Why is indium used in dsc

Analyte Name Indium. CAS Number Analyte Name Melting temperature. Concentration Enthalpy of fusion. CAS Number -. Accurate Mass -. Accurate Mass Smiles [In]. IUPAC indium. Melting temperature. Shipping Temperature Room Temperature. Punchout session timeout warning. Select "Continue session" to extend your session.

Owing to its narrow fusion peak and high purity, indium In is one of the metals used for temperature and enthalpy calibration of differential scanning calorimeters not only during heating but also during cooling [ 6 ].

Therefore, In may also be considered an ideal substance to study crystallization from melt and the influence of foreign substances on crystallization kinetics. The knowledge of the solidification kinetics of a material in the presence of a low thermal conductivity polymer is of great potential interest not only to predict the effect on the crystallization rate and hence on the processing time [ 7 ], but also to control the grain size [ 8 ] and intragrain defect density [ 9 ], which are very important when high quality crystals are required.

Moreover, understanding the effect of polymers on metals crystallization allows realizing that also interpretative analysis of thermal data concerning polymers and polymer composites may fail when the temperature of the furnace instead of that of the samples is used [ 5 , 7 , 10 , 11 ] and, therefore, that the advice of the authors of [ 2 ] about the DSC calibration substances is not a choice but a need.

For instance, it is still accustomed to compare the crystallization kinetics at a constant furnace temperature of a polymer matrix with that of much higher thermal conductivity composites, whereas for a reliable discussion of results it would be more reasonable to report the actual temperature values of samples [ 7 ]. Indeed, the necessity to account for thermal inertia and temperature gradients originated in samples during differential thermal analysis and DSC has been highlighted by many authors and also equations for peak corrections have been proposed [ 12 , 13 ], although these rectifications are mainly ignored in the recent state of thermal analysis.

The concomitant use of a metal and a polymer is not new in DSC measurements [ 11 ]. The solid-liquid transition of In placed onto polymer sheets has been exploited for several purposes, as evaluations of thermal gradients inside polymers and thermal conductivity [ 14 , 15 ].

However, previous studies address the estimation of polymer properties and did not investigate the effect of a polymer on the phase transition kinetics of metals. On the contrary, the present paper focuses on In crystallization, in order to establish the change of In phase transition kinetics in the presence of a low conductivity polymer [ 1 , 16 ] and to evidence that nonconductive substances at ordinary scan rates cannot follow temperature programs in thermal equilibrium conditions with the DSC platform.

As crystallization is an exothermic process, as observed first by Gibbs the heat evolved must be carried away from the crystal growth front for advancement of solidification [ 17 — 20 ]. Indeed, during solidification metals may show recalescence increase in brightness and temperature observed in solidification of undercooled metals because of latent heat release as consequence of grains growth [ 21 ].

The rate of heat dissipation during crystallization, reflected by DSC crystallization peaks, has to be evidently linked to the crystallization rate. Le Bot and Delaunay investigated the DSC solidification of pure indium on cooling from melt and showed that crystallization starts at a temperature slightly lower than the In melting point The rate of heat removal during crystallization and therefore the shape of the crystallization peak depend not only on the thermal conductivities of the solidifying substance and DSC furnaces, but also on the thermal properties of substances interposed between the cooling source and the crystallizing material.

The presence of a low thermal conductivity and high thermal capacity substance, like polytetrafluoroethylene PTFE , on the bottom of the DSC pan affects the crystallization rate of In since the heat flow from In towards the DSC platform is hindered by the high resistance of the polymer sheet.

Consequently, the crystallization kinetics of In, obtainable from the DSC crystallization peak, will change. As mentioned above, this work has been undertaken to assess the mathematical dependence on time of the solidification kinetics of In when the heat pathway includes a polymer film and to draw conclusions on the influence of DSC calibrations on polymer measurements.

PTFE has been chosen amongst polymers because of its excellent thermal stability. In conformity with literature, the crystallization of In has been here considered to occur via nucleation, growth, and cessation of growth because of grains impingement [ 21 ].

Indeed, the rate of crystallization is recognised to follow a bell-shaped trend relative to time. From mathematical considerations it can be shown that the crystallization rate of a polycrystalline substance increases rapidly before impingement not only because of nucleation but also because of growth [ 22 ], achieving the maximum soon after the start of coalescence.

Then, because of the high length of interfaces built between crystallites, the growth is hindered in most of spatial directions and the rate of crystallization falls quickly to zero. The apparatus was calibrated with pure indium, lead, and zinc references at various scanning rate. From a film of PTFE 0. The solid-liquid transition of a pure metal occurs by heat delivery at the thermodynamic melting temperature, which remains constant until the whole solid has been transformed in liquid.

If a polymer sample contained in a DSC pan was in thermal equilibrium with the platform during heating at constant rate i. This is even more the case of a metal, such as In, used as a reference substance for temperature calibration. Furthermore, at the melting temperature of In the contact resistance between In and a solid polymer is negligible [ 14 , 23 ] namely, contact resistances are defined only for contact between two solids and, because of the high thermal conductivity of In, only the polymer limits the heat exchange rate.

Plotting the heat flux as function of temperature, the difference between the thermodynamic melting point of In and the transition temperature readable on the DSC curve in the presence of PTFE, may be ascribed to the thermal lag accumulated by PTFE during heating up to the fusion point of In [ 15 ]. Thermal lags may occur not only during heating [ 5 ] but also during cooling and may be evidenced even during more complex thermal programs, as in procedures for isothermal crystallization [ 15 ].

In Figure 2 a the DSC curve of pure In, according to the thermal program described in the experimental part, is shown. Comparing Figures 2 a and 2 b it emerges that in the presence of PTFE the crystallization of indium at the same furnace temperature is not immediate but requires a longer time to start and to be completed. As the temperature of the DSC platform during the two crystallization processes is the same, this difference can only be explained by assuming that the bottom layer of PTFE causes a delay in the cooling of the top layer of In.

The isothermal crystallization peak of In in the presence of PTFE, shown in Figure 2 b , may be described by two equations. In the initial part of the liquid-solid phase transition the heat flow rate is given by where is the heat flux during the phase transformation, is a constant, and is the time constant proportional to the product of the thermal capacity and the thermal resistance of the system, which determine its dynamic behaviour [ 24 ].

It is well established that there is an analogy between thermal and electrical phenomena and relative quantities [ 14 ]. The electrical equivalent of formula 1 describes the charge of a capacitor in a RC circuit including also a voltage source.

In the central part of the transition, the heat flow rate still expressed by 1 is almost constant with time, whereas at the end of the transition the tailed portion of the peak see Figure 2 b may be represented by the following exponential decay: Formula 1 is the thermal equivalent of the potential difference across the capacitor in an electrical RC circuit driven by a voltage source, whereas 2 is equivalent to the discharge of a capacitor through a resistor in a RC circuit deprived of a voltage source.

Within the framework of the crystallization theory based on nucleation and growth, the exponential increase of the heat evolution rate may be related to the growth of crystalline nuclei; meanwhile other nuclei of critical dimension may be still generated.

The subsequent almost constancy of the heat flow rate indicates that the equilibrium melting temperature of In has been achieved. Since nucleation needs overcooling, the constancy of the heat flow rate also indicates that the nucleation process has achieved saturation and that, therefore, only growth of grains occurs at an almost constant rate. This latter stage, which is very fast in absence of PTFE, is here prolonged as consequence of the slowdown of the heat removal through the polymer film.

Finally, the exponential decay of the heat flow rate at the end of crystallization is consistent to a slowdown mainly due to impingement of crystalline grains. In conclusion, the presence of PTFE slows down the crystallization of In and it is possible to determine not only the time needed for the overall crystallization but also the time for the completion of the nucleation process directly from DSC curves. It is worth observing that the slowdown of crystallization by using a polymer sheet may be an ingenious contrivance to achieve higher perfection of solids for particular applications, since rapid crystallization often leads to many dislocations and defects [ 25 ].

The most common approach for description of phase transformation of materials is due to Kolmogorov and Avrami, which independently derived an equation by considering the mathematical relationship between the number of nuclei, supposed to be spherical, and the volume of the new growing phase under isothermal conditions, modelling nucleation as a stochastic process and growth as a deterministic phenomenon [ 26 , 27 ].

Although Kolmogorov published in Russian the statistical theory on nucleation several years before Avrami, this latter is the most popular and quoted author in articles on polymers and, therefore, the phase transformation theory will be herein associated to Avrami. The application of the Avrami equation requires estimation of the degree of liquid-solid transformation with the time and the definition of two constants and , depending on the nucleation and growth of crystals, respectively.

Specifically, the constant depends on the nucleation rate whereas the so-called Avrami index depends on the type of nucleation and the number of dimensions of the growing crystals. Actually, 3 is also valid if the nucleation rate at the zero time is infinite and then suddenly falls to zero, which means that all nuclei appear simultaneously and their number remains constant up to impingement.

The nucleation constant depends on the temperature according to an Arrenhius type dependence: where is the activation energy for nucleation and the universal gas constant. According to the original theory, should be an integer from 1 to 4. However, is often not integer and even higher than four [ 28 ]. The parameter is proportional to the crystallization rate and is obtainable from logarithmic linearization of 3 by performing isothermal DSC crystallizations of In.

The crystallinity fraction originated at a definite temperature as a function of time is generally calculated by the formula where the numerator is the heat developed during crystallization from zero time taken at the onset of crystallization up to time , whereas the denominator is the heat generated on complete crystallization.

These two values are proportional to the area of the portion of the crystallization peak in the time interval and to the total area of the crystallization peak, respectively. Equation 3 is based on postulates which often result in overestimations of the volume of the transformation product [ 29 ].

Amongst the assumptions, there is also that of uniform randomly nucleation [ 30 ]. This condition may not be respected even for rapidly quenched thin metal layers [ 31 ], meaning that practically nucleation is a process time and space dependent [ 28 , 32 ] and therefore the Avrami equation can be only seemingly fitted by data collected in limited time intervals for specimens with very small thickness.

It is well recognised that the temperature detected by DSC is the temperature at the bottom pan [ 1 , 12 , 13 , 24 ] and that the sample temperature depends on the thermal characteristics of the definite substance included in the pan and on the scan rate [ 1 , 11 — 13 , 21 ].

However, most of DSC manufactures advertise their efforts to reduce the thermal conductivity of the furnaces and they do not seem to take into account that the rate of the DSC response depends also on thermal conductivity of samples. High thermal conductivity substances, like metals, are also usually recommended for DSC temperature and enthalpy calibration, independently of the samples to be investigated, with the assumption that low scan rates and masses are sufficient to minimize temperature gradients even inside low thermal conductivity samples.

Even though this assumption was really applicable, it must be noted that most of the thermal procedures on polymers, like crystallization from melt, need instead fast cooling and relatively high masses of samples to overcome kinetic barriers or instrumental limits [ 15 , 28 , 33 , 34 ]. Contrivances such as low mass and scan rates allow obtaining accurate and reproducible melting points and enthalpies even for polymers, but they are not useful for predictions of kinetic parameters relative to high masses of specimens.

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