. Basing on the presented measurement results four stages of the process were identified (Fig. 12). In the endothermic stage I, proceeding with a weight loss, the release and desorption of volatile products, contained in the samples after the first stage, occurred. In the exothermic, second stage, marked with the symbol II, proceeding with a samples weight gain, the oxidation of noncarbonised nc-TiCx/C by oxygen present in trace amounts in argon occurred. In the endothermic third stage, labeled by III, simultaneously proceeded the pyrolysis of organic admixtures and carbonisation of nc-TiCx/C. The pyrolysis and carbonisation were treated as concurrent reactions. Stage IV (exothermic) proceeded with sample weight gain. It concerns the oxidation of carbonised nc-TiC with oxygen contained in argon. The process proceeded at temperature above 1573 K.

Methodology of Thermal Research in Materials Engineering 157

stage IV

**Figure 12.** Plots of normalized TG functions in time for β=10 K/min and 6h at 1673 K recorded during

0 100 200 300 400 500

stage III

t [min]

**Figure 13.** Mass spectra of CO2 and normalized TG function plots. Heat treatment of nc-TiC/C in argon

The analysis of mass spectra allowed the more accurate characterisation of distinguished stages. In stage I volatile products were evolved. CO2, CO and NO were also formed. In this stage H2, NH3, HCN and CH3–CH3, CH2=CH2 and CH≡CH also evolved. In stage II oxidation of non-carbonised nc-TiCx/C occurred. During the course of stage III CO and CO2 were emitted. They were attributed to the oxidation of released hydrocarbons. HCN, CH3–CH3,

The correct kinetic description was hindered by evolution of secondary products. Therefore

the values of α(T) determined for each stage required an independent evaluation.

the heating of nc- TiCx/C in argon.

0,85

0,90

TGu [mg/mg]

0,95

1,00

stage II stage I

at the heating rate 10 Kmin-1.

CH2=CH2, and CH≡CH were also formed.

**Figure 11.** Plots of T, normalized TG function and DTG and HF in time. Heat treatment of nc-TiC/C in argon. a) temperature, b) normalized TG, c) DTG, d) HF

During the thermogravimetric measurements the evolved gaseous products were identified by mass spectrometry method. The compounds produced in the reaction with the oxygen present in trace amounts in argon, are described in detail, because these processes could affect the quality of the obtained, carbonised nc-TiC. CO2-m/e44, CO-m/e28 NO-m/e30 and NH3-m/e17 have been identified. NH3 formed as a result of pyrolytic decomposition of carbon compounds was the precursor of nitric oxide. CO2-m/e44 mass spectrum, is shown in Figure 13 as an example. To facilitate the analysis of the results the normalized TGu function is also plotted.

156 Heat Treatment – Conventional and Novel Applications

at temperature above 1573 K.

admixtures and carbonisation of nc-TiCx/C. The pyrolysis and carbonisation were treated as concurrent reactions. Stage IV (exothermic) proceeded with sample weight gain. It concerns the oxidation of carbonised nc-TiC with oxygen contained in argon. The process proceeded

**Figure 11.** Plots of T, normalized TG function and DTG and HF in time. Heat treatment of nc-TiC/C in

During the thermogravimetric measurements the evolved gaseous products were identified by mass spectrometry method. The compounds produced in the reaction with the oxygen present in trace amounts in argon, are described in detail, because these processes could affect the quality of the obtained, carbonised nc-TiC. CO2-m/e44, CO-m/e28 NO-m/e30 and NH3-m/e17 have been identified. NH3 formed as a result of pyrolytic decomposition of carbon compounds was the precursor of nitric oxide. CO2-m/e44 mass spectrum, is shown in Figure 13 as an example. To facilitate the analysis of the results the normalized TGu function

argon. a) temperature, b) normalized TG, c) DTG, d) HF

is also plotted.

**Figure 12.** Plots of normalized TG functions in time for β=10 K/min and 6h at 1673 K recorded during the heating of nc- TiCx/C in argon.

**Figure 13.** Mass spectra of CO2 and normalized TG function plots. Heat treatment of nc-TiC/C in argon at the heating rate 10 Kmin-1.

The analysis of mass spectra allowed the more accurate characterisation of distinguished stages. In stage I volatile products were evolved. CO2, CO and NO were also formed. In this stage H2, NH3, HCN and CH3–CH3, CH2=CH2 and CH≡CH also evolved. In stage II oxidation of non-carbonised nc-TiCx/C occurred. During the course of stage III CO and CO2 were emitted. They were attributed to the oxidation of released hydrocarbons. HCN, CH3–CH3, CH2=CH2, and CH≡CH were also formed.

The correct kinetic description was hindered by evolution of secondary products. Therefore the values of α(T) determined for each stage required an independent evaluation.
