In the general approach, the thermal parameters of nanocrystalline 3C-SiC particles have been studied to some extent. However, the physical processes on the surface of the 3C-SiC nanocrsytals have not explained in the previous studies [12, 13]. The study of surface processes, the effect of temperature on such processes and the adsorption cases from the atmosphere are extremely relevant in the use of materials [34–39]. Its important to note that nanocrystalline 3C-SiC particles has a very large spaecific surface area (120 m2∙g− 1) like other nanomaterials. This causes to extremely active surface of the nanocrystalline 3C-SiC particles. As a result, 3C-SiC nanocrystals adsorb water molecules with high sensitivity when in contact with the atmosphere, causing the formation of O and H groups on the surface. The analysis showed that the hydroxyl or OH groups formed on the surface are not sufficiently stable at relatively high temperatures. Thus, as shown in Fig. 1, the OH groups begin to leave the surface of the nanomaterial depending on the heating rate, starting from the temperature values of about 467-483K. This process ends at a temperature of about 740-755K, and the energy supplied to the system is used to increase the Gibbs energy, entropy, and enthalpy (Figs. 2, 3, and 4). It is important to note that, heating rate directly affected to the hydroxyl groups dispersion time on the surface and temperature range of dispersion of OH radicals which is collected on the surface of 3C-SiC nanocrystals. Therefore, if this process occurs in the temperature range of 467-740K by low heating rate (5K/min), there is a shift in this process at relatively high heating rate (20K/min), and OH groups leave the nanomaterial in the temperature range 483-755K.
The temperature may increase with a constant rate or with some fluctuations during thermal analysis depending on the state of the system. Although the software of the devices provides a constant rate of thermal processing in real experiments, there are more or less fluctuations observed in the increasing of temperature. In this case, there is a very small difference in the temperature of the sample and the program of device, as well as other physical parameters [12, 40]. We can calculate the specific heat capacity of the system according to the heat flow if consider that in the classical approach [12, 13, 40, 41]:
From Eq. (1), the specific heat capacity can be easily calculated in accordance with the heat flux in the experimental DSC curve.
Normally, DSC spectra are analyzed at constant pressure, and because the nanocrystalline 3C-SiC particles used in this study are solid, the notion of constant pressure or volume is generally eliminated by a very small error. In this case, we can calculate the specific heat capacity from the DSC spectra as follow:
where, Φ - is the heat flux in the DSC spectra, β - is the thermal processing rate, m - is the mass of the sample. The enthalpy and entropy of the system can be calculated in the given temperature range according to the calculated heat capacity by the following equations [12, 41]:
The free Gibbs energy of the system can be determined with a simple approach according to the calculated enthalpy and entropy values:
In the present study, the specific heat capacity, free Gibbs energy, enthalpy, and entropy of nanocrystalline 3C-SiC particles were calculated at different temperatures using the equations (2), (3), and (4).
The analysis showed that 3C-SiC nanocrystals are extremely resistance materials to temperature. Nanocrystalline 3C-SiC particles have a very high melting point around 3103K. Therefore, 3C-SiC nanocrystals have extremely strong stability under heating up to 1200K. Simultaneously, HRTEM, SAED and EDP analyzes showed that 3C-SiC nanocrystals do not undergo structural changes in extreme environments [25]. On the other hand, it has been noted that very small amounts of oxidation can occur on the surface of 3C-SiC nanocrystals at temperatures above about 1000K [12, 13].
Nanocrsytalline 3C-SiC particles were investigated with the four different heating rates (5 K/min, 10 K/ min, 15 K/ min and 20 K/min) in the temperature range of 300 K – 1200 K. The heat capacity, Gibbs energy, enthalpy and entropy of nanocrystalline 3C-SiC particles at all thermal processing rates (5 K/min, 10 K/min, 15 K/min and 20 K/min) were calculated theoretically based on experimental results. Figure 1 briefly describes the spectra corresponding to 5 K/min and 20 K/min thermal processing rates. In the initial approach, as can be seen from the spectra, water or other additives adsorbed from the atmosphere are released from the system. Unlike conventional bulk materials, 3C-SiC crystals in nanoscale have an extremely large surface area and adsorption capacity. Previous experiments have shown that this feature is sharply distinguishes 3C-SiC nanocrystals from 3C-SiC wafer [42–45]. It is known that nanomaterials have a very large specific surface area (Specific Surface Area (SSA)) and these types of materials are surface active, which makes water or other compounds dependent on the nanoparticle surface immediately upon their contact with the atmosphere. Active surface is chemisorbed from the environment by weak interaction with H2O and OH groups. Linear increase in temperature breaks the weak reciprocal effect. From the observation of thermal curves, it can be concluded that as the temperature rises, the water or other impurities existed in the nanomaterial begin to leave the system. This process completed at about 450–500°C temperature. There is almost no change in the initial approach to the thermal spectra of nanocrystalline 3C-SiC particles from 500°C to 1000°C.
The temperature dependence of the specific heat capacity of nanocrystalline 3C-SiC particles at different thermal processing rates are given in Fig. 2. Specific heat capacity is proportional to the heating rate in the selected low temperature range (temperature range of 300 K − 350 K) (Fig. 2a). However, chaoticity is observed on the temperature dependence of the specific heat capacity in the wide temperature range (300K − 1200K) (Fig. 2b). The numerical value of the specific heat capacity is around the characteristic value (750 J·kg− 1K− 1) for SiC in the low temperature region. However, there are sharp deviations with increasing temperature. Numerical value of the specific heat capacity is negative at T ≥ 800K of temperature. This suggests that exothermic effects are observed in nanocrystalline 3C-SiC particles at temperatures ≥ 800K. Thus, in this case, the temperature of the sample container in the experimental device is lower than the temperature of the sample. Moreover, the numerical value of the specific heat capacity is positive in the temperature range 300K-800K or corresponds to endothermic processes in the general approach.
The temperature dependences of the enthalpy of nanocrystalline 3C-SiC particles at different thermal processing rates are shown in Fig. 3. As can be seen from the figure, the numerical value of enthalpy decreases in the low temperature region in proportion to the thermal processing rate (Fig. 3a). The enthalpy of the system is chaotic, similar to the heat capacity at relatively high temperatures. However, in the general approach, the enthalpy of the system decreases with increasing thermal processing rate in the entire temperature range (Fig. 3b).
Based on the experimental results, the calculated entropy of the system for nanocrystalline 3C-SiC particles is shown in Fig. 4. As can be seen from the temperature dependences of the entropy of the system, in this case, according to the enthalpy and heat capacity, the entropy of the system decreases with increasing thermal processing rates in the low temperature range. The entropy of the system is a negative after the temperature is approximately T ≥ 800K. This, in the general approach, can be explained by exothermic effects, similar to the specific heat capacity.
According to the experimental results, temperature dependences of the free Gibbs energy were calculated (Fig. 5). As can be seen from the figures, the numerical value of the free Gibbs energy is inversely proportional to the thermal processing rate (Fig. 5a). The numerical value of free Gibbs energy increases with increasing thermal processing rate, which is an indication that the system is more stable when heated at relatively low speeds. Obtained dependencies in a wide temperature range has shown that the numerical value of the free Gibbs energy increases almost in direct proportion to the temperature at relatively large temperature values. An increase in the numerical value of the free Gibbs energy for a system is, in a sense, an increase in the potential energy of the system (chemical potential). Any system tends to minimize its potential energy over time, and an increase in the value of free Gibbs energy in any system can reduce the stability of that system. Physically, this explains why the resistance of the system naturally decreases at high temperatures.
The numerical value of the free Gibbs energy calculated according to the experimental results is negative in the low temperature range. This means that the processes occurring in the system are spontaneous and the system can move towards equilibrium. Note that in the general approach at temperatures T < 800K, the numerical value of the free Gibbs energy varies around zero, which is an indication that the system is in equilibrium. Numerical value of the free Gibbs energy at temperatures T > 800K is positive. In this case, the processes in the system are not spontaneous, but changes can be observed in the opposite direction to the system. In the general approach, changes in temperature around 740K can be explained to some extent by the Debye temperature [12, 46, 47]. However, more analytical investigations are needed to give an exact opinion.