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a p p l i c at i o n N o t e Differential Scanning Calorimetry Authors Phil Robinson PerkinElmer, Inc. Seer Green, U.K. Practical Specific Heat Determination by Dual Furnace DSC Introduction This application note outlines the practical considerations when making specific heat determinations by Dual Furnace (Power Compensation) DSC and explains the way that specific heat may be calculated from DSC data. This will give thermal analysts sufficient information to make successful and accurate specific heat measurements using their calorimeters and understand how to avoid some of the common errors encountered. Background The DSC 1 of 1963 was used by Michael J. O’Neill of the Perkin-Elmer Corporation, who discussed measurement of specific heat (Cp) by DSC as an alternative to using drop calorimeters and adiabatic calorimeters. Shortly after the introduction of the DSC 1, O’Neill detailed determination of Cp data,1 and modern software is based on this methodology. Applicability Specific heat determination may be made on most, if not all, types of solid and liquid material, although it is necessary that the material is thermally stable over the temperature range used for the determination and that no mass loss occurs. The temperature range normally specified is between -100 °C and 600 °C although it is possible to work beyond these values. For a broader discussion of experimental parameters and applicability, see ASTM® standard test method E1269. Overview of Methods of Determining Cp by DSC Experimental Considerations There are three basic methods of determining specific heat, each with advantages, differing levels of accuracy achievable and ease of use. However it should be noted that designs of DSC analyzer differ and some types are inherently more accurate than others. The accuracies mentioned below are based on using a dual furnace DSC: measurements using single furnace (heat-flux) DSC require additional care to produce acceptable data. As with any analytical technique, the care exercised by the user can have a large influence on the overall accuracy of the determination, and it is a combination of small factors which must be considered for successful measurements to be made. For all methods of specific heat determination, including modulated temperature DSC, the same consideration apply, and these are: 1.Traditional 2-Curve Method • The sample pan and the sample presentation Here, the same sample pan is used for two identical tests consisting of: – Sample pan flatness • A baseline run using an empty pan and cover • A sample of the material whose specific heat is being determined run using the same pan and cover as the baseline run. With care, specific heat data with around 1.5% absolute accuracy or slightly better should be obtained. • Accuracy in sample mass measurement – Sample shape and flatness – Sample pan placement in furnace – Reference pan • Furnace cover placement – Furnace cover shape and “fit” to the furnace (avoids baseline errors) • Experiment design 2.Traditional 3-Curve Method This method uses the same approach as the 2-curve method except that a third curve is generated using a reference material, the one most often used is a sapphire disk. This reference data is used to “correct” sample data at every temperature, giving improved accuracy which, with care, can be up to 0.8% absolute accuracy. This method also allows different sample pans to be used for each part of the test. 3.Modulated-Temperature DSC Method This newest approach to specific heat determination involves subjecting the material being tested to a nonlinear temperature program, with the specific heat data extracted from the resulting heat-flow curve. Data can be obtained with an absolute accuracy of up to 0.5% of the literature value with suitable care and correct use of baselines. The method is generally extremely slow due to the nature of the temperature program and the requirement to have a baseline. However the improvement in accuracy makes it worth considering for some situations, although it is not discussed further in this application note. The influences shown above are discussed in more detail below and explain how, when all factors are correctly implemented, accurate specific heat measurements can be obtained. Required Accuracy in Measurement of Sample Mass If the weight of the sample is not recorded with sufficient accuracy, an unacceptable error in specific heat measurement can result. An example is shown in Table 1. Since errors are cumulative, each individual error will increase the error of the final specific heat measurement. Note that in circumstances where the sample pan is weighed separately, (e.g. when a different sample pan is used for the baseline, sample run and reference run) errors from weighing would approximately double or worse, significantly affecting the overall accuracy of the specific heat determination. It is recommended that samples and sample pans are always weighed to an accuracy of 0.001 mg to maximize the accuracy of the specific heat data. Table 1. Example of a measured heat-flow at 95 °C = 13.735 mW with an actual mass of 5.133 mg. 2 3-place grams 4-place grams 5-place grams 6-place grams Sample Mass (mg) 5 mg 5.1 mg 5.13 mg 5.133 mg Possible Error (mg) ±1 mg ± 0.1 mg ±0.01 mg ±0.001 mg Specific Heat (J/g/°C) 0.915667 0.897712 0.892463 Accuracy of Specific Heat 102.66% 100.6471% 100.0595% 0.891941 100.0000% The Sample Pan Instrumental Conditions Choice of sample pan will have an influence on accuracy, and also how well the sample is prepared into the pan. In most cases, It is useful if the sample pan can be used without the need for crimping, allowing the same sample pan to be used for the baseline test and the sample test. In this way, the specific heat contribution from the sample pan mass is eliminated. Where this is not possible, then the pans should be weighed at least as accurately as the sample. For thermally stable solid samples, it is recommended to use standard aluminum sample pans 0219-0041 which can be used without crimping. To make successful specific heat measurements, it is vital that the analyzer is clean and stable in temperature and environment before attempting to make a measurement. Preparation of the DSC analyzer before starting such testing might involve the following: Sample Pan Flatness • Ensure that the Sample Holder is Clean It is recommended that a furnace clean is carried out before starting measurements, removing both sample and reference pans before starting. This will do two things. Firstly, it will clean any sample residues from the furnace, and secondly it will “condition” the sample holder, giving a highly repeatable baseline. Since all measurements involve heat flow between the sample holder sensor system through the bottom of the sample pan and into the sample, it is clear that the flatness of the sample pan will have an influence on the measurement. Correct use of either a standard crimper or universal crimper will encapsulate the sample and produce a pan with a flat bottom. It is recommended that you inspect the pan visually to ensure that the bottom of the pan is flat after samples have been sealed in the pan before loading them into the analyzer. • Ensure that the Sample Holder Block Temperature is Stable Sample Form and Shape • Ensure that the Platinum Covers Fit Well It follows that if the sample pan bottom must make good contact with the heat source, then the sample itself should make good contact with the sample pan in order to minimize the thermal resistance. This suggests that the sample should ideally be flat and thin, allowing a large surface to touch the sample pan in relation to the thickness of the sample. It is recommended that the sample is thin and flat e.g. 1 mm thick by 5-6 mm diameter with flat surfaces top and bottom. Where this is not possible, at least the bottom of the sample should be made as flat as possible. Sample Size The sample size makes a difference to the measurement since errors resulting from the accuracy of weighing become significant with very small samples. In general, sample sizes should be between 5 mg and 100 mg, with the preferred weight between 20 mg and 40 mg. It may be necessary to reduce the heating rate when using larger samples to minimize temperature gradients within the sample. Reference Pan The pan used in the reference furnace should generally be of the same type as the pan used for the sample. The reference sample pan should not be disturbed or changed in any way until all tests have been completed, otherwise errors in specific heat determination may be introduced. If the temperature of the sample holder is not stable, baseline drift will be seen from run to run, leading to significant errors in specific heat determinations. Ideally, the analyzer should be left turned on overnight, and the cooling accessory should be switched on at least 2 hours before measurements commence. A cooling accessory of some type should always be operational during specific heat determinations. If the covers to not fit easily, then the external “shape” of the furnace can vary from run to run as the lids tilt slightly. Some surprisingly small effects in the “fit” of the lids can cause some large errors in the repeatability of the baseline. It is recommended that you use the manually fitted platinum furnace covers and not the autosampler covers. The autosampler covers do not always “sit” in the same orientation or flatness, and this results in changes to the baseline, which affects the accuracy Reform the normal covers (the type with two tweezer holes) using reforming tool 0319-0030 or fit a pair of new covers 0419-0299 before starting to make measurements for best accuracy. • Keep each Platinum Cover on the Same Furnace It is particularly important that each cover remains on the same furnace throughout the measurement as differences in their mass would give errors in the measured specific heat. Recent changes to the platinum cover design provide an indent in the center of one of the lids when purchasing a pair of covers (0419-0299) which allows easy identification of each cover. New furnace covers are made from a platinum-iridium alloy which is less prone to bending through mishandling than the earlier pure platinum covers of the type. 3 • Platinum Cover Orientation For most work on a DSC, the orientation of the platinum furnace covers has little effect if any. However for specific heat work, orientation matters rather more, especially if the covers are deformed slightly. For maximum repeatability, use the tweezer-holes in the cover as a guide to orientate the covers in the same way each time. • Purge Gas The settings will depend on which analyzer you are using. Diamond DSC, P1, DSC and DSC 7 Ensure that your purge gas is flowing by using an inlet pressure of about 2 bar, giving a flow of about 30 mL/ min through each of the two furnaces. Note that the total flow from the exhaust of the analyzer will be around 60 mL/min. DSC 8000 and DSC 8500 Use a flow of 60 nL/min since the mass flow control regulates the total flow into the analyzer. Sample Preparation and Encapsulation As with any thermal analysis measurement, correct sample presentation is of paramount importance, and the sample should be: • A thin flat layer of sample to minimize temperature gradients within the sample. • Evenly distributed over the base of the pan to ensure good contact with the pan base and the sample holder sensor. • Sandwiched by the sample pan lid to promote fast heatflow through good thermal contact between the pan and the sample. This also minimizes sample movement which can also cause some undesirable effects on the measurement. • A flat bottom to the sample pan to ensure good thermal contact with the furnace sensor system. To meet the above criteria, it is recommended that the sample size should be typically between 10 mg and 100 mg, and that sample pans are selected as follows: The amount of time required to purge the system (and therefore achieve a stable heat-flow) may vary with the type of purge gas being used. a). Standard Aluminium Pans (0219-0041) I. Nitrogen or Air – no purging time is necessary after closing the sample holder cover (see note below concerning oxygen). b). Universal Crimper Aluminium Pans (Robotic Crimper) II. Helium or Argon – To completely displace air or nitrogen after closing the sample holder cover, a purging time of 6-8 minutes is necessary (at the recommended flow rates above) since helium and argon both have a much higher thermal conductivity than either air or nitrogen. After closing the sample holder cover, the changing concentration of these higher-thermal conductivity gases have the effect of causing some apparent drift in the heat-flow reading from the sample holder until purging has completed. III. Oxygen – Due to the high reactivity of oxygen, it is likely that samples will be changed by the effects of heating in this atmosphere; therefore it is recommended that specific heat determinations should not be carried out in the presence of oxygen. This comment will also apply to air with sample types of sample. Use sample pan bases and cover, but do not seal or crimp them for the baseline run or sapphire disk. If heating the sample to temperatures above 100 °C, consider piercing the top pan in the center to allow the air inside the pan to expand. Alternatively, consider using a pan with holes as the top pan. Note that having the top pan with holes will allow any material ejected from within the pan to be collected in the well at the top of the pan, avoiding the need to clean out the sample furnace after such a spillage. This “spillage” can also indicate an interpretation of any anomalies seen on the DSC curve. Using a sample pan as a cover can fulfill the requirement of “sandwiching” the sample between the pan base and the cover. c). Other Pan Types Other pans should be selected where circumstances require their use, but allowance should be made for the differing thermal resistances. Pans made of material other than the aluminum types above are acceptable but it should be noted that aluminum has a relatively higher thermal conductivity compared with other metals and this will affect the thermal resistance constant (Ro) values obtained. See Appendix 1 for Ro determination. The prime reason for considering using pans of different design is to prevent volatilization (or sublimation) occurring. In this case it may be necessary to use a large volume stainless steel pan in order to contain the sample in the temperature range required, and, if necessary, determine Ro for this type of pan. 4 Experimental Design for the 2-Curve and 3-Curve Cp Determinations Starting Temperature and Isotherm Figure 1 The starting temperature chosen should be at least 30 °C above the “block” temperature of the DSC (i.e. the background temperature from the cooling accessory); any lower and the furnace control is not reliable, resulting in inaccurate heat-flow readings. It may be necessary to wait for several minutes before starting to record data while the heat-flow reading stabilizes. Ending Temperature and Ending Isotherm The maximum temperature span for a single specific heat test is best kept to between 150 °C and 200 °C since small changes in the instrument stability may become significant with larger temperature ranges. Where specific heat over a larger temperature range is required, it is recommended that a series of overlapping temperature ranges is used, and it is suggested that a 50 °C overlap should be made. With each portion treated as a separate test, the overlapping portions can be used to confirm that the data is continuous, The length of the ending isothermal at the upper temperature should be selected so that a good steady isothermal heat-flow reading is obtained, and it should be recognized that this may take between three and five minutes, with larger samples requiring even longer than this. It is generally found that three to five minutes is sufficient for acceptable stability, although the accuracy can be significantly compromised if the heat flow is not completely stable at the end of this isothermal period. Figure 2 is a display of the ending isothermal for an empty sample pan and a sample pan containing a sapphire disk (0219-1269) 6 mm diameter x 1 mm thick and weighing approximately 130 mg. It is recommended that the heat-flow reading should be stable to 0.001 mW over one minute for the best specific heat data. The data displayed in Figure 1 shows two curves – one where the heat flow is completely stable at the start of the test (red) and where the analyzer was not completely stable (blue). Heating Rate The size of the signal from which the specific heat is calculated is directly proportional to the heating rate used, so it follows that heating faster rates will produce larger signals, which would give more accurate data. However if the heating rate is too high, the temperature gradients in the sample will be large and this may introduce other errors in the measurement. It is normal to use heating rates between 5 °C/min and 20 °C/min, with 20 °C/min being preferred in most circumstances. If low heating rates are required, larger samples should be considered to produce heat-flow signals of a reasonable size and minimize measurement errors from this source. While for 3 curve Cp technique it is not so important, for all other Cp methods it is essential that the instrument be calibrated using two temperature calibration standards to ensure that the scan rate is accurate. Figure 2 It will be seen that the empty pan with much lower mass achieves a steady baseline much sooner than the much larger mass sapphire test, and that the isothermal drift from the sapphire test is much greater despite the 4 minutes at isothermal. The significance of this drift is that it will introduce an error into the measurement, and it is suggested that a longer time is used to allow better stability. It should also be noted that the Y difference between the two curves (~15.7 mW – ~15.57 mW) is related to the difference in mass between the empty pan and the pan plus sapphire. 5 Typical Temperature Program Testing Based on the guidelines above, a typical experiment to measure heat capacity from 20 °C to 200 °C would be as follows: With 0219-0041 pans and covers, the same aluminum pan is used for all three tests, so the aluminum weight is a constant and therefore can be ignored. Therefore each measurement, consisting of three data sets, uses only one single pan for all three tests. • Stabilize the analyzer at 0 °C for each test before starting to record data – It may be useful to use the “Start Run” criteria in Pyris™ software under the “Initial Conditions” settings to automatically stabilize the heat-flow signal before data collection starts. Use the “right mouse button” to “change” the “start run” parameters. • In Pyris, use the following method: Test 1 – Baseline Run Sample furnace contains an empty aluminum pan and lid (do not crimp) with furnace cover fitted. Sample weight in Pyris = 0 mg. Reference furnace is not disturbed. 1. Isothermal at 0 °C to 1 minute 2. Heat from 0 °C to 200 °C at 20 °C/min Test 2 – Sapphire Reference Run 3. Isothermal at 200 °C for 5 minutes Sample furnace contains the sapphire reference in the same aluminum pan as for Test 1, with the flat aluminum lid on top of sapphire (do not crimp), and furnace cover fitted. Sample weight in Pyris = weight of sapphire disk. 4. End condition = “go to load” Although the data is required from 20 °C, the test must start from 0 °C since the first minute of the heating data is required for stabilization, and at 20 °C/min, this is required to be 20 °C below the temperature at which the first measurement is needed. Typical Test Procedure The best pans to use are the standard aluminum sample pans (0219-0041) for solid samples such as powders or polymers since there is no need to weigh the pan itself. This is because the pan used is a constant since the same pan is used for all three tests. Using this pan type is assumed in these instructions: Initial Situation At the start of the test, the situation should be like this: Sample Furnace Empty standard pan plus lid (0219-0041) – not crimped. Furnace cover fitted. Reference Furnace Empty standard pan plus lid (0219-0041) – it does not matter if the lid is crimped or not crimped. Furnace cover fitted. It is important not to disturb the reference furnace in any way during testing (do not move either the pan, lid or the furnace cover) since this will affect the “balance” of the two furnaces and give errors in heat capacity measurement. 6 Reference furnace is not disturbed. Test 3 – Sample Run Sample furnace contains the sample in the same aluminum pan and lid (crimped with sample) used for Test 1 and Test 2, with furnace cover fitted. Sample weight in Pyris = weight of sample. Reference furnace is not disturbed. Placing the furnace cover each time the sample is changed is important – the orientation of the platinum lid and how flat it sits on the furnace will affect the heat capacity values obtained. If you use universal crimper pans, then you must crimp each pan (which cuts off the ring of aluminum) and weigh the amount of aluminum as well as the amount of the sample. The heat capacity software has a place to enter the weight of the aluminum pan if you use the 3-curve method (i.e. with sapphire as reference). Data Analysis Following the DSC Tests Pyris software automatically optimizes the data during the Cp calculation, performing shifting and sloping of the data in the process. For manual calculations the user must make these corrections, which should involve the following steps: Use “Shift Y” to move the start of all curves to a nominal milliwatt value (Figure 3). Verification of Data Quality Figure 6 shows data from a sapphire disk (0219-1269) collected at 20 °C/min. The data has been optimized by aligning the start and ending isothermal using the “slope” function in Pyris software. Figure 3 Use the “slope” command on the “rescale” toolbar and select “align endpoints” option. The slope should normally be set from the first data point of the lower temperature isothermal and the last data point of the upper temperature isothermal (Figure 4). Figure 6 The calculated difference between the two curves seen in Figure 6 is shown in Figure 7. Figure 4 The baseline curve is then subtracted from the sample curve, giving the heat-flow difference due only to the sample (Figure 5). This must then be further calculated to take into account the mass of the sample and the heating rate used for the test. Figure 7 It is normal to remove the heat-flow curves to make the display more clear, and to switch the x-axis to temperature before creating a table of results using <Tools> <Tables> from the Pyris top menus. Figure 5 7 Specific Heat Calculation Table 2 The calculation is extremely straightforward. The difference between the sample curve and the baseline curve is measured in milliwatts and converted to specific heat as follows: Temperature (°C) Measured Cp NBS (1971) Error (J/g*°C) (J/g*°C)% 42 0.449 0.809239 -44.5 47 0.791 0.818851 -3.4 52 0.832 0.828168 0.5 57 0.845 0.837289 0.9 62 0.856 0.846214 1.2 67 0.864 0.854845 1.1 Mass mg 72 0.873 0.863182 1.1 77 0.882 0.871322 1.2 82 0.890 0.879364 1.2 87 0.898 0.887113 1.2 97 0.913 0.902020 1.2 Identifying Common Problems in Specific Heat Determination 100 0.917 0.906532 1.2 Problem: Measured Specific Heat Data is Inaccurate 107 0.927 0.916144 1.2 117 0.941 0.929580 1.2 127 0.953 0.942330 1.1 137 0.966 0.954492 1.2 147 0.980 0.966065 1.4 157 0.992 0.977050 1.5 167 1.003 0.987544 1.6 177 1.014 0.997548 1.6 • Sample weights are not measured accurately enough 187 1.025 1.007062 1.8 • Platinum covers exchanged The data in the table is taken from Figure 7. Points to note are: • First minute of the data is inaccurate (in this instance equivalent to 20 °C) and should be discarded. This is related to heat-flow equilibration at the start of heating. Specific Heat = Heat Flow Mass x Heating Rate Units: Heat Flow Heating rate mW (=mJ/s) ˚C/second The specific heat is then given in J/g/°C. Likely causes: • Insufficient time for instrument stabilization after start-up • Cooling accessory not stable • More equilibration time needed at initial isothermal or ending isothermal • Instrument not in control at the starting temperature • Pan weights are not measured accurately enough (only important if different pans are used for different parts of the test) • Instrument has not been calibrated using two temperature standards • Data above 150 °C shows an increasing error. This demonstrates that the span used for the specific heat determination should be kept to no more than 150 °C to maximize accuracy. Problem: Initial Isothermal Drifting at Start of Test • Column 3 data is based on published information from the National Bureau of Standards in the U.S. in 1971. –Cooling system has not been fully stabilized, e.g. Intracooler not left for minimum 1.5 hours When used as an internal reference in the three curve method, sample data is corrected according to the values determined from the reference material test, reducing the error in the sample determination. Likely causes: • Instrument not fully stable at start of test • Program not sent to initial test temperature and held isothermal before test start –Manually go to start temperature, wait for stability and then start test • Purge gas concentration still changing after sample loading (helium or argon) –Increase waiting time after loading samples 8 Problem: End Isothermal of the Test Still Drifting When Test Ends Likely causes: • Ending isothermal wait is too short –Increase end isothermal time Reasons to Adjust Ro Ro will require adjusting if the pan type is changed from the standard aluminum sample pan to, for example, alumina or graphite, which have different thermal resistances. It may also be necessary to check Ro if working below -50 °C or above 400 °C with any pan type. • Sample is very large and requires more time to stabilize –Increase end isothermal time • Instrument not stable –Cooling system unstable or not fully stabilized Problem: Baselines Drift During Test Likely causes: • Sample movement in sample pan –Improve crimping and sample loading • Platinum covers not replaced flat –May need to be re-shaped using the reforming tool 0319-0030 • Cooling accessory not fully stable –Need to wait longer before using the analyzer There are of course many other possible reasons; however these are among the more common. Conclusion The specific heat of a material may be determined with reasonable accuracy if sufficient care is taken. The primary advantage of the DSC approach to measuring specific heat is the simplicity of the sample preparation, the speed of data collection and analysis. References 1.Measurement of Specific Heat Functions by Differential Scanning Calorimetry, M.J. O’Neill, Perkin-Elmer Corporation. Reprinted from Analytical Chemistry, Vol. 38, page 1331, September 1966. Appendix 1. Determination of Thermal Resistance Constant, Ro The thermal resistance constant, Ro, is a measure of the speed of heat transfer between the sample holder of the DSC and the sample itself. The speed of heat transfer is affected, among other things, by the flatness of the base of the sample pan, the thermal conductivity of the sample pan material, sample pan type and by the heating rate. Once determined, it is used in the specific heat calculation to correct the measured temperatures in relation to the height of the specific heat “peak”. It is important that Ro be measured accurately under the conditions that the sample is to be analyzed. Figure 8. Calculation of Ro from an Indium Curve run at 0.8 ˚C/min. To measure the thermal resistance constant, prepare a 3 mg piece of indium metal and flatten it on a sheet of glass before encapsulating it in a suitable DSC pan. The indium sample should then be analyzed at the same heating rate used for the specific heat determination. The plot in Figure 1 shows a Delta Y calculation performed on the leading edge of an indium melting peak. The Ro is calculated as follows: Ro = DT DY = (155.58 – 155.35) ˚C (19.36 – 7.37) mW = 0.23 ˚C 11.98 mW = 0.0192 ˚C/mW This value of 0.0192 °C/mWatt for Ro is shown in Figure 8 as a label. Typical values are between 0.02 °C/mWatt and 0.08 °C/mWatt but these values are for guidance and are very sample pan type dependent. Some software (Pyris for Microsoft® Windows® for example) requires that Ro is given in °C/Watt. To obtain this value, convert the Delta Y calculation from milliwatts to Watts by dividing by 1000. 9 PerkinElmer, Inc. 940 Winter Street Waltham, MA 02451 USA P: (800) 762-4000 or (+1) 203-925-4602 www.perkinelmer.com For a complete listing of our global offices, visit www.perkinelmer.com/ContactUs Copyright ©2012, PerkinElmer, Inc. All rights reserved. PerkinElmer® is a registered trademark of PerkinElmer, Inc. All other trademarks are the property of their respective owners. 010612_01