Quasi-isothermal Conditions Research Articles

Evaluation of heat capacity measurements by temperature-modulated differential scanning calorimetry

Temperature-modulated differential scanning calorimetry (TMDSC) is known to have the ability to measure heat capacity of materials more accurately than the conventional differential scanning calorimeter. However, the accuracy of the measured heat capacity displays significant dependence on various experimental parameters such as period of modulation (p), amplitude of modulation (a), geometry of sample (g), heating rate (r), etc. One of the key features of this system is the ability to measure heat capacity under quasi-isothermal conditions. In the present investigation, heat capacity of a well-established system namely sapphire and thoria was measured by TMDSC under dynamic mode and also under quasi-isothermal mode. The experimental parameters, mentioned above p, a, g, and r are varied to establish the conditions for measuring heat capacity accurately.

Oxidation resistance of TiAl 3–Al composite coating on orthorhombic Ti 2AlNb based alloy

The TiAl 3–Al composite coating on orthorhombic Ti 2AlNb based alloy was prepared by cold spray. Oxidation in air at 950 °C indicated that the bare alloy exhibited poor oxidation resistance due to the formation of TiO 2/AlNbO 4 mixture and intended to scale off at the TiO 2 rich zone. A nitride layer about 2 µm was formed under the oxide layer. The oxygen invaded deeply into the alloy and caused severe microhardness enhancement in the near surface region. The TiAl 3–Al composite coating exhibited parabolic oxidation kinetics and showed no sign of degradation after oxidized up to 1098 h at 950 °C in air under quasi-isothermal condition. No scaling of the coating was observed after oxidized at 950 °C up to the tested 150 cycles. The major oxide in the oxidized coating was Al 2O 3. The AlTi 2N, TiAl and small amount of TiO 2 were also observed in the oxidized coating. The EPMA and microhardness tests showed that inward oxygen diffusion was prevented by the interlayer, which was formed between the composite coating and the substrate during heat-treatment. Microstructure analyses demonstrated that the interlayer play a major role in protecting the substrate alloy from high temperature oxidation and interstitial embrittlement.

Theory and practice in the thermoanalytical kinetics of complex processes: Application for the isothermal and non-isothermal thermal degradation of HDPE

Abstract The criterion according to which the best mechanism of a complex process and the associated kinetic parameters correspond to the best fitting of TG and DTG curves recorded at several heating rates is discussed. The main problem is if the application of such a criterion for a complex process will lead to the mechanism and corresponding kinetic parameters to be used for predictions. To this end, the thermal degradation of HDPE has been investigated by TG/DTG + DSC simultaneous analysis performed in Ar flow, at five heating rates as well as in four quasi-isothermal conditions. An algorithm for correct determination of the mechanism and the corresponding kinetic parameters of a complex process from non-isothermal and quasi-isothermal data was suggested.

High-pressure destruction kinetics of Clostridium sporogenes ATCC 11437 spores in milk at elevated quasi-isothermal conditions

The high-pressure sterilization establishment requires data on isobaric and isothermal destruction kinetics of baro-resistant pathogenic and spoilage bacterial spores. In this study, Clostridium sporogenes 11437 spores (10 7 CFU/ml) inoculated in milk were subjected to different pressure, temperature and time ( P, T, t) combination treatments (700–900 MPa; 80–100 °C; 0–32 min). An insulated chamber was used to enclose the test samples during the treatment for maintaining isobaric and quasi-isothermal processing conditions. Decimal reduction times ( D values) and pressure and temperature sensitivity parameters, Z T (pressure constant) and Z P (temperature constant) were evaluated using a 3 × 3 full factorial experimental design. HP treatments generally demonstrated a minor pressure pulse effect (PE) (no holding time) and the pressure hold time effect was well described by the first order model ( R 2 > 0.90). Higher pressures and higher temperatures resulted in a higher destruction rate and a higher microbial count reduction. At 900 MPa, the temperature corrected D values were 9.1, 3.8, 0.73 min at 80, 90, 100 °C, respectively. The thermal treatment at 0.1 MPa resulted in D values 833, 65.8, 26.3, 6.0 min at 80, 90, 95, 100 °C respectively. By comparison, HP processing resulted in a strong enhancement of spore destruction at all temperatures. Temperature corrected Z T values were 16.5, 16.9, 18.2 °C at 700, 800, 900 MPa, respectively, which were higher than the thermal z value 9.6 °C. Hence, the spores had lower temperature sensitivity at elevated pressures. Similarly, corrected Z P values were 714, 588, 1250 MPa at 80, 90, 100 °C, respectively, which illustrated lower pressure sensitivity at higher temperatures. By general comparison, it was concluded that within the range operating conditions employed, the spores were relatively more sensitive to temperature than to pressure.

Analysis of adiabatic heating and its influence on the Garofalo equation parameters of a high nitrogen steel

Torsion tests at high temperatures and high strain rates were conducted on a high nitrogen steel (HNS). Under these conditions, adiabatic heating influences its flow behavior. This work focus on a new algorithm for conducting the adiabatic heating correction of stress–strain curves. The algorithm obtains the stress–strain curves at quasi-isothermal conditions from those at adiabatic conditions. The corrections in stress obtained can be higher than 15% and increase with increasing strain rates and decreasing temperatures. On the other hand, an upper bound for the temperature rise was found using a dynamic material behavior approach. Finally, the influence of adiabatic heating correction on the Garofalo equation parameters of HNS was analyzed. High values of activation energy and stress exponent were attributed to reinforcement by dispersed particles and the high amount of alloying elements.

Fine microstructure of wire rods manufactured from Sv-08G2S high-plasticity steel

This paper presents the results of transmission-electron-microscope and scanning-electron-microscope studies of the fine structure of wire rod made from Sv-08G2S silicon-manganese welding steel, including electron microscope images used to identify the structural components of the wire-rod metal and determine the dislocation density. The wire-rod microstructure was compared against that obtained when a new, optimized process was developed for heat-treatment of the wire rod in a Stelmor line. We show that the wire rod has the highest ductility when the production flow includes long-term holding of the wire-rod coils under quasi-isothermal conditions (under insulated hoods with a cooling rate 0.2-0.3°C/sec) in such a way as to minimize the number of bainite-martensite areas when the structureless martensite (hardenite) content does not exceed 5%. The resulting process and optimized microstructure enabled us to achieve a relative deformation of 98% when direct-drawing wire rod from Sv-08G2S alloy steel.

Structural and energetic properties of carbosils hydrothermally treated in the classical autoclave or the microwave reactor

Carbon/silica adsorbents (carbosils) prepared by pyrolysis of CH2Cl2 at 823 K and the reaction time from 0.5 to 6 h on the mesoporous silica gel surface (Si-60, Merck, granule size 0.2–0.5 mm) and then hydrothermally treated at 473 K with steam or liquid water by using the classical autoclave with traditional heating way or in the microwave reactor were studied by means of TG, adsorption and Q-TG methods. Changes in the structural characteristics of hybrid adsorbents before and after hydrothermal treatment were analyzed on the basis of nitrogen adsorption. Thermal properties of initial and modified samples as well as concentration of carbon deposit were studied using thermogravimetry (TG) in the range of 293–1273 K. The adsorbed water layers were investigated by means of thermodesorption of water under the quasi-isothermal conditions (Q-TG) in the range of 293–573 K. Concentration of weakly and strongly adsorbed water and the surface free energy on the interphase of adsorbent/water were calculated. It was stated that hydrothermal treatment in the microwave reactor, contrary to that in the classical autoclave, allows to obtain adsorbents with noticeably higher values of total pore volume in relation to the initial adsorbents and in majority with a higher specific surface area. Application of microwave energy allows to obtain adsorbents with lower values of surface free energy in relation to the initial adsorbents and those modified in the autoclave.

Temperature oscillation calorimetry for the determination of the heat capacity in a small-scale reactor

This contribution describes a method for heat capacity determination in a small-scale reaction calorimeter under quasi-isothermal conditions ( ± 0.16 ∘ C to ± 0.77 ∘ C ). The heat capacity of the reactor content is calculated from the amplitudes and the phase shifts of the reactor temperature, of the electrical heater and of the cooling rate when forced temperature oscillations are applied. The heat capacities of eight solvents (water, ethanol, methanol, acetone, 1-octanol, diethylenglycol, toluene, and 1-butanol) covering a wide range of viscosity were calculated for various experimental conditions, including reactor volume and stirrer speed. Systematic deviations were detected when compared to the corresponding literature values. Straight line calibration with the total heat transfer coefficient and two modern multivariate calibration techniques (partial least squares and neural network) were applied to correct for these deviations. The different calibrations show similar precision and allow for an online determination of the heat capacity with an accuracy comparable to other published methods. Successful applications include the determination of the heat capacities for n-heptane, for various homogenous ethanol/water mixtures, and during the course of the hydrolysis of concentrated sulfuric acid.

Increase in effective activation energy during physical aging of a glass

The novel technique of multi-frequency temperature modulated differential scanning calorimetry is employed to measure a decrease in the heat capacity under quasi-isothermal conditions. The effect is used to monitor the kinetics of physical aging in glassy maltitol at ∼5–8 °C below the glass transition temperature. The heat capacity relaxation is analyzed by an isoconversional method that yields effective activation energy for different extents of relaxation. The early stages of aging demonstrate an activation energy ∼145 kJ mol −1 that is significantly smaller than the activation energy of α-relaxation (413 kJ mol −1), which is closely approached at the final stages of aging.

Oxidation behaviour of gamma titanium aluminides with EB-PVD thermal barrier coatings exposed to air at 900 °C

The oxidation behaviour of two γ-TiAl-based alloys coated with thermal barrier coatings (TBCs) was investigated in air at 900 °C for up to 1000 h under quasi-isothermal conditions. Samples bare or with a 4 μm thick Ti–51Al–12Cr (at.%) protective layer were coated with yttria-stabilized zirconia using electron-beam physical vapour deposition. Prior to TBC deposition, the specimens were pre-oxidised in air and low-pressure oxygen. Thermal barrier coatings on samples of the alloy Ti–45Al–8Nb–0.2B–0.15C (at.%) did not spall off during the maximum exposure length, whereas failure occurred with pre-oxidised specimens of the alloy Ti–45Al–6Nb–1Cr–0.4W–0.2B–0.5C–0.2Si (at.%). Scanning electron micrographs revealed an outer oxide scale with a columnar structure grown by outward diffusion of Ti and Al cations. Below this columnar oxide layer, a broad porous zone formed consisting predominantly of titania. A discontinuous nitride layer was observed in the transition region between oxide scale and substrate. Spallation of the TBC was associated with cracking in the porous titania-rich layer. The zirconia top coat exhibited an excellent adherence to the thermally grown oxide scales. Pre-exposure in low-pressure oxygen did not improve the oxidation resistance. Post-oxidation microstructural examinations of specimens coated with Ti–Al–Cr revealed that the protective layer was severely deteriorated. On the degraded Ti–Al–Cr coating, an outer scale with columnar structure formed beneath the TBC.

Thermal decomposition of nickel nitrate hexahydrate, Ni(NO 3) 2·6H 2O, in comparison to Co(NO 3) 2·6H 2O and Ca(NO 3) 2·4H 2O

The thermal decomposition of Ni(NO 3) 2·6H 2O ( 1), Ca(NO 3) 2·4H 2O ( 2) and nitryl/nitrosyl nitrato nickelate(II), NO 2/NO[Ni(NO 3) 3] ( 3), was investigated by thermogravimetric measurements with quasi-isothermal conditions and compared to Co(NO 3) 2·6H 2O. The respective decomposition processes of 1 and 2 differ from each other showing that at one hand anhydrous Ca(NO 3) 2 was obtained whereas anhydrous nickel dinitrate has not be formed due to redox and condensation reactions. Instead basic nickel compounds have been formed. In reducing atmosphere nickel metal can be obtained. Anhydrous Ni(NO 3) 2 results by the thermal degradation of nitryl/nitrosyl nitrato nickelate. FT-Raman spectra have been of help in the judgement of the decomposition processes.

Expanding ring experiments to measure high-temperature adiabatic properties

High-temperature mechanical properties for metals are usually measured under quasi-isothermal conditions, where thermal equilibrium has been achieved by slowly preheating the samples to a predetermined temperature. However, there are several situations of practical interest where the material is heated rapidly, so thermal diffusion is negligible during the time of heat deposition and mechanical deformation. Hypervelocity impact and penetration, shaped charge jet formation, and pulsed heating in electromagnets are some such situations. Experimental evidence from electron beam heating indicates that high-temperature mechanical properties significantly depend on the rapidity of heat deposition. As the time duration of heating is reduced, the amount of local heat transfer decreases due to limited thermal diffusion. Thus, the thermodynamic process deviates from the isothermal process and approaches the adiabatic process for pulsed heating conditions. Therefore, it is imperative that mechanical properties be measured under appropriate thermodynamic conditions. We propose that the electromagnetically driven expanding ring experiment be used to measure the adiabatic mechanical properties of metals. While earlier expanding ring experiments were conducted primarily to obtain high-strain-rate strength and fragmentation data, our primary goal is to obtain high-temperature data under pulsed heating conditions approaching the adiabatic process. Our preliminary data suggest that the adiabatic mechanical properties are quite different from isothermal properties.

Measurement and modelling of kinetics of hydrogen sorption by LaNi 5 and two related pseudobinary compounds

The hydriding/dehydriding rates and the pressure–composition isotherms were measured for LaNi5, LaNi 4.85Al 0.15 and LaNi 4.75Fe 0.25 under quasi-isothermal and variable pressure conditions. Isothermal conditions were obtained by reducing the thermal time constant of the experimental device. Empirical rate equations to describe the sorption reaction kinetics were derived. These rates are expressed as a function of temporal composition, saturated composition, temperature, applied pressure and essentially the initial operating conditions which were not considered in most of all the previous studies related to the reaction kinetics of metal hydrides. Besides, the rate equations presented in this work can be integrated easily in the numerical models that predict dynamic flow and heat and mass transfer within realistic metal–hydrogen devices. This paper also discusses the effects of Fe and Al as substituents for Ni on P–C isotherms and reaction rates of LaNi 5 alloy.

Studies of surface properties of Na- and La-montmorillonites

This paper presents possible applications of thermal analysis, sorptometry and porosimetry to study physico-chemical properties of Na- and La-montmorillonite samples, especially for determination of total surface heterogeneity. The quasi-isothermal thermogravimetric (Q-TG) mass loss and its first derivative (Q-DTG) curves with respect to temperature and time obtained during programmed liquid thermodesorption under quasi-isothermal conditions have been used to study adsorbed layers and heterogeneous properties of the Na- and La-montmorillonites. Calculations of the desorption energy distribution functions by analytical procedure using mass loss Q-TG and differential mass loss Q-DTG curves of thermodesorption under quasi-isothermal conditions of polar and non-polar liquid vapours preadsorbed on a material surface are presented. Parameters relating to porosity of samples were determined by sorptometry, mercury porosimetry and atomic force microscopy (AFM). From nitrogen sorption isotherms from sorptometry and porosimetry methods, the fractal dimensions of montmorillonites have been calculated. Moreover, a new approach is proposed to calculate fractal dimensions of materials obtained from Q-TG curve; this is compared with values obtained by the above methods. The total heterogeneous properties (energy distribution function and pore-size distribution functions) of samples studied were estimated. The radius and pore volume of the tested samples calculated on the basis of thermogravimetry, sorptometry and porosimetry techniques were compared and good correlations obtained.

Studies of liquids diffusion in the chosen material samples

The new method of diffusion coefficient calculations from thermogravimetry Q-TG data has been presented. Programmed thermo- desorption of polar and apolar liquids from aluminium oxide and montmorillonite-Na and -La samples in quasi-isothermal conditions has been made. The result from above method was compared with literature data and good correlation were obtained.

Studies of liquids diffusion in the chosen material samples

The new method of diffusion coefficient calculations from thermogravimetry Q-TG data has been presented. Programmed thermodesorption of polar and apolar liquids from aluminium oxide and montmorillonite-Na and -La samples in quasi-isothermal conditions has been made. The result from above method was compared with literature data and good correlation were obtained.

AC calorimetric study of the thermoelastic martensitic transformation in Cu–Al–Ni alloys

We present an accurate study by AC calorimetry on the thermoelastic martensitic transformation in Cu–Al–Ni shape memory alloys , both under quasi-isothermal conditions and by using a dynamic method. The AC calorimetry results suggest that the martensitic transformation exhibits some kinetic effects strongly dependent on the temperature rate, in agreement with internal friction results. Calorimetric results obtained by DSC do not exhibit this behaviour, but dynamic measurements obtained under oscillating fields show a net dependence on the temperature rate.

Mechanochemical Activation of Aluminum: 3. Kinetics of Interaction between Aluminum and Water

Two regimes of oxidation by water are revealed for nanocrystalline aluminum prepared by the mechanical activation of its mixture with graphite and distributed in the matrix of amorphous carbon. At the temperatures 50°C < T < 90°C, nanosized aluminum particles interact with water under quasi-isothermal conditions. The main products are hydrogen and pseudoboehmite AlOOH; a low content of bayerite Al(OH)3 is also formed. After the induction period, the kinetics of interaction can be satisfactorily described by the law of a diminishing sphere. The effective activation energy of the reaction is equal to 61 ± 10 kJ/mol and is identical for the samples of submicron aluminum prepared by different procedures. At temperatures above 90–95°C, the oxidation of mechanically activated aluminum by water is transformed into a thermally self-accelerated explosion process. Under these conditions, the oxidation of aluminum to α-Al2O3 is accompanied by an exothermal reaction between the metal and the carbon matrix during which aluminum carbide Al4C3 is formed.

Thermal decomposition of bismuth laurates

Two bismuth salts, Bi 6 O 4 (OH) 4 (C 17 H 35 COO) 6 and Bi 6 O 4 (OH) 4 (C 17 H 35 COO) 6 .nC 17 H 35 COOH, were synthesized. The thermogravimetry under quasi-isothermal conditions denotes the multi-step character of decomposition processes for both compounds. The thermogravimetry under linear heating was used for kinetic studies. Thermogravimetric data (obtained at different rates of linear heating) were processed with 'Netzsch Thermokinetics' computer program. Kinetic parameters were calculated only for the first decomposition step for both salts, the process is described by different nucleation equations (with and without autocatalysis).

Protective coatings on orthorhombic Ti2AlNb alloys

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)2 Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α2-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.