dsc测量比热容

Abstract1.Introduction2.Materials3.Methodology4.Results5.Conlusions abstract This study presents a methodology to determine (Cp) of materials for thermal energy storage (TES) by DSC.Three different DSC methods are considered to be applied in the methodology, and are explained and compared in this study in order to select the most proper one for Cp determination. To perform this study, the Cp of three materials commonly used in sensible TES systems, slate, water, and potassium nitrate (KNO3), is determined. 一、introductionllack of fossil energetic resourceslits pricelneed to decrease CO2 emissionslead to dedevelop new energy systems able to take profit of renewable energy and be environmentally friendly and less expensiveTES systems are a possible solutionSolar energy abundant clean and easy accessible.problem:as the hours of maximum energy demand match with the hours of no solar irradiationwith the aim to fill this existing energy supply gap, TES systems are presented as the solution to store the energy and use it anytimeTES technologieslsensible heat storage the process by which the heat is accumulated due to the increase of the material temperature without experimenting structural changes, no phase changellatent heat storagephase change , thermochemical energymaterial selectionA high storage capacity Therefore,need a high specific heat capacity.dTTfTiTmCPQ(1) 定义：在定义：在程序控制温度程序控制温度下，测量下，测量物物质与参比物之间的能量差质与参比物之间的能量差随随温度变温度变化关系化关系的一种技术。的一种技术。 DSC曲线峰包围的面积正比于热焓曲线峰包围的面积正比于热焓的变化的变化 可以测定多种热力学和动力学参数可以测定多种热力学和动力学参数，例如比热容、反应热、转变热、，例如比热容、反应热、转变热、相图、反应速率、结晶速率、高聚相图、反应速率、结晶速率、高聚物结晶度、样品纯度等。该法使用物结晶度、样品纯度等。该法使用温度范围宽（温度范围宽（-175725）、分辨）、分辨率高、试样用量少。适用于无机物率高、试样用量少。适用于无机物、有机化合物及药物分析。、有机化合物及药物分析。差示扫描量热法差示扫描量热法（DSC） （Differential Scanning Calorimetry） 采用试样和一标准物质在相同条件下进行扫描，然后量出二者的纵坐标进行计算。要求标准物质在所测温度范围内没有发生物理化学变化，且比热容已知。常用的标准物质是蓝宝石。具体作法是在DSC内先用两个空样品皿，以一定的升温速度作一条基线，然后放入蓝宝石样品在相同条件下作一条DSC曲线，最后在同样条件下作未知试样的DSC曲线，如图three DSC methods for Cp measurementlThe dynamic method has traditionally been used to measure PCM latent heat but also for Cp measurements applying high heating rates in the temperature range of study lThe areas method is based on the Cp value, thus, how much heat flux is needed to heat up a material for temperature increase, and consists of consecutive isothermal stages differing 1 with no heating segments amidlThe isostep method is an intermediate between these two, as it is a succession of dynamic methods applied to heat up the material just 1 between isothermal segments. This method has been tested in glass transition pharmaceutical studies with good results obtained , but it has not been used for TES materials Cp determination, thereby its selection for this studyTo ensure the performance of the methodology in a representative variety of material phase forms and chemical structures, three different materials widely used in sensible heat storage systems have been chosen to perform this study.lWater. Its thermal properties are well known, and therefore, these values can be used as a reference to see the approach on the measurements each method has. Commercial Bi-distillated water from Panreac has been used to perform the experimentslSlate. It is a widely used construction material and its performance depends on its specific heat capacity, therefore authors find it an interesting material to be tested. The slate samples used in this study were taken from a quarry in the Catalan Pyrenees.lPotassium nitrate. It is an inorganic salt mainly used as molten salt in concentrated solar power plants (CSP Plants). It has high melting point (320) and, as all inorganic salts, a complex chemical structure, and that is why it is also found to be useful for this papers purpose.It is important to remark that this material selection responds to the need to test the methodology and not to tie it to a concrete material type or a specific TES application. Therefore, materials used in different TES systems and with which authors commonly work have been selected.determined by differential scanning calorimetry (DSC)Standard 40 uL aluminium crucibles weigh with a analytical balance with a precision of 0.01 mg.The procedure to determine the specific heat capacity of a material consists of three different measurements, all done under the same conditions, thus, using the same DSC method:Blank measurement. It is necessary to run an experiment with an empty crucible to measure the heat flux that corresponds to the crucible material in order to subtract this signal and consider only the sapphire and material sample ones.Sapphire measurement. Sapphire is the material used as reference as its specific heat capacity is well known at every temperature and its signal is hugely stable over temperature, data necessary for the materials Cp calculation.Material measurement. Needed to determine the Cp value of the material under study.三、MethodologyPCmQmWK/S(2)The sensitivity of the analysis is tied to the heating rate and the sample mass. literature studies found high heating rates of about 1020 K/min are desiredEq. (2) shows, minor sample masses are needed.The Cp was measured between 15 and 16 , 2526 and 35 36 for all the materials under study. These measurement ranges were selected in order to ensure that water was in liquid state and to be able to take the certain known water 4.18 J/gCp value at 25 as a reference to evaluate each method performance.3.1. DSC measurement methods3.1.1.Dynamic methodIt is a temperature controlled method that achieves the thermal equilibrium before and after a controlled heating segment ,it consists of three segments.1015 min to homogenize the material temperaturehigh heating rateanother isothermal stage again for 1015 minthe lower and higher temperatures of the method can be 150 smspsmQmmCQCpm（3）Similar to the previous explained dynamic method, the isostep methodThis method uses Eq. (3) to calculate the Cp of the material along each heating segment.3.1.2.isostep method3.1.3.Areas methodThe areas method consists of consecutive isothermal segments with no heating stages amid.Each of the peaks corresponds to every temperature step of the method. The temperature difference between isotherms is of 1 .psssCmQAspmmmCmQAmsmpspmAACCwhere As J/g is the integrated peak area for the sapphire curveIn this study, results are given with an error of 3%, including both equipment and calculation errors.The dynamic method gives a continuous signal along the whole temperature study range, as shown in redThree different continuous signals are obtained with the isostep method, one for each heating segment for the 1516 C, 25 26 C and 3536 C steps. However, as the measurement temperature range is of just 1 C and the material is heated at a really fast rate,四、resultsresultsthe three signals are really steep and no clear tendency can be seen in the results, fact that already shows the low sensitivity this method has for Cp calculation. Nevertheless, and in order to compare the methods, an average value of the most constant parts of these three signals is presented in the graphics. Contrary, the areas method provides just three points, one for each temperature increment between the isothermal segments, hence, one at 16 C, one at 26 C and one at 36 C.154.1waterareas method gets closer values to the theoreticai valuesThe dynamic method inconsistent values are found in the begining and the end,that indicates sensitivity problems in these two measurement parts.the isostep method are the average of the most constant parts of the obtained signal,being the standard deviation for water of 0.08 J/g .The sensitivity problems. Due to the abrupt temperature change on both initial and end points, the DSC temperature sensor cannot react fast enough to read the real temperature.This fact has greater significance in the isostep method results due to the higher relation between heating rate and temperature increase. results show that the areas method is the method that provides Cp close to the theoretical rangedynamic methodthe isostep method, with a standard deviation of 0.03 J/g .most inconsistent4.2.slate4.3. potassium nitratesame conditionsame reason both dynamic and isostep method measurements are limited by the high heating rates applied, that low down the sensitivity of the analyses and increase the error of the results. comparing the three material results analysis, at this point it is clear that the areas method is, by far, the best method applied in this study to determine the specific heat capacity of a material.4.4. results discussion and summaryno specific value for the slate used was found in the literature五、conclusions A new methodology for specific heat capacity determination of materials for TES systems by DSC is proposed in this paper. It has clearly been stated in both dynamic and isostep method results that the DSC instrument requires of certain stabilization time after switching the scanning at high rates, obtaining wrong measures during the first 12 min of experiment. The isostep method has been found as a useless method for Cp determination of materials for TES systems The areas method has been found as the method with best performance of the three tested. It presents really low relative errors (3%) with respect to the theoretical Cp of the materialsthanks