1、DEUTSCHE NORM June 1994 I I General principles of differential thermal analysis DIN 51 007 - ICs 71.040.40 Thermische Analyse (TA); Differenzthermoanalyse (DTA); Grundlagen In keepina with current practice in standards published by the International Organization for Standardization (/SO), a comma ha
2、s been used throughout as the decimal marke,: Contents 1 Scope and field of application . . 1 Page 2 Concepts 3 Principles of methods 4 Apparatus . 5 Procedures 6 Calibration 8 Specimen preparation 9 Preparation of equipment . 1 O Evaluation 11 . Test report . Standards and other documents referred
3、to Explanatory notes . 1 Scope and field of application This standard describes several methods for analyzing solid and liquid materials using differential thermal ana- lysis (DTA) or differential scanning calorimetry (DSC). The methods presented in this standard can be used to examine any physical
4、or chemical process involving thermal effects in order to determine transition tempera- tures or heats, reaction temperature ranges, heats of reaction, the purity of eutectic systems (eutectic purity, for short), specific heat capacities and reaction kinetics. Some of these measurements are performe
5、d under con- trolled atmospheres. DTA or DSC can also be used in conjunction with other measurements. For instance, DTA can be carried out in conjunction with thermogravimetric measurements to determine changes in mass as a function of temperature or time. The principles of thermogravimetric analysi
6、s (t.9.a.) are covered by DIN 51 006, the determination of melting temperatures by DIN 51 004. 2 Concepts See DIN 51 005 for general concepts regarding thermal analysis. 3 Principles of methods 3.1 Differential thermal analysis (DTA) and heat flux differential scanning calorimetry (DSC) A test speci
7、men and a reference specimen are sub- jected to a controlled temperature programme (e.9. heat- ing), and the temperature difference between the two Continued on pages 2 to 11. euth Verlag GmbH, Berlin, has the exclusive right of sale for German Standards (DIM-Normen). DIN 51 007 Engl. Price group 9
8、Sales No. O109 08.96 Page 2 DIN 51 007 Low range Middle range High range Extremely high range specimens is measured as a function of temperature or time. With DTA, this difference is used to determine char- acteristic temperatures; with heat flux DSC, the tempera- ture difference is used to determin
9、e calorific values. - 180 Room temperature Room temperature Room temperature 3.2 Power compensation DCC With this technique, the temperature difference between a test specimen and a reference material is determined by subjecting them to a controlled temperature pro- gramme (e.g. heating) while at th
10、e same time equalizing the temperature of both specimens. The energy input re- quired for this process is measured as a function of tem- perature. Characteristic temperatures and calorific values can be obtained by using this method. 4 Apparatus The following shall be used for DTA and heat flux DSC:
11、 an oven with specimen holders and a temperature re- cording system (e.g. differential thermal analyser or dif- ferential scanning calorimeter). The following shall be used for power-compensation DSC: a calorimeter as above, but with separate ovens, each having its own specimen holders, temperature
12、recording system and compensating device, as well as heaters and thermocouples which maintain the temperature pro- gramme and control the power input. DTA and DSC equipment shall also be capable of gener- ating and maintaining a controlled atmosphere when necessary. The temperature signal of calorim
13、eters shall have a noise not exceeding 10 yW r.m.s*) in low temperature ranges; in higher ranges, this level shall not exceed 1 O0 yW r.m.s.*). See table 1 for suggested temperature ranges. Table 1 : Temperature ranges Minimum temperature, in “C Temperature range Maximum temperature, in “C 700 1 O00
14、 1600 1600 and above 4.1 Ovens Ovens shall be capable of maintaining heating or cooling rates to the accuracies given in subclause 4.2. Rates of 1, 2, 5, 10, 20 or 50“C/min are recommended. Experi- ence has shown that high cooling rates can only be achieved in some temperature ranges. The temperatur
15、e field of the oven shall meet the follow- ing requirements: The vertical plane shall extend out from the surface of the specimen holder to a distance at least 1,5 times the holders length; temperatures along this plane shall not deviate from each other more than I5“C. The horizon- tal plane shall e
16、xtend out from the surface of the holder to a distance at least 1,5 times the holders diameter; temperatures along this plane shall be constant to within + 0,5 “C. NOTE: Changes to the temperature gradient dur- ing the temperature programme can lead to drifts in the baseline, poor resolution and/or
17、erroneous results. If a controlled atmosphere is required, ovens shall be ca- pable of maintaining the required atmospheric conditions throughout their entire temperature ranges. If a gas is to be introduced, this gas should have reached specimen temperature by the time it reaches the specimen. Gas
18、samples (e.g. for mass spectrometric or gas chro- matographic analysis) shall be taken in such a manner that the thermal measurement is not affected. 4.2 Temperature control The temperature control device shall be capable of maintaining a constant heating rate between 5 %/min and 50“C/min to an accu
19、racy of 10 %. During isothermal measurements, the temperature shall not deviate more than 0,5 % from the specified isothermal temperature. For temperatures from - 100 “C to +lo0 “C, the tempera- ture shall be accurate to within * 0,5 “C after calibration. The damping which occurs when switching to a
20、n isothermal programme shall not cause an increase in the preset temperature of more than 20 % of the numeri- cal value of the heating rate (in“C/min); the decay time of the damping shall not exceed five minutes. NOTE: During some measurements, the tempera- ture programme is controlled by the specim
21、en in such a way that a constant rate of reaction or of decomposition is obtained. 4.3 Equipment for producing a controlled atmosphere In some cases, controlled atmospheric conditions are re- quired during measurement. For instance, if the speci- men reacts with air, testing shall be carried out und
22、er an inert gas atmosphere. In this case, elements connecting the gas source and the specimen chamber shall be pro- vided with control valves and a flowmeter. For measurements during which the reaction of the specimen with a gas or gas mixture is to be examined, equipment for preparing the gas is re
23、quired. Elements connecting the gas source and the specimen chamber shall be provided with control valves and a flowmeter. If measurements are to be carried out at a pressure above or below atmospheric pressure, the test chamber must be air-tight and equipment for establishing and maintaining the de
24、sired pressure is necessary. Sealed, pressure-resistant holders (e.g. autoclaves or glass am- poules) shall be used when testing at a pressure lower than that above the specimen. The effect of the gas flow rate on thermal measurements should be kept to a minimum. Any gas introduced into the chamber
25、should have reached specimen tempera- ture by the time it reaches the specimen. 4.4 Recording device Devices for automatically recording differential tempera- tures or heat flows shall have a limit of error of no more than 0,5 %. To meet this requirement, analog devices *) r.m.s.: root mean square.
26、DIN 51 007 Page 3 / -_ Baseline type 1 Baseline type 2 -. .-. .- Extrapolated baselines -.-.- I Tp T, Temperature, T Time. t _t Ti Onset temperature Tf End temperature T, Extrapolated end temperature Te Extrapolated onset temperature Tp Peak melting temperature Figure 1 : Example of a melting (fusio
27、n) endotherm with two types of interpolated baselines *) must be of class 0,5 or better. The speed of the paper feed should correspond to the heating or cooling rates given in subclause 4.1. Digital devices should be capable of recording at least one result for every temperature change of 1 “C; for
28、isothermal measurements, a rate of one result per minute is sufficient. All devices shall be capable of re- cording temperature as well as time values. Heat flow recording devices shall be capable of record- ing at least three results per unit of time. All devices shall be capable of recording tempe
29、rature changes smaller than 1 “C. For DTA measurements, dif- ferential temperature sensitivity should be sufficient to provide readability of temperature changes smaller than 5 * “C. For DSC measurements, heat flow sensitiv- ity shall be greater than the signal noise level. When testing in conjuncti
30、on with thermogravic measure- ments, the recording device shall have sufficient sensi- tivity to meet the requirements in DIN 51 006. In order to limit the amount of data recorded, devices should be so designed that results are recorded only when necessary. 5 Procedures 5.1 First order phase transit
31、ions include polymorphic transi- tions (solid to solid), melting (or fusion) (solid to liquid), crystallization (liquid to solid), evaporation (liquid to gas) and sublimation (solid to gas). Investigating first order phase transitions *) See translators note on page 10. NOTE 1 : First order phase tr
32、ansitions are charac- terized by a sudden change in the phase transi- tion curve. Heating a specimen causes it to melt (or undergo a polymorphic transition), resulting in an endothermic peak in the recorded curve (see figure 1). Cooling a specimen causes it to crystallize (or undergo a polymorphic t
33、ransi- tion), producing an exothermic peak (see figure 2).*) In either case, the transition temperature is taken from the extrapolated onset temperature, Te, which is defined as the point of intersection of the extrapolated baseline at the beginning of the curve and either the inflectional tangent o
34、r the tangent of the greatest slope drawn along the leading edge of the peak. NOTE 2: A temperature obtained at a heating rate of 1 “C/min can deviate from a temperature extra- polated at a zero heating rate by anywhere be- tween 0,05 “C and 0,15 OC. l NOTE 3: When testing polymorphous materials, ov
35、erheating or supercooling can occur due to a lag phenomenon, which is in turn a result of a re- tardation in the rate of the formation of crystal nu- clei. In this case, the measured transition temper- ature does not reflect the temperature at equili- brium, but depends heavily on the thermal histor
36、y of the specimen and the heating or cooling rate used. NOTE 4: If the crystalline region, or nucleation site, expands slightly, the shape of the melting peak will be determined by the size of the area where crystallite is present. In such cases (e.g. when testing polymers), the transition tempera-
37、ture shall be derived from 1;. Page 4 DIN 51 007 Baseline type 1 Baseline type 2 _ -.-.- - . . - . . - Extrapolated baselines T; Te Tp Tc Tf -Temperature, T Time. t - Ti Onset temperature TI End temperature T, Extrapolated end temperature Te Extrapolated onset temperature Tp Peak crystallization tem
38、perature Figure 2: Example of a crystallization exotherm with two types of interpolated baselines t k I a _ Baseline type 1 Ti Onset temperature -.-.- Baseline type 2 T, End temperature -. .-. .- Extrapolated baseline T, Any temperature between Ti and Tf F Total peak area between Ti to Tf (with base
39、line type 1) F, Partial peak area between Ti to T, (with baseline type 1) s, Distance from baseline extrapolated at onset to baseline type 2 at T, sf Distance from baseline extrapolated at onset to DTA curve at Tf Figure 3: Constructing the baseline of an endothermic peak using tangents DIN 51 007 P
40、age 5 Baseline type 1 Baseline type 2 - -.-.- T, TP Tt Temperature, T Time, t Ti Onset temperature Tp Peak reaction temperature Tf End temperature Figure 4: Characteristic temperatures and constructed baselines for a chemical reaction exotherm Transition heats measured using a calorimeter are pro- p
41、ortional to the peak area (formed by the peak itself and an interpolated baseline), and are determined by inte- grating the peaks over time. There are two ways to construct an interpolated baseline (see figures 1 to 4): Baseline type 1: Draw a straight line joining the onset temperature and end temp
42、erature (linear interpolation). Baseline type 2: Construct a baseline using a point of inflection, whereby a point on the baseline will be deter- mined for every temperature, Tn, between Ti and T, (see figure 3). The distance of the interpolated baseline from the baseline extrapolated at the onset,
43、s, should thus fulfil the condition s, = F, sf/E The peak area, E and the partial peak area, Fn, are then determined with the help of the interpolated baseline. Baseline type 1 should be used for phase transitions of pure substances, type 2 for kinetically-dominated phase transitions. 5.2 Chemical r
44、eactions Chemical reactions of specimens subjected to a temper- ature programme are graphically depicted by a charac- teristic peak (see figure 4). Such reactions may also in- volve unwanted decomposition. During dynamic measurements, the temperature at the onset of the chemical reaction cannot be d
45、etermined, due to the fact that reactions (albeit very slow ones) take place even at low temperatures (cf. the Arrhenius equa- tion, which states that the rate of a chemical reaction varies with the temperature). This phenomenon also ap- plies to the determination of the onset time during isothermal
46、 measurements. The temperature at the first noticeable deviation of the curve from the baseline, Ti, represents the onset tem- perature of the reaction or decomposition. This onset temperature is a function of the heating (or cooling) rate, the initial mass of the specimen, any special atmos- pheric
47、 conditions present, the characteristics of the in- strument, and the characteristics, concentration(s) and thermal history of the specimens. Thus, the onset tem- perature is only representative when parallel tests have been carried out under identical conditions. In other ap- plications, factors su
48、ch as manufacturing or storage con- ditions should be taken into consideration. The methods described in subclause 5.1 can also be used to determine heats of reaction, whereby baseline type 2 should be used. 5.3 Glass transition temperatures Glass transition is the reversible transition in an amor-
49、phous material (or in amorphous regions of a partially crystalline material) from a hard, brittle state to a viscous or rubbery one, and vice versa. Glass transition occurs over a broad temperature range (e.g. 20 to 30“C), and involves a rise in the specific heat capacity. A glass transition effected by heating produces an endothermic DSC curve (figure 5). If an amorphous or partially crystalline substance is stored at a temperature directly below the glass transi- tion temperature or is stored for a long period at a tem- perature far below the glass transition temperature
