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The heat flow should reach near-steady-state conditions during each segment. This means that each segment should last at least one minute. The evaluation of the specific heat capacity using steady-state ADSC requires the sample and blank measurements to be performed with the same temperature program. The blank-subtracted heat-flow curve of the sample is shown in the upper diagram and the resulting specific heat capacity curve c p curve below.

Step 2: Setup

The evaluation procedure calculates one point per segment. Steady-state ADSC requires relatively long measurement times and accurate heat-flow adjustment in the relevant temperature range. In contrast to the techniques I have already discussed, it allows measurements under quasi-isothermal conditions, that is, at an underlying heating rate of zero. The temperature program is shown in the upper diagram and is defined by the underlying heating rate, the amplitude, and the period.

ADSC achieves the best heat capacity accuracy by using measurements of sample, blank and a reference material, in this case aluminum. The upper diagram shows the measured heat-flow curves and the lower diagram the resulting specific heat capacity. An advantage of this technique is that the specific heat capacity c p can be determined at very low heating rates and even under quasi-isothermal conditions. This improves the sensitivity and resolution of the measurement. In addition to the specific heat capacity, the method determines the reversing and non-reversing heat flow.

This allows overlapping effects in complex thermal events to be separated. Three measurements are required to achieve the best accuracy. This makes the technique rather time consuming. However, if only the relative change of the specific heat capacity is required, the ADSC evaluation can be performed using just sample and blank measurements. For a relatively fast survey, only the sample measurement needs to be evaluated. If thermal events occur, the resulting specific heat capacity may be frequency dependent. In contrast to all other temperature-modulated techniques, it uses a stochastic modulation function.

Introduction

The method delivers a maximum of information about the sample in one single measurement. The patented evaluation procedure allows the best possible separation of sensible and latent heat capacity and facilitates a consistency check of the resulting curves. This significantly improves the quality of the results. This technique can be also used to measure the specific heat capacity.

The upper diagram shows a typical temperature program. The measured heat-flow curve is plotted below. Typical TOPEM parameters for specific heat capacity c p measurements are heating rates between zero point five and two Kelvin per minute 0. In principle, only one measurement is necessary to determine the specific heat capacity c p. The result can however be improved by measuring a sapphire reference standard.

In contrast to the other techniques, the sapphire technique can be used for long periods and for different measurement conditions. The upper diagram shows the heat-flow curve of the sample and part of the sapphire curve measured at a faster heating rate.

The lower diagram shows the specific heat capacity curve as the result of the evaluation. In principle, the TOPEM technique offers similar advantages as the other temperature-modulated techniques. The measurement accuracy is comparable to that of the IsoStep technique. In contrast, quasi-static measurements are possible and the measurement time is shorter because only one measurement is needed. The diagrams in the next two slides compare the results obtained using the techniques I have discussed so far in this seminar.

The points in this first diagram show the data for the specific heat capacity of the polystyrene given by NIST. This diagram now includes all the results measured using the different methods in addition to the data from NIST. The direct and sapphire methods show a larger deviation at lower temperature.

This is due to the initial start-up transient of the conventional DSC. Finally, I want to point out that the direct and sapphire methods measure the sum of the latent and sensible specific heat capacities whereas the modulated methods determine the sensible specific heat capacity. This is not important for the example I have just discussed because the latent specific heat capacity is zero. This situation changes fundamentally if a thermal event occurs. The specific heat capacity is an important property for characterizing energy storage and insulation materials. It is also important for improving processes in production, storage and transportation.

Rapid and reliable measurement of the specific heat capacity is required in many industries, for example in the chemical, pharmaceutical, and food industries. High-sensitivity sensors have been developed to measure low-energy transitions. In practice, specific heat capacity data is used to obtain detailed information about materials, for example enthalpy, degree of crystallinity, content in blends, alloys or copolymers, storage heat capacity of phase-change materials, and many others. In the first application, TOPEM is used to measure the sensible specific heat capacity curve during the curing of a two-component epoxy system.

The black curve is the total heat-flow curve and is almost identical to a conventional DSC curve. In contrast, the blue curve shows the change of the sensible specific heat capacity with higher resolution and sensitivity. The reaction rate then decreases. In a curing reaction, the size of the molecules increases. This is accompanied by an increase in viscosity and an increase in the glass transition temperature of the partially cured mixture. At a certain time, the glass transition temperature reaches the sample temperature.

As a result, the reaction mixture transforms into the glassy state, in other words, the sample vitrifies. The reaction rate slows considerably due to this vitrification process and the mechanism of molecular network formation changes in the partially cured material. Following this so-called devitrification process, the material is in the rubbery state and final curing occurs. This detailed interpretation was obtained from one single temperature-modulated DSC measurement of the specific heat capacity.

The measurement of water and dilute aqueous solutions by conventional DSC is somewhat of a problem because of evaporation, which occurs even at temperatures far below the boiling point. This is the reason for significant experimental errors.

TOPEM is the best technique to use because the sensible specific heat capacity is measured with the good accuracy in one single run. The result shows the correct temperature dependence. The third application example illustrates the determination of the temperature-dependent crystallinity of polymers during heating.

The sample is prepared from pellets of polyethylene terephthalate or PET for short. The red curve is the specific heat capacity curve of the first heating run of the original sample. The second heating curve after rapid cooling from the melt is colored blue. The second heating curve clearly indicates three thermal events. The measured specific heat capacity c p curves can be used to determine the specific enthalpy curves by integration.

This calculation can be easily performed with the STAR e software. The resulting curves of the measured specific enthalpy can be used to determine the crystallinity. I will explain this in the next slide. The specific enthalpy of an amorphous or liquid material is larger than that of a semicrystalline material. The contribution of the crystalline phase is the difference between the specific enthalpies of the liquid phase and the measured specific enthalpy curve.

The crystallinity, X, is the ratio between this difference and the difference between the specific enthalpies of the liquid and crystalline phases. The diagram shows the resulting crystallinity curves of the original material and the rapidly cooled material. During melting, the crystallinity decreases to zero. The crystallinity decreases during melting. The initial crystallinity and differences in the crystallinity curve during heating provide information about modifications in the molecular structure, the effectiveness of additives, and the thermal and mechanical history of materials.

The measurement of good quality quantitative heat capacity data is a challenging task for DSC. Nevertheless, it is a common and useful technique because of the availability of DSC instruments, simple sample preparation, relatively short measurement time, acceptable accuracy, and easy evaluation using commercial software. Allow the instrument sufficient time to stabilize. This means that the gas flow is constant, that the measuring system has been switched on for some time, and that the sensor and furnace lid are properly installed and not contaminated.

Usually the results are improved by blank curve subtraction. Use similar crucibles for the sample, blank and reference measurements and enter the crucible masses in the software. Do not deform the bottom of the crucible during sample preparation. The safest way is to not use of the sealing press and manually place the lid on the crucible. For organic substances, we recommend the forty microliter mL crucible and a sample mass between ten 10 and twenty milligrams 20 mg.

If sapphire is used as a standard or for calibration, use similar heat capacities for sample and sapphire.

Breadcrumb

If possible, compact samples of low-density foams and powders, but avoid mechanical stress in the crucible. This can cause artifacts. The heat capacity : This is a characteristic property of the sample and depends on the sample size; and. The specific or molar heat capacity : These quantities are independent of the sample size. The specific heat capacity contains two components: The sensible component, which is related to rearrangements of the molecules as a whole, and the latent component which is determined by all other thermal events.

The specific heat capacity is important for the advanced evaluation of DSC curves, detailed material analysis, and improvement of all processes in the life-cycle of products beginning from their production to their disposal, and finally the recycling of the product. The method chosen depends on the sample, the experimental setup, the measuring time available, and the required accuracy. For proper measurement of specific heat capacity c p , the experimental setup should be carefully calibrated and installed.

Attention should also be paid to sample preparation. Precision and reproducibility can be more than one order of magnitude better. This depends largely on the sample. Finally, I would like to draw your attention to information about specific heat capacity that you can download from the Internet.

Back issues can be downloaded as PDFs from www. In addition, you can download information about webinars, application handbooks or information of a more general nature from the Internet addresses given at the bottom of this slide. This concludes my presentation on specific heat capacity. Thank you for your interest and attention. Transport and Logistics. Expertise Library. Literature: White Papers, Guides, Brochures. Technical Documentation. On Demand Webinars.

Specific Heat Capacity Problems & Calculations - Chemistry Tutorial - Calorimetry

Live Events. Live Webinars. Browse our product offerings here. On Demand Webinar. Determination of specific heat capacity. Heat capacity vs specific heat capacity The measured heat capacity depends on the sample size and does not characterize the material. Register for the Specific Heat Capacity Webinar. Specific heat capacity.

Lab 4 - Calorimetry

Slide 1: Contents First, I would like to define what we mean by the specific heat capacity and then briefly discuss the technical importance of this property. Slide 2: Definition of Heat Capacity The heat capacity of a sample is a property that can be measured under different experimental conditions, normally either at constant pressure or at constant volume. Slide 3: Definition of Specific Heat Capacity The heat capacity consists of two components: the sensible heat capacity and the latent heat capacity.

Slide 4: Importance of c p This slide lists several practical applications of the specific heat capacity, c p. It is also an important quantity for the advanced evaluation and interpretation of DSC curves.

Specific heat capacity questions and equation

Slide 6: c p Measurement Methods I now want to discuss several different methods used to measure the specific heat capacity based on DSC. Slide 7: Direct method The simplest method to determine the specific heat capacity is the Direct method. Slide 8: Direct method This slide displays the result of the Direct method. This method requires accurate calibration of the heat flow in the relevant temperature range. Slide IsoStep DSC The upper diagram shows the blank-subtracted heat-flow curves of the sample and the sapphire standard.

Slide ADSC The upper diagram shows the measured heat-flow curves and the lower diagram the resulting specific heat capacity. The method is less influenced by possible instrumental drift. Slide 18 TOPEM The upper diagram shows the heat-flow curve of the sample and part of the sapphire curve measured at a faster heating rate.

Slide 22 Application 1: Curing and Vitrification I would now like to describe three application examples. Slide 23 Application 2: Water The second application shows the measurement of the specific heat capacity of water. Slide 24 Application 3: PET Crystallinity X T The third application example illustrates the determination of the temperature-dependent crystallinity of polymers during heating.

Slide 25 Application 3: PET Crystallinity X T The specific enthalpy of an amorphous or liquid material is larger than that of a semicrystalline material. The slide lists a number of tips and hints for obtaining good data: Allow the instrument sufficient time to stabilize. Check the repeatability and reproducibility by repeating measurements. Averaging of replicate measurements improves accuracy.

Position the crucibles accurately on the sensor. Take care to ensure good thermal contact between sample and crucible. First, we distinguish between: The heat capacity : This is a characteristic property of the sample and depends on the sample size; and The specific or molar heat capacity : These quantities are independent of the sample size. Several UserCom articles relating to specific heat capacity are also listed on this slide. Related Products. Thermal Analysis Excellence offers a comprehensive portfolio of differential scanning calorimeters, thermogravimetric analyzers, dynamic mechanical an Library Literature.


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