Papers by Keyword: Non-Isothermal

Abstract: Non-isothermal DSC has been used to investigate the curing kinetics of epoxy adhesives (DGEBA-cycloaliphatic amine). The epoxy samples were scanned on DSC with five heating rates (5°C/min, 7.5°C/min, 10°C/min, 12.5°C/min, and 15°C/min). The curing kinetics were obtained through ASTM standards E2890 and E698 (the Ozawa and Kissinger methods). The kinetic parameters obtained include Ea (activation energy), A (pre-exponential factor), and n (reaction order). The activation energy calculated from the Kissinger and Ozawa method was slightly different but insignificant. The reaction rate (dα/dt) and degree of curing/conversion (α) relationship towards temperature (T), and time (t) was also investigated. The curing process's reaction rate (dα/dt) has maximum value; it can no longer increase after a specific conversion rate (α).

Abstract: Microalloying elements Ti and Nb are commonly added to high-strength Dual Phase steels as they can provide efficient means for additional strengthening due to grain refinement and precipitation strengthening mechanisms. In the form of solute elements or as fine carbonitride precipitates, Ti and Nb are also expected to have a significant effect on the microstructural changes during annealing and especially on recrystallization kinetics. The present work investigates the influence of microalloying elements Ti and Nb on recrystallization in various cold-rolled Dual Phase steel grades with the same initial microstructure but different microalloying contents. Using complementary experimental and modeling approaches makes it possible to give some clarifications regarding both the nature of this effect and the comparative efficiency of Ti and Nb on delaying recrystallization. It is shown that niobium is the most efficient micro-alloying element to impede recrystallization and that the predominant effect is solute drag.

Abstract: The thermotropic liquid crystal copolyester P-BPA2.5 and P-BPAF2.5 with low melting temperature and high glass transition temperature were obtained by introducing 2.5mol% bisphenol A (BPA), bisphenol AF (BPAF) and terephthalic acid (TA) receptively into the molecular chain of poly (oxybenzoate-co-oxynaphthoate). The isothermal and nonisothermal degradation behavior of the two copolyesters P-BPA2.5 and P-BPAF2.5 were studied by thermo-gravimetry analysis (TGA) under nitrogen atmosphere. The degradation kinetics of the copolyesters under dynamic and isothermal heating conditions was evaluated byKissinger-Akahira-Sunose (KAS) and iso-conversional methodrespectively. Results showed that the activation energy (E) values followed the order P-BPAF2.5>P-BPA2.5 under dynamic heating atmosphere. However, the P-BPA2.5 showed a better thermal stability as compared with P-BPAF2.5 under isothermal heating conditions. The differences of degradation mechanism between the two copolyesters may be a reason for this phenomenon, which was discussed in detail in the text.

Abstract: Reduction of iron oxide by hydrogen is important in the production of direct reduced iron. This method of iron production is gaining increasing significance as an alternative route to the blast furnace technology with the many difficult issues facing the latter, the most important being the problem related to environmental. In order to reduce the emission of greenhouse gases CO₂, particularly for iron making, the production of Direct Reduced Iron (DRI) using hydrogen as the reducing gas instead of carbon monoxide is being considered. Reduction of pure hematite by hydrogen was studied at the laboratory scale, varying the experimental conditions like temperature (700oC and 800oC) and porosity (20% and 40%). Then, a Kinetic Modelling was conducted using Matlab software based on independently measured physical and thermodynamic properties of the reaction system and experimentally measured properties of the reactant solid (Fe₂O₃), gas phase (H₂) and reactant product (Fe). There is a gap that occurs between the predicted result and the experimental result although the model explicated the trend and the behaviour of the reduction rate of Ferric Oxide and indicated a good homogeneity to the experimental conditions used in this research.

Abstract: The oxidation resistance of SiAlONs is one important property. In our work we present a test method to characterize its anti-oxidation property with the non-isothermal oxidation process using Thermo Gravimetric Analyzer (TGA). The oxidation starting temperature (T_s), maximum oxidation rate temperature (T_m) and the value of the oxidation weight gain per unit area (G_a) are three key parameters to describe the antioxidant properties of the material. Experimental results prove this test method is easy and reasonable and it will be developed as the national standard of China.

Abstract: The low density polyethylene (LDPE)/ethylene-octene copolymer (POE) blend films and LDPE/POE/inorganic particle composite films were prepared. By means of DSC method, the crystallization capability and the non-isothermal crystallization kinetics of LDPE composite films were investigated, the results show that the addition of DE can reduce the interfacial free energy of the crystal and the active energy of a nucleus, resulting in stable nuclei can be formed at lower supercooling, which makes the polymer crystallized successfully at high temperatures. By selecting data processing method of Mo ZS and modified Jeziorny, the results indicate that the DE play a role of heterogeneous nucleation agent in the process of crystallization and increased the crystalline rate of LDPE.

Abstract: The curing kinetic analysis is an important technique for the characterization of the curing behavior of reactive polymeric systems. In this study, fluoroepoxy oligomer was synthesized from trifluoromethyl aniline and epichlorohydrin. The epoxide equivalent weight (EEW) and the number average molecular weight (M_n) of the systhesized fluroepoxy oligomer determined from acid titration and gel permeation chromatography were found to be 312.16 g/eq and 534 g/mol, respectively. The mixtures of the fluoroepoxy oligomer were mixed with the cycloaliphatic amine in various stiochiometric ratios (1:1, 1: 1.5 and 1:2). The effects of the stiochiometric ratio on the curing behaviors were studied using both isothermal and non-isothermal DSC methods. Ozawas, Kissingers and Friedmans methods were employed to investigate the kinetic parameters. The results showed that the peak temperature (T_p) increased with the increasing heating rate. The activation energy (E_a) calculated from Ozawas and Kissingers methods were much larger than that from Friedmans method.

Abstract: A modified model describing the austenite reaction was developed that took into account the effect of heating rate. The model considered the variation of activation energy during non-isothermal heating and one set of model parameter was adequate to predict the formation of austenite. To verify the theoretical model, the process of austenite formation during continuous heating in Cr5 roller steel with pearlite and ferrite mixed initial microstructure was analyzed by dilatation experiment. The results show that a strong logarithmic relationship between apparent activation energy and heating rate. Experimental kinetic transformations as well as critical temperatures of austenite reaction are in good agreement with the calculations. The model can be used to describe the transformation kinetics at an intermediate heating rate.

Abstract: Nanocrystallization of anatase phase was established in BaO-TiO2-B2O3 glass system. Crystallization kinetics of anatase phase in these glasses were investigated using non-isothermal differential scanning calorimetry (DSC) at three different heating rates (10, 20 & 30 K/min). Scanning Electron Microscopy (SEM) carried out on heat treated (at 920 K) glasses confirmed bulk nucleation and three-dimensional growth. Johnson-Mehl-Avarami model could not be applied for this system suggesting considerable overlap of the nucleation and growth involving complex transformation process. However, modified Kissinger and Ozawa models were used to calculate the effective activation energy associated with anatase crystallization. The kinetic exponent n was found to be temperature dependent indicating the change in the crystallization mechanism. This was attributed to the high entropy fusion of anatase phase, fast crystallization rate and nano dimension of the anatase phase.

Abstract: A general analytical phase transformation model has been proposed and successfully applied to describe the crystallization of amorphous alloys. The “additivity rule” is proved to be compatible with the analytical model; the effects of anisotropic growth based on Monte Carlo (MC) simulations is reinterpreted using the analytical approach; and an improved temperature integral is also proved to be compatible with the analytical model. Kinetic analysis basing on the analytical model declares the transformation mechanism, e.g. nucleation, growth and impingement mode. On this basis, the kinetic behaviors of isothermal and non-isothermal crystallization of amorphous Zr₅₀Al₁₀Ni₄₀ are analyzed.