Heating Rate Differential Scanning Calorimetry Research Articles

Synthesis and Thermal behaviors of 1, 8-Dihydroxy-4, 5-Dinitroanthraquinone Nickel Salt

A novel energetic combustion catalyst, 1, 8-dihydroxy-4, 5-dinitroanthraquinone nickel salt (DHDNENi), was firstly synthesized by the process of metathesis reaction in a yield of 91%, and its structure was characterized by IR, element analysis, thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). The thermal behavior and non-isothermal decomposition reaction kinetics of DHDNENi were studied by means of different heating rate differential scanning calorimetry (DSC). The kinetic equation of major exothermic decomposition reaction for DHDNENi is obtained. The self-accelerating decomposition temperature (TSADT) and critical temperature of thermal explosion (Tb) are 574.4K and 593.4K, respectively. The thermal stability of DHDNENi is good.

Influence of Graphene Oxide on Crystallization Behavior and Chain Folding Surface Free Energy of Poly(vinylidenefluoride‐co‐hexafluoropropylene)

Graphene oxide (GO) filled poly(vinylidene fluoride‐co‐hexafluoropropylene) (PVDF‐HPF) copolymer nanocomposites as promising piezoelectric materials are developed, and their crystallization behavior and chain folding free energy are examined. Appropriate distribution of GO nanoparticles in the copolymer is confirmed by transmission electron microscopic analysis. The crystalline structure of nanocomposites is analyzed by wide‐angle X‐ray scattering and Fourier transform infrared spectroscopy. The results show an increase in the β‐phase content of PVDF‐HPF copolymer in the presence of GO. Non‐isothermal crystallization kinetics of the neat polymer and corresponding nanocomposites are studied by a multiple heating rate differential scanning calorimetry using modified Avrami–Jeziorny and Liu models. Moreover, barrier energy of crystallization is calculated by Friedman and Kissinger models. It is found that addition of GO to the copolymer increases nucleation activity of the nanocomposites. Investigation on linear crystal growth via Hoffman's theorem indicates an enhanced nucleation activity upon increasing GO content. The surface folding free energy of the neat polymer is changed from 5.79 × 10−2 to 7.27 × 10−2 J m−2 upon addition of 5 wt% of GO. A bundle‐like mechanism is proposed, which can explain the crystallite growth. Analysis based on Vyazovkin theorem reveals that the crystallization activation energy is independent of GO at elevated temperatures.

Investigating aging effects for porous silicon energetic materials

When infused with an oxidizer, on-chip porous silicon (PS) shows tremendous potential as an energetic material. However, applications such as fuzing and propulsion require long-term material stability. The present work utilizes accelerated lifetime testing for PS samples with sodium perchlorate (NaClO4) oxidizer. Devices were exposed to elevated temperatures for various time intervals, then subjected to an electrical pulse through an integrated bridgewire to test for ignition. The pass/fail data from ignition testing was then analyzed to determine the mean and median failure point at each temperature using an Arrhenius aging model and a lognormal probability density function (PDF). Samples were heated over a temperature range of 185–300 °C for up to 46 h with median failure times ranging from 0.4–20 h. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were used to evaluate chemical changes of PS/NaClO4 during aging. Comparison of DSC traces for aged samples showed differences for heating above and below 250 °C, suggesting two different aging mechanisms. FTIR results suggest that for low temperature aging (below ∼250 °C) the hydrogen termination layer of the porous silicon was still intact, but backbond oxidation did occur. Above ∼250 °C the hydrogen termination layer was no longer evident in FTIR and considerable oxidation occurred, suggesting reaction or dissociation of the hydrogen layer. This corresponds with an exothermic peak shown during DSC at 300 °C. Thermogravimetric analysis/mass spectroscopy (TGA/MS) was also performed, and confirmed hydrogen gas release beginning at ∼280 °C. A DSC peak at ∼400 °C is evident for fresh samples, but is greatly reduced or not evident for all aged samples. We attribute this peak to backbond oxidation, which is shown to occur in FTIR for all aged samples. Variable heating rate DSC experiments were also performed to determine activation energy of fresh and aged samples. Activation energy was calculated using the Kissinger Method for the first two exothermic peaks in DSC experiments. For the first peak, corresponding to hydrogen desorption (∼300 °C), activation energy for fresh and aged samples was 132.1 kJ/mol and 132.7 kJ/mol, respectively. The activation energy of the second exothermic peak, corresponding to backbond oxidation, for fresh and aged samples was 183.8 kJ/mol and 159.6 kJ/mol, respectively. Device failure during accelerated aging tests was also used to predict the mean lifetime of porous silicon/sodium perchlorate at room temperature (25 oC) as 100 years with a 90% confidence interval of 31 to 328 years.

Curing of bisphenol A-aniline based benzoxazine using phenolic, amino and mercapto accelerators

The curing of bisphenol A-aniline based benzoxazine was studied applying different accelerators (4,4-thiodiphe- nol, o-dianisidine, 2-mercaptobenzimidazole and 4-mercaptophenol) to initiate the catalytic ring-opening of benzoxazine. Possible pathways of benzoxazine ring-opening, polymerization and cross-linking without and with the addition of differ- ent accelerators are presented. The curing kinetics was investigated by model-free kinetic analysis of experimental data obtained by differential scanning calorimetry (DSC). The addition of different accelerators significantly reduced the onset temperature of curing in dynamic experiments. The effects of accelerators on the results of isothermal conversion predic- tion were studied and discussed in detail. Among the used accelerators, thiodiphenol showed the best accelerating effi- ciency and was consequently used in further studies, where its amount was varied. By low heating rate DSC analysis the catalytic ring-opening, thermally accelerated ring-opening and the diffusion-controlled steps were identified. The amount of added accelerator affected particularly the ring-opening and diffusion-controlled steps.

Synthesis and thermal behavior of 1,8-dihydroxy-4,5-dinitroanthraquinone manganese salt

A novel energetic combustion catalyst, 1,8-dihydroxy-4,5-dinitroanthraquinone manganese salt (DHDNEMn), was synthesized by virtue of the metathesis reaction in a yield of 91%, and its structure was characterized by IR, element analysis and differential scanning calorimetry(DSC). The thermal decomposition reaction kinetics was studied by means of different heating rate DSC. The results show that the apparent activation energy and pre-exponential factor of the exothermic decomposition reaction of DHDNEMn obtained by Kissinger’s method are 162.3 kJ/mol and 1011.8 s−1, respectively. The kinetic equation of major exothermic decomposition reaction of DHDNEMn is \(\frac{{d\alpha }} {{dT}} = \frac{{10^{11.8} }} {\beta }\frac{2} {5}(1 - \alpha )[ - \ln (1 - \alpha )]^{3/5} \exp ( - 1.623 \times 10^5 /RT) \). The entropy of activation(ΔS≠), enthalpy of activation(ΔH≠) and free energy of activation(ΔG≠) of the first thermal decomposition are −24.49 J·mol−1·K−1, 185.20 kJ/mol and 199.29 kJ/mol(T=575.5 K), respectively. The self-accelerating decomposition temperature(TSADT) and critical temperature of thermal explosion(Tb) are 562.9 and 580.0 K, respectively. The above-mentioned information on the thermal behavior is quite useful for analyzing and evaluating the stability and thermal safety of DHDNEMn.

Synthesis, crystal structure and thermal behavior of 4-amino-3,5-dinitropyrazole copper salt

A novel energetic combustion catalyst, 4-amino-3,5-dinitropyrazole copper salt ([Cu(adnp)2(H2O)2]), was synthesized in a yield of 93.6% for the first time. The single crystal of [Cu(adnp)2(H2O)2] was determined by single crystal X-ray diffraction. It crystallizes in a triclinic system, space group P−1 with crystal parameters a = 5.541(3) Å, b = 7.926(4) Å, c = 10.231(5) Å, β = 101.372(8)°, V = 398.3(3) Å3, Z = 1, μ = 1.467 mm−1, F(0 0 0) = 243, and Dc = 2.000 g cm−3. The thermal behavior and non-isothermal decomposition reaction kinetics of [Cu(adnp)2(H2O)2] were studied by means of different heating rate differential scanning calorimetry (DSC). The kinetic equation of major exothermic decomposition reaction for [Cu(adnp)2(H2O)2] was obtained. The entropy of activation (ΔS≠), enthalpy of activation (ΔH≠), free energy of activation (ΔG≠), the self-accelerating decomposition temperature (TSADT) and the critical temperature of thermal explosion (Tb) are 59.42 J mol−1 K−1, 169.5 kJ mol−1, 1141.26 kJ mol−1, 457.3 K and 468.1 K, respectively.

Synthesis and thermal behaviors of 1,8-dihydroxy-4,5-dinitroanthraquinone barium salt

A novel energetic green combustion catalyst, 1,8-dihydroxy-4,5-dinitroanthraquinone barium salt (DHDNEBa), was firstly synthesized by the process of metathesis reaction with a yield of 95.7%, and its structure and thermal behavior were characterized by IR, elemental analysis, thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). The thermal decomposition kinetics and mechanism were studied by means of different heating rate DSC, thermolysis in situ rapid-scan FTIR and simultaneous TG–MS technology. The results show that the apparent activation energy and pre-exponential factor of the exothermic decomposition reaction of DHDNEBa obtained by Kissinger's method are 177.3kJmol−1 and 1013.5s−1. The entropy (ΔS≠), enthalpy (ΔH≠) and free energy of activation (ΔG≠) of the first step thermal decomposition are found to be 8.16Jmol−1K−1, 168.0kJmol−1 and 163.4kJmol−1, respectively. The self-accelerating decomposition temperature (TSADT) and critical temperature of thermal explosion (Tb) are 555.0 and 570.2K. The decomposition reaction process of DHDNEBa begins with four-molecule combined water lost in the DHDNEBa structure. The gaseous products such as H2O, CO, CO2 and NO are detected from decomposition of DHDNEBa at 606K. The residue is mainly composed of BaO and some carbon black. The above-mentioned information on thermal behavior is quite useful for analyzing and evaluating the stability and thermal safety of DHDNEBa.

Synthesis, thermal behavior, and application of 4-amino-3,5-dinitropyrazole lead salt

Lead salt of 4-amino-3,5-dinitropyrazole (PDNAP) was synthesized from 4-amino-3,5-dinitropyrazole by the process of metathesis reaction, and its structure was characterized by IR, element analysis, TG, and DSC. The thermal decomposition kinetics and mechanism were studied by means of different heating rate differential scanning calorimetry (DSC) and thermolysis in situ rapid-scan FTIR simultaneous. The effects of PDNAP as an energetic combustion catalyst on the combustion performance of the solid propellant were studied. The results show that the peak temperature is 319.2 °C on DSC curve. The kinetic equation of major exothermic decomposition reaction is $$ \frac{{\text{d}}\alpha}{{\text{d}}T} = \frac{{10^{15.45} }}{\beta }4(1 - \alpha )[ - \ln \left( {1 - \alpha } \right)]^{{{3 \mathord{\left/ {\vphantom {3 4}} \right. \kern-0pt} 4}}} \exp ({{ - 1.972 \times 10^{5} } \mathord{\left/ {\vphantom {{ - 1.972 \times 10^{5} } {RT}}} \right. \kern-0pt} {RT}}). $$ The PDNAP is shown by IR spectroscopy to convert to PbO during the decomposition process. Combustion experiments show PDNAP can reduce the burning rate pressure exponent of the double-base or composite-modified double-base propellant.

Synthesis and thermal behaviors of 4-amino-3,5-dinitro-1H-pyrazole

4-Amino-3,5-dinitro-1H-pyrazole (LLM-116) was synthesized by vicarious nucleophilic substitution (VNS) reaction, and its structure was characterized by NMR, IR and element analysis. The thermal decomposition kinetics and mechanism were studied by means of different heating rate differential scanning calorimetry (DSC), thermolysis in situ rapid-scan FTIR and simultaneous TG–MS technology. The results show that the apparent activation energy and pre-exponential factor of the exothermic decomposition reaction of LLM-116 obtained by Kissinger method are 106.97kJmol−1, 1010.47s−1 in first decomposition stage and 138.98kJmol−1, 1012.24s−1 in second decomposition stage. The decomposition of LLM-116 begins with one molecular water lost by NO2 and NH2 forming furazan in the LLM-116 structure. The furazan compound is unstable to generate NO. At the same time, another NO molecular is released as ONO is broken at approximately 173°C in the first stage, and then HCN and unsaturated poly hydrocarbon are produced as pyrazole decomposes at 236°C in the second stage. The above-mentioned information on thermal behavior is quite useful for analyzing and evaluating the stability and thermal safety of LLM-116.

Application of an independent parallel reactions model on the annealing kinetics of BEPO irradiated graphite

Stored energy release rates have been determined for neutron irradiated graphite samples machined from an early air-cooled nuclear reactor (British Experimental Pile Zero or BEPO). The rate of release of stored energy was measured for both isothermal and linear rise heating rate differential scanning calorimetry experiments. The rate of release data were analysed using a thermal kinetics, independent parallel reactions model. The effect of annealing on the graphite crystalline structure was evaluated by investigating changes to X-ray diffraction spectra. A correlation between the calculated crystallite size and stored energy release is presented. A method for calculating the kinetic parameters for the annealing reaction is proposed and tested against the data. The method shows excellent consistency for both the isothermal and linear heating rate experiments (with less than 3% standard deviation).

Kinetics of the curing reaction of selected epoxy resin-amine systems

The paper is a review of authors' research works on curing reaction of some commercial and synthesized by them epoxy resins (among others - with liquid crystalline fragments). As curing agents 5 aromatic diamines differing in chemical structure were used. The results of investigation of curing kinetics by differential scanning calorimetry (DSC) confirm the significant effect of heating rate (up to 20 oC/min) on the process. It was found, for example, that for small heating rate (2-5 oC/min) DSC thermograms of epoxy resins with liquid crystalline fragments show double exothermic peak contrary to the thermograms of typical epoxy resins showing single exothermic peaks. This phenomenon is attributed to the formation of smectic structure during the curing (at temp. 132-162 oC). Usefulness of three various kinetic models (by Barret, Borchardt-Daniels or Kissinger) describing the curing process investigated have been compared.

1,3,3-trinitroazetidine (TNAZ). Study of thermal behaviour. Part II

Thermal behavior of TNAZ (1,3,3 - trinitroazetidine) was studied by using differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimetryc analysis (TGA). It was found out that TNAZ is thermally more stable than RDX, but less stable than HMX and TNT. The reaction of intensive thermal decomposition starts at 183–230 °C, depending on heating rate, while the first exothermic reaction was observed at 178 °C at the heating rate of 1 °C/min. By applying multiple heating rate DSC measurements and Ozawa's method the activation energy of 161.3 kJ/mol and pre-exponential factor of 8.27·1013 1/s were calculated from DSC peak maximum temperature-heating rate relationship. By the same method the activation energy of 157.5 kJ/mol and pre-exponential factor of 4.55·1013 1/s were calculated from DTA peak maximum temperature. By applying Flynn-Wall isoconversional method it was calculated from DSC measurements that the activation energy equals between 140 and 155.6 kJ/mol at degrees of conversion ranging between 0.3 and 0.7, while pre-exponential factor ranges between 7.8·1012 and 1.92·1013 1/s.

Determination of Kinetic Parameters of Bi2Se3 Thin Films by Computation

Bi2Se3 thin films are prepared by thermal evaporation method onto well-cleaned glass substrates. From the XRD analysis, the film is found to be amorphous in nature. Thermal analysis of Bi2Se3 sample scraped off from the glass substrate is performed using the technique differential scanning calorimetry (DSC) at different heating rates. Kissinger method is used to analysis these constant heating rate spectra and the activation energy 'Ea′ corresponding to the phase transformation is calculated through the examination of the DSC peak temperature shift induced by variation of the heating rate. An analytical equation with three kinetic parameters (activation energy (Ea), transformation order (n) and the frequency factor (v)) is derived from the Johnson-Mehl-Avrami (JMA) theory to describe the constant heating rate DSC spectra. Using the activation energy from Kissinger plot, theoretical constant heating rate DSC traces are generated for arbitrary values of 'v' and 'n' using the analytical equation. Finally, theoretically generated DSC spectra is compared with that of the experimental one and the unknown values 'v' and 'n' are determined.

Amorphous and Crystalline Phase Formation in Ni/Al Multilayer Thin Films

ABSTRACTWe have investigated reactive phase formation in magnetron sputter-deposited Ni/Al multilayer films with a 1:3 molar ratio and various periodicities ranging from 320 nm to a codeposited film with an effective periodicity of zero. The films were studied by x-ray diffraction, differential scanning calorimetry, electrical resistance measurements, and transmission electron microscopy. We find that a reaction which results in the formation of an amorphous phase has taken place during the multilayer deposition process. This reaction substantially reduces the driving force for subsequent reactions and explains why nucleation kinetics become important for these reactions. The mode of transformation for a film with 10 nm periodicity was investigated, in detail, by applying the Johnson-Mehl-Avrami analysis to data obtained from isothermal and constant heating rate differential scanning calorimetry, in combination with electron microscopy studies of the transformation microstructure.

Intermetallic formation in copper/magnesium thin films—kinetics, nucleation and growth, and effect of interfacial oxygen

In situ resistance measurements, x-ray diffraction, Rutherford backscattering spectrometry, transmission electron microscopy, isothermal and constant heating rate differential scanning calorimetry and Auger electron spectrometry depth profiles have been used to investigate the interactions in copper and magnesium thin films leading to the growth of Cu2Mg and CuMg2 intermetallics. The effect of exposing the reacting interfaces to controlled exposure of oxygen on the nucleation and growth kinetics of such intermetallics was also investigated. It is found that the first phase to form is CuMg2, at about 200–215 °C. It is determined that the formation of CuMg2 occurs by a two step process consisting of nucleation and growth. The nucleation of CuMg2 takes place in a region composed of a Cu/Mg solid solution. The nuclei form at certain preferred sites and grow in directions both parallel and perpendicular to the surface, eventually leading to a continuous CuMg2 layer. The growth of CuMg2 nuclei in the plane of the original interface occurs at a constant rate, whereas the growth in a direction perpendicular to the original interface is found to be diffusion limited. In the presence of excess copper Cu2Mg forms at higher temperatures, with complete conversion to Cu2Mg occurring at about 380 °C. When the Cu surface is dosed with oxygen prior to Mg deposition, ramp rate differential scanning calorimetry (DSC) shows that the nucleation and growth of CuMg2 as well as the growth of Cu2Mg are not disturbed. Dosing the Mg surface with oxygen results in significant changes in the growth of the two phases. In this case a thin MgO layer is formed at the oxygen dosed surface, lateral growth of CuMg2 is unaffected, but vertical growth of CuMg2 across the oxygen dosed interfaces is delayed by 25–30 °C. The growth of Cu2Mg is also shown to be delayed, by 22–54 °C due to the interfacial oxygen dose.

An Experimental Study of Phase Transformations in an Al–Zn–Mg–Zr Alloy: DSC and Hot Microhardness Measurements

Isothermal hot microhardness and multi-scan differential scanning calorimetry (DSC) experiments were carried out on an ALZnMgZr, 7000 series aluminum alloy to characterize the evolution and dissolution of GP zones and the formation of the η′ phase. Three stages of hardness evolution with aging time were observed for aging in the 25–100°C temperature range corresponding to GP zone formation, simultaneous GP zone dissolution and η′ formation, and η′ formation and coarsening. The isothermal activation energy for the GP zone formation from hot microhardness measurements was about 0.35 eV per atom comparable with that from previous hardness measurements reported in the literature. From variable heating rate DSC scans, the activation energies for GP zone formation and dissolution were 0.75 and 0.92 eV per atom, respectively. The activation energy for η′ phase formation and growth of the asquenched specimens as well as aged specimens was about 0.59 eV per atom, and the Avrami index for the transformation was inferred to be in the range 2.3–2.8 (± 20%). The activation energies are discussed in terms of various diffusional processes in the Al matrix and are compared with values reported in the literature. Résumé Des expériences comportant des mesures de microdureté isothermes à chaud ainsi que de la calorimétrie differentielle par balayage (DSC) ont été enterprises sur un alliage d'aluminium de la série 7000 (Al, Zn, Mg, Zr) afin de caractériser l'évolution et la dissolution des zones G.P. et la formation de la phase η′. Trois stades d'évolution de la durete sont observes en fonction du temps de vieillissement dans un intervalle de température de 25 à 100°C correspondant à celui de la formation des zones G.P., à la dissolution simultanée des zones G.P. et la formation de η′ , et à la formation et la croissance de η′. L'energie d'activation isothermique pour la formation des zones G.P. a été évaluée à 0.35 eV/atome à partir des mesures de microdureté et cette valeur est comparable à des mesures antérieures rapportées dans la littérature. Par des balayages calorimétriques à différents taux de chauffe, l'énergie d'activation pour la formation et la dissolution des zones G.P. est évaluée respectivement à 0.75 et O.92 eVjatome. L'énergie d'activation pour la formation et la croissance de la phase η′ dans des échantillons trempés ainsi que des échantillons vieillis est environ 0.59 eV/atome l'indice d'Avrami pour la transformation se situant dans L'intervalle 2.3–2.8 (20%). Les énergies d'activation sont interprétées en fonction des différents processus diffusionnels dans la matrice d'aluminium et sont comparées aux valeurs trouvées dans la littérature.

Calorimetric and other studies of intermetallic phase formation in Ag/Al bilayer thin films

Silver and aluminium bimetallic thin film couples have been studied using constant heating rate differential scanning calorimetry both on cleaned glass substrates and freshly cleaved NaCl crystals. The most striking feature was the occurrence of two maxima in the reaction rate during the formation of a single product phase, Ag2Al, suggesting a two-step growth process. The activation energy for this reaction was 0.98 eV in the first step and 0.93 eV in the second step. These values are in good agreement with values obtained by a different technique, i.e. Rutherford back-scattering. Transmission electron microscopy, thin film X-ray diffraction, the change in resistance of bilayer films with temperature and thermodynamic and kinetic analyses have been used to investigate the intermetallic phase formation. It was observed that substrate plays an important role in the kinetics of thin-film reaction.

Phenol–formaldehyde resin curing and bonding in steam‐injection pressing. I. Resin synthesis, characterization, and cure behavior

AbstractTwo different phenol–formaldehyde (PF) resole resins are serving as models in a study aimed at establishing the effects of moisture, temperature, pressure, and time on resin cure and bonding during the pressing of wood flakeboard. This phase of the program had two goals: first, to characterize the two resins in terms of their structure and chemistry during synthesis, aging, and cure—using viscosity measurement, gel permeation chromatography (GPC), nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA); second, to make a preliminary evaluation of the utility of DSC, FTIR, and DMA for measuring the degree of resin cure. The two resins differed significantly in relative amounts of hydroxymethyl groups and methylene linkages (NMR), in molecular weight and its distribution (GPC), and in reaction rate (as measured by viscosity, DSC, FTIR, or DMA). The degree of cure developed during constant heating rate DSC scans was calculated for a series of maximum DSC temperatures from both the loss in hydroxymethyl groups (FTIR) and the decrease in available exothermic heat (DSC). Agreement between the two methods was quite good, considering the inherent difficulties in quantifying infrared data. For comparison, the degree of cure developed during constant heating rate DMA scans was calculated for a series of maximum DMA temperatures from both the increase in storage modulus (DMA) and the decrease in exothermic heat (DSC after rewetting). Samples that apparently achieved complete cure in the DMA still exhibited significant residual cure potential in the DSC. We attribute the lower apparent cure in the DMA to loss of moisture from samples during the DMA scan, with consequent loss in plasticization and molecular mobility.

Gaussian activation energy spectra in reversible and irreversible structural relaxation

The concept of an activation energy spectrum (AES) has been successfully applied by many authors in modelling the kinetics of structural relaxation displayed by many physical properties of metallic glasses. In this paper it is shown that a combination of a constant, physically reasonable value of v 0 with broad gaussian spectra of activation energies can provide a good fit to observed isothermal kinetic data. It is also found that temperature-dependent gaussian spectra can be used to model the results of non-isothermal experiments, e.g. constant heating rate differential scanning calorimetry. The origin of the temperature dependence of these spectra is related to the behaviour of a distribution of two level systems. Finally it is confirmed from a study of reversible and irreversible changes of electrical resistivity and from a comparison of the relaxation kinetics of several properties of one glass, that separate spectra are needed to account for reversible and irreversible processes.

Non-isothermal transformation kinetics: Application to metastable phases

Constant heating rate differential scanning calorimetry is frequently employed to study the kinetics of transitions from metastable phases and, in particular, crystallization of metallic glasses. Such data are analyzed by the Kissinger method, which was originally devised for the study of homogeneous reactions. The consensus in the literature, until recently, was that such applications (i.e. to heterogeneous solid state transformations) of the Kissinger method are not valid. We address the principal objections to these applications and provide alternative derivations of recent theoretical results which demonstrate that the Kissinger method is valid for heterogeneous reactions of the type described by the Johnson-Mehl-Avrami equation in the isothermal case. Isothermal and constant heating rate data on transformation of metastable NiP powders are presented. These experimental results and the discussions presented here help to clarify the effects of incubation times in non-isothermal transformation kinetics and provide a further demonstration of validity of the generalized Johnson-Mehl-Avrami theory for the description of heterogeneous solid state transformations.