Constant Heating Rate Experiments Research Articles

In situ observation of the multistep process of cold sintering

AbstractA compact cold sintering stage was constructed to densify ceramic samples in capillary tubes for the purpose of conducting in situ experiments designed to elucidate fundamental cold sintering mechanisms. The stage was used to successfully densify samples composed of ZnO/CH₃COOH mixtures under a range of pressures (25–175 MPa) over a modest temperature range (room to 150°C) in capillary tubes (0.75 mm inner diameter). The progression of the cold sintering process of ZnO was monitored by both piston displacement and a camera. These measurements allowed the strain rate and fluid extraction during densification to be recorded. The results of these measurements are compared to existing models of cold sintering and pressure solution creep, and direct correlations are observed between abrupt changes to the densification rate during constant heating rate experiments and the observation of solvent extraction from the powder compact.

Kinetic evaluation of the crosslinking of a low-temperature cured biobased epoxy-diamine structure

Bis(oxiran-2-ylmethyl) adipate, synthesized by adipic acid, was investigated for its capability to polymerize at ambient temperature when blended with 1-(2-aminoethyl) piperazine. The object of the current work is associated to the kinetic analysis of the aforementioned epoxy-amine system, qualifying it as a potential candidate in the adhesion field of manufacturing industries. Polymerization kinetics were followed by Differential Scanning Calorimetry measurements, carried out both under dynamic and constant temperature conditions. The first gave rise to the total heat released through polymerization and the glass transition temperatures of the initial monomers (Tgo) and the crosslinked network (Tgoo), found to be 356 J/g, −69.7°C and 1.8 °C, respectively. The later revealed the effective crosslinking of the system at 25, 40 and 60 °C under reasonable periods of time. Cyclical dynamic-dynamic and isothermal-dynamic DSC experiments were further performed, giving results that follow the Di Benedetto equation, with Tgo and Tgoo values very close to the experimentally calculated. According to Vyazovkin isoconversional method applied on dynamic DSC measurements, the activation energy found to be independent of the conversion degree throughout the curing process and equal to 61.4 kJ/mol, indicating a single-step reaction. Various autocatalytic reaction models were selected to reproduce constant heating rate experiments, all displaying very accurate results, while fitting parameters from the aforementioned simulations, along with kinetic parameters derived from Vyazovkin method, were utilized for the estimation of isothermal curing predictions. Isothermal predictions were validated through comparison to both solely isothermal DSC measurements and data gathered in cyclical isothermal-dynamic experiments. Consequently, the nth order with an autocatalytic constant (Kcat) reaction model Cn was selected for the construction of a Time-Temperature-Transformation plot, revealing optimum curing conditions for the investigated temperature range. The superiority of the present epoxy thermoset system lies in the monomers' organically originated nature, along with its low-temperature curing cycle, enhancing environmental consciousness and saving energy for generations to come.

Flash sintering behaviour of 8YSZ-NiO composites

Flash sintering is an electric field/current assisted sintering technique, which is reported to lower the furnace temperature and to reduce sintering time significantly. In this work, we have studied the processing of 8YSZ/NiO composites by flash sintering, for the first time. Two composites, with different amount of NiO (one below the percolation limit and another one above it) were processed in two different sintering atmospheres. Constant heating rate experiments were performed to know the minimum furnace temperature required to flash sinter the samples for a given applied electric field. Subsequently, isothermal flash sintering experiments were performed at different current densities. The flash onset temperature of the composites was lower in the reducing atmosphere compared to in air. The power dissipated in stage III of the flash was strongly influenced by the composite composition and the sintering atmosphere. The extent of densification in the composites was controlled by the current density. The composites were densified up to a relative density of ∼90% in 30 s when flash sintered in air. In reducing atmosphere, there was in-situ reduction of NiO to Ni. As a result, for composites containing NiO above the percolation limit, the current preferentially flew through the in-situ formed metallic phase and there was no densification in the composite in reducing atmosphere. Phase and microstructure evolution in the composites was studied through XRD, SEM and EDS. With proper control of the electrical parameters (electric field and current density), composites with controlled porosity can be processed through flash sintering which may have applications for solid oxide fuel cells.

Influence of TiO2 on the densification behaviour of Yb2O3

The effect of temperature and heating rate on the densification of ytterbia (Yb2O3), with and without titania (TiO2) doping was investigated. It is shown that up to a certain doping level, titania doping enhances the densification behaviour of ytterbia. The effect of titania doping on crystal structure confirms that titania is substitutionally incorporated in ytterbia up to the solubility limit, which corresponds well with the densification results. The increased densification rate of titania-doped ytterbia is attributed to the formation of cation vacancy and lattice distortion. Using constant heating rate experiments, the activation energy for densification has been calculated and it is shown that in the intermediate density range (60 % to 85 %), the activation energy is independent of the density. Titania doping increases the activation energy for densification.

Influence of flash sintering on phase transformation and conductivity of hydroxyapatite

We report on the influence of current density limits on phase transformation, density, and microstructure of Hydroxyapatite (HAp) subjected to flash sintering experiments. Arrhenius plots of the electrical conductivity during the constant heating rate experiments showed changes in the ionic conductivity mechanisms during the incubation time, contrasting behavior from other materials presented so far. Flash sintering also suppressed the formation of β-tricalcium phosphate, which occurs during conventional sintering. We report herein that low current density limits were able to successfully sinter dense HAp with minimal phase transformations under lower furnace temperature and significantly shorter time processing.

Predicting transformations during reactive flash sintering in CuO and Mn2O3

AbstractReactive flash sintering has been demonstrated as a method to rapidly densify and synthesize ceramic materials, but determining the extent of chemical reactions can be complex since the maximum temperature reached by the sample may be brief in time. The black body radiation (BBR) model has been shown to accurately predict the sample temperature during the steady state of flash (stage III). This work demonstrates situations where the BBR model alone does not accurately predict when a phase transformation will occur. We examine the model reactions of CuO reduction to Cu2O during stage II and Mn2O3 reduction to Mn3O4 in stage III. In CuO, highly resistive samples result in initially localized current flow, a stochastic process resulting in inhomogeneous heating and error in the BBR model during stage II. CuO reduction does not occur in constant heating rate experiments with 6.25 V/mm fields, even though the sample temperature momentarily exceeds the phase transformation temperature. Increased furnace heating to 950°C before application of a field is required to drive the transition. In Mn2O3, the calculated sample temperature of the gauge is less than the transformation temperature, but localized heating at the contact will exceed the transformation temperature, causing the transformation to propagate away from the electrode during stage III. This work demonstrates two forms of inhomogeneity (local, stochastic current flow, and local contact resistance) that result in a complex thermal profile of the sample. This profile should be interrogated to understand reaction kinetics, and can be beneficial when engineered.

Melting Kinetics of Nascent Poly(tetrafluoroethylene) Powder.

The melting behavior of nascent poly(tetrafluoroethylene) (PTFE) was investigated by way of differential scanning calorimetry (DSC). It is well known that the melting temperature of nascent PTFE is about C, but reduces to C for once molten material. In this study, the melting temperature of nascent PTFE crystals was found to strongly depend on heating rate, decreasing considerably for slow heating rates. In addition, during isothermal experiments in the temperature range of C, delayed melting of PTFE was observed, with complete melting only occurring after up to several hours. The melting kinetics of nascent PTFE were analyzed by means of the isoconversional methodology, and an apparent activation energy of melting, dependent on the conversion, was determined. The compensation effect was utilized in order to derive the pre-exponential factor of the kinetic model. The numerical reconstruction of the kinetic model was compared with literature models and an Avrami-Erofeev model was identified as best fit of the experimental data. The predictions of the kinetic model were in good agreement with the observed time-dependent melting of nascent PTFE during isothermal and constant heating-rate experiments.

A new thermogravimetric application for determination of vapour pressure curve corresponding to average boiling points of oil fractions with narrow boiling ranges

The average boiling point of oil fractions with narrow boiling ranges or pseudo-components is an important parameter in thermodynamic calculations in the fields of process design and environmental protection. The present work provides an experimental approach, involving a method based on the thermogravimetric analysis, for determination of the pressure dependence of average boiling points (or in other words for determination of “the vapour pressure curve corresponding to average boiling points”) of oil fractions with narrow boiling ranges, pre-prepared by distillation. This method can be used for determination of the atmospheric average boiling points (corresponding to atmospheric pressure) of thermally unstable fractions from the pressure dependence curves, obtained from the tests conducted at lower pressures. The method uses the principle of ASTM E1782 “Standard Test Method for Determining Vapor Pressure by Thermal Analysis”, where the material is vaporised at pre-determined pressure from a hermetically sealed capsule through a pinhole using a constant heating rate experiment. The accuracy of the method was evaluated on the basis of the average boiling point values of narrow boiling ranges fractions determined by rectification both at atmospheric pressure and in vacuum (determined as the arithmetic mean of the lower and upper temperature limits of the fraction). The pressure dependence curves of the average boiling points, formed here on the basis of average boiling points obtained in the pressure range from 5 kPa to atmospheric pressure, can be reliably described by the integrated Clausius-Clapeyron equation. To increase the accuracy in determining the atmospheric average boiling points of thermally unstable fractions, obtained in extrapolation from the Clausius-Clapeyron curve of the average boiling points in the tests conducted at lower pressures, a higher number of tests, on the basis of which to conduct the extrapolation, is desirable.

On the role of Debye temperature in the onset of flash in three oxides

Higher electric fields lower the temperature for the onset of flash. We explore the question “what can be the lowest temperature for initiating flash?”. Constant heating rate experiments at increasing electric fields reveal a surprising characteristic: the Debye temperature emerges as a the lower bound for the onset of flash. Data for flash temperature for three oxides are shown to exhibit this behavior in a universal plot.

Effective Activation Energy for the Solid-State Sintering of Silicon Carbide Ceramics

Effective activation energy for densification of SiC in the presence of C and B4C additives is determined by constant heating rate experiments through analysis of shrinkage in hot pressing. The activation energy (AE) for sintering increased linearly with the relative density (RD) in two different regimes. The effective AE increased from 407 ± 50 kJ/mole at 75 pct RD to 1132 ± 75 kJ/mole at 95 pct RD. Lattice diffusion is proposed as the predominant mechanism for SiC densification at higher density. This is also validated by the uniform distribution of sintering additive through electron probe microanalysis. The low AE in the regime of lower density could be attributed to the pressure-assisted particle rearrangement during hot pressing. The relative contribution of both the mechanisms between two density limits resulted in the change in AE for sintering. The mechanisms of defect generation resulting in densification are also discussed.

Towards a meaningful non-isothermal kinetics for biomass materials and other complex organic samples

The literature of the kinetics in thermal analysis deals mainly with models that consist of a single reaction equation. However, most samples with practical importance are too complex for such an oversimplified description. There is no universal way to overcome the difficulties, though there are well-established models that can express the complexity of the studied reactions for several important types of samples. The assumption of more than one reaction increases the number of unknown parameters. Their reliable estimation requests the evaluation of a series of experiments. The various linearization techniques cannot be employed in such cases, while the method of least squares can be carried out at any complexity of the models by proper numerical methods. It is advantageous to evaluate simultaneously experiments with linear and nonlinear temperature programs because a set of constant heating rate experiments is frequently not sufficient to distinguish between different models or model variants. It is well worth including modulated and constant reaction rate temperature programs into the evaluated series whenever they are obtainable. Sometimes different samples share some common features. In such cases one can try to describe their reactions by assuming parts of the kinetic parameters to be common for the samples. One should base the obtained models and parameter values on a sufficiently large amount of experimental information, in a reliable way. This article is based on the authors’ experience in the indicated directions from 1979 till the present. Though the examples shown are taken from biomass research, the models and methods shown in the article are also hoped to be relevant for other materials that have complicated structure or exhibit complicated thermal reactions, or both.

Mechanical strength and defect distributions in flash sintered 3YSZ

The influence of preexisting defects and the generation of new ones during flash sintering was studied in 3YSZ. Weibull statistics was used to analyse mechanical testing data from flash and conventionally sintered specimens with equal densification. The obtained values for Weibull modulus and characteristic strength are 6.09 and 371MPa for the conventionally sintered samples, respectively, and 5.92 and 506MPa for the flash sintered samples. It can be inferred that flash sintering does not alter significantly the defect distribution on the studied material. Preexisting defects were manufactured with rice starch as a pore forming agent, in 5, 10 and 15vol%. Those samples were flash sintered in isothermal and constant heating rate experiments. The experiments show that there is no noticeable difference on the incubation time and the onset temperature for flash sintering.

Flash Sintering of (La, Sr)(Co, Fe)O3–Gd‐Doped CeO2 Composite

Sintering of composites constituted by two nonstoichiometric phases (La0.6Sr0.4)(Co0.2Fe0.8)O3 (LSCF) and Gd0.1Ce0.9O2 (GDC) under constant electric field in constant heating rate experiment is studied in this work. The requirements of field and temperature for composite systematically increase with GDC amounts this indicating the importance of material conductivity. Sintering/grain growth rate is higher in the composite compared to pure LSCF phase. Flash‐sintering phenomenon in the composite is explained on the basis of three factors: (1) large and continuous increase in conductivity of LSCF acts as source of defects, (2) maintenance of sufficient local temperature because of GDC during continuous conductivity increase facilitates the cationic diffusion, and (3) reduction reactions of LSCF, during polaron hopping conduction, and of GDC phase at higher temperature activate the sintering process.

Kinetics and Mechanisms of Isothermal Devitrification in Amorphous Cu50Zr50

The crystallization kinetics and microstructural dynamics associated with devitrifying a melt-spun Cu50Zr50 metallic glass were investigated using isothermal treatments, in situ high-energy synchrotron X-ray diffraction, conventional and high-resolution transmission electron microscopy, and differential scanning calorimetry. The analysis of isothermal transformations allows us to more clearly unravel the complex interplay between nucleation and growth of competing stable and metastable phases. The isothermal devitrification response was found to involve the Cu10Zr7, CuZr2, and CuZr phases, consistent with previously reported constant heating rate experiments, but here we have resolved the phase evolution and structural characteristics of the transformation, including the very early stages of crystallization. At 671 K (398 °C), the isothermal transformation starts with the formation of the Cu10Zr7 phase, which grows in a generally equiaxed morphology. At a size of approximately 100 nm, the growth of the Cu10Zr7 particles is interrupted by the precipitation of a thin layer of the CuZr2 phase, upon which the metastable CuZr (B2) grows epitaxially. Crystallization kinetics are quantified here though in situ measurements (HEXRD, DSC) and ex situ microstructural analysis (TEM, HRTEM). Finally, the influences of chemical partitioning, diffusion, and crystallographic orientation on this sequence are examined.

Thermoanalytical study of crystallization process in metallic glass of Co69Fe3Si18B10

The non-isothermal crystallization kinetics of amorphous Co69Fe3Si18B10 metallic glass has been studied using various thermal analytical models. The DSC curve of amorphous sample in a constant heating rate experiment showed the appearance of two crystallization events at peak temperatures 761 and 803 K, respectively. The activation energy for the secondary crystallization process was found to be lower compared to that of primary process which indicated the ease of secondary crystallization process from modified matrix due to the evolution of the primary phase. The activation energy remained constant during the primary crystallization process over the range of crystallization fraction (0.1–0.9), however, the local Avrami exponent for this crystallization process manifested complex variation with crystallized volume fraction. The validity of the JMA model for the primary crystallization process has been studied. This crystallization process was found to show autocatalytic behavior which has been given a mathematical description using a two-parameter model of Sestak–Berggren.

Studies on sintering kinetics of ThO2–UO2 pellets using master sintering curve approach

Three different compositions of thoria–urania pellets, namely, ThO2–4%UO2, ThO2–10%UO2 and ThO2–20%UO2 (all compositions are in wt% containing natural uranium) were fabricated by Coated Agglomerate Pelletization (CAP) process. The compositions studied in the current paper are the proposed fuels for the forthcoming Indian Advanced Heavy water Reactor (AHWR) and its variant based on low enriched uranium. Sintering kinetics of ThO2–x%UO2 (x=4, 10, 20) green pellets, thus fabricated, were evaluated using constant heating rate experiments in a vertical dilatometer. Activation energies of sintering (Q) were estimated using Arrhenius plot as proposed by Wang and Raj. Master Sintering Curves (MSC) for the above three compositions were constructed using shrinkage data. A FORTRAN program, employing optimization based numerical algorithm for fitting relative density vs. work of sintering data with sigmoid function, was used for this purpose. The apparent activation energies, evaluated using MSC principle, appear to be consistent with the values obtained by Arrhenius plot.

An experimental study of the effect of ionic liquids on the low temperature oxidation of coal

Fourier transform infrared spectroscopy (FTIR) and constant heating rate experiments were performed to study the low temperature oxidation of coal treated by an ionic liquid, 1-allyl-3-methylimidazolium chloride. The inerting effect of the ionic liquid toward the low temperature oxidation process is discussed. The results show that: (1) The hydroxyl content associated with hydrogen bonds, the aliphatic methyl content, the methylene group content, and the ether oxygen bond content are reduced in the treated coal. At the same time the content of aromatic CC bonds is constant but these chemical bonds weaken and some substituted aromatic hydrocarbon content increases while other types decrease. This demonstrates that (AMIm)Cl dissolves and destroys the coal surface microstructure; (2) The oxygen consumption of the treated coal is less than what is seen in raw coal. The CO, CO2, C2H4, and C2H6 content from the treated coal is reduced compared to the untreated coal; (3) The apparent activation energy for the oxidizing reaction is different in the treated and raw coals. Micro-structural changes and macroscopic gas production allow us to conclude that (AMIm)Cl can effectively inhibit low temperature oxidation of coal.

Thermokinetic model of sample response in nonisothermal analysis

A thermokinetic model for the reaction rates of solids measured in constant heating rate differential thermal analysis is derived from heat transfer and chemical kinetics. An explicit result is obtained for the maximum reaction rate in a constant heating rate experiment in terms of the thermal properties of the solid, the activation energy of the thermal process, the heat of reaction, the sample mass and the heating rate in the test. The theoretical predictions compare well with numerical simulations and experimental data for polyoxymethylene and polystyrene obtained by two different thermal analysis methods using three different instruments over a wide range of sample mass and heating rate. Based on these results, an accuracy criterion for thermal analysis measurements is proposed that is consistent with recommended practices.

Enhanced Sintering Rate of Zirconia (3Y‐TZP) Through the Effect of a Weak dc Electric Field on Grain Growth

We show, for the first time, that a dc electric field of 20 V/cm shifts the densification curve to a lower temperature in constant heating rate experiments with yttria‐stabilized tetragonal zirconia powder (3Y‐TZP). The enhanced sintering rate is ascribed, at least in part, to the reduced rate of grain growth under the applied field, consistent with earlier experiments on the influence of such fields on grain size in superplastic deformation and isothermal grain growth in zirconia polycrystals.

Phase evolution in reaction sintered zirconium titanate based materials

Zirconium titanate materials are proposed for structural components for which fully reacted and relatively large pieces are required. In this work the phase evolution in slip cast compacts constituted by equimolar mixtures of TiO 2 and ZrO 2 stabilized with 3 mol% of Y 2O 3 at high temperature is studied, to establish the basis to design suitable thermal treatments for ZrO 2(Y 2O 3)–TiO 2 materials. The temperatures at which the processes involved in the reaction sintering occurred were identified by constant heating rate experiments. Phase and microstructure analyses have been performed on specimens treated at the identified temperatures and air quenched. Then the adequate temperature range to get fully reacted and dense materials has been deduced. Materials treated at 1500 °C to 2 h were constituted by Zr 5Ti 7O 24 as major phase, a solid solution of TiO 2 and Y 2O 3 in c-ZrO 2 as secondary phase and a ZrO 2–TiO 2–Y 2O 3 non-stoichiometric compound with pyrochlore structure as minor phase. Pyrochlore was demonstrated to be a metastable phase at 1500 °C.