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Chemical Instrumentation
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Edited by GALEN W. EWING, Seton Hall University, So. Orange, N. J. 07079
These articles are intended to sene the readers O ~ T H I JOURNAL S by calling attention to new developments i n the theory, design, or availability of chemical laboratory instrumentation, or by presenting useful insights and ezplanations of topics that are of practical importance to those who use, or teach the use of, modem instrumentation and instrumental techniques. The editor invites correspondence from prospective contributors.
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LXVII. Thermal Analysis Techniques: W. W. Wendlandt, Department o f Chemistry, University of Houston, Houston, Texas 77004
SIMULTANEOUS TG-DTA INSTRUMENTS There is a time-saving convenience in recording simultmeously the TG and DTA curves of the sample under identical conditions of furnace heating rate and other pyrolysis conditions. Commercial instruments are mailable which will permit these measurements to be made under a wide range of experimental conditions. Not only can TG-DTA measurements be made hut other parameters such as differential thermogravimetry (DTG), gss evolution detection (EGD), gas pressure (thermommometry), electrical conductivity, dilatometry, and so on, as well. Only instruments capable of the simultaneous recording of T G D T A curves will be discussed here.
I . Hungarian Optical Works; Derivatograph The Derivatogaph is a. multift~nction thermal analysis system which can record the TG, DTG, DTA, and T curves of a sample on n single chart. By means of an accessory attachment, the T D (thermal dilation) and DTD (derivat,ive of T D ) curves can also he recorded. Evolved gas analysis may be carried out under certain conditions. The instrument, as shown in Figure 23, consists of an analytical balance, two furnaces, a hunace temperature programmer, sample and reierence crucibles, voltage regulator, and a galvanometric light beam-photographic paper recorder. The balance is nn air-damped analytical type wit,ll a basic sensitivity of 20 i- 0.2 mg full-scale deflection with n working mass
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range of 10 mg to 10 g. The derivative of the TG curve is obtained by means of a simple device consisting of a magnet and induction roil. ~ The ~ former~is susoended . . ~ ~ ~ on one arm of the balance beam with hoth of its poles surrounded by two indoction coils. When a change of mass occurs, movement of the magnet induces a voltage in the coils whichis proportions1 to the rate of mass chance. This induced coil voltarre is memured by one of the light beam ga-Ivanometers and recolded on the chart a5 one of the curves. Maximnm temperature of the furnace is llOOaC; N., COS, Ar, 02,etc., may be used as the furnace atmosphere at atmospherio pressure only ~
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Figure 2 3 . The Derivotograph: Id schematic, Ib) photograph. 1, Crucible for the romple; 2 , Crucible far the inert substonce; 3 , 4, porceloin tube; 5, electric furnace; 6, 7, thermo~ouples; 8, balance; 9, optic01 slit; 10, lompr; 1 1 , coil; 1 2 , permanent magnets; 1 3 , 1 4 , 1 5 , gakanometen; 1 6 , photographic paper.
2. Mettler Thermoonolyzer The Mettler Thermoanalyser is the most elaborate, versatile and expensive of all the TG-DTA systems commercially available. I t is a. universal research instrument which will simultaneously plot on a single chart the TG, DTG, DTA, the pressure, and the gm flow velocity, if so desired. The instrument is shown in Figure 24. The balance consists of an a111minum beam substitution type with sapphire kniie edges and plan& Change t,he sample mass tames a beam deflection which moves s. light shutter interrupting a light beam disphy on two photodiodes. The unbalance in photadiode current is zmplified and fed back t o a coil attached to the balance beam as a restoring force. Tho electrical weight indication hss a d ~ t a l weighing range with three different sensitivities as standerd wibh a. fourth as optional. Two con~.ecotive sensitivities, in the ratio of 1:10, are always recorded.
One range is 0-1000 mg, recorded as 100 mg/in.; the i.econd is also 0-1000 mg hut recorded as 100-mg full-scale deflection, or 10 mg/in. A more sensitive mass range of 0-100 mg is recorded in a similar manner. (Continued on page A8641
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ChernjCQ/lnstmrnentQtion
3. MeMer Thermoanalyzer 2 The Mettler Thermoanalyaer 2, a5 shown in Fieure 25. is a modified version of t,he Mettler Thermoanalyaer previously discussed. Although mare compact in size, it offers the same advantages of simultaneous TGUTA-UTG on t,he same sample. The weighing system is housed in a stainless-steel balance chamber. By using an inert carrier gas, it is possible t o work with corrosive gases without damage t o the weighing system. The temperature programmer has 26 heating and cooling rates from 1 to 100mC/min. Maximum
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Mettler Thermaonalyzei 2 furnace.
A unique design feature is the gas flow C O , , ~ ~ O I system. Corrosive noncorrosive gases may be emplo,ved with provisions in the ease of the former to keep i t from coming in contact with the balance mechanism. Two vacuum and diffusion pumps me employed t o evacuate the furnace chamber to pressures of the order torr. of 5 X lowThree furnaces are itvailahle-a i.empernture unit with a. range up t o 1050°C; a high-temperature unit for use up t o 1600°C; and a super-high-temperature model with a maximum temperat,ure of 2400°C. Numerous sample holders may be obtained, depending upon the application desired.
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temperature of the furnace is 1000°C. This system uses the same type of sample holders as the other instnlment.
4. Rig&
Denki
The Differential Thermal and Thermogravimetric Analyzer will record simultaneously the T G D T A curves for a s m ple. The insbniment uses a suspensiontype electric balance with no knife edges or holder. Mass changes of l0pg in a 500 mg sample can he detected. The DTA part of the system is the same as that previously described.
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Part II. Differential Thermal An,alysis and Differential Scanning Calorimetry DIFFERENTIAL THERMAL ANALYSIS-INTRODUCTION
The technique of dilierential thermal analysis consists of the measurement of the temperature of a sample, compared t o a. thermally inert reference materid, a5 both are heat,ed or cooled in a furnace. The measorement is a differential one in that tthe differencebetween the sample and reference materials, as phown in Figure 26, is detected and recorded. Thetemperature difIerence, T , - T,, where T , is the
sample temperature and T, is the reference temperature, is referred t o as AT; this parameter is recorded either a5 a function of time, T., T,, or furnace temperature, T,. Since the AT signal is of the order of microvolts (for a Chromel-Ahlmel thermacouple, 4OPV.deg-'), it must be amplified, using a goad quality dc amplifier, t o the millivolt level and then recorded on either a strip-chart potentiometric recorder or a. X-Y recorder. Both types of recorders
(Continued on page A626)
Chemical Instrumentation are in general use although the latter aonears t o b e the mast nonulax &t the present time on commercial instrumentation. The DTA technique is employed t o detect thermal transitions in which changes in enthalpy or entropy (secondorder transitions) occur. The most common enthalpy changes observed are: (a) phase transitions such as fusion, vasolidp, and so porization, boiling, solid, on; ( b ) chemical reactions of vsrious kinds; and ( c ) oxidation-reduction rescbions. I n the case of second-order transitions, the most common is the glass transition observed in polymeric materials. Wide use is made of DTA for the q~ialitative and qusntitative analysis of chemical systems; these applications, however, will not be discussed here since they have been adequately described elsewhere (15, 15, 2'3-38).
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DTA lnsfrumsnfation A schematic diagram of a typical DTA apparatus is illnstrated in Figure 27. The apparatm consists of sample and reference holders, fnrnare, two temperature sensors ( ~ ~ s u a l lthermocouples), y microvolt dc amplifier, furnace temperature programmer, furnace atmosphere control, and recorder. There is great variation in the type of sample and reference holders
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operations, photodiodes have been employed as temperatore sensors hnt they have not been mcorporated into commercis1 instruments. There are many diflerent types of furnaces available in DTA instruments. I t appears that the most common is the Kanthal wire heater element wound on a ceramic tube core which can attain a. maximum temperature of about 1250°C. To reach higher temperatures, platinum, rhodium, silicon carbide, or tungsten heating elements are used. Several low-temperature furnaces have been described which use stainless-steel heater cartridges, with a Nichrome wire heater element. The furnaces may be mounted vertically or horizontally. The amount of thermal insulation and the physical size determine
Figure 26. Differentid temperature measure. mentor wed in DTA.
employed; these may consist of cylindrical cavities in metal or ceramic blocks, met,al crucibles of diflerent geometrical configurations, or glass, silica, or ceramic tubes, and so on. They are in physical contact, with a temperature sensor which is generally a thermocouple although r e sistance thermometers, thermistors, thermopiles, and thermoelectric discs m e also used. The most commou types of thermacouples are those constructed of ChromelAlumel, Platinel-I1 alloy, or platinumrhodium alloys. For high-temperature
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Figure 27. parotur.
recorder
Sehemotic diagram of a DTA op-
(Continued on page A628)
Chemical Instrumentation t,he cooling period of the furnace. This is an important factor if multiple runs are t o he made during the average working day. Small, quick-cool h m n c c s are sametimes very desirable for certain types of applications. To control the temperature rise of the furnnces, many different temperature programmers have been described, ranging from simple motor.advanced variable voltage transformers t o thermocouple actuated feedback types ( 1 4 ) . Heat,ing or cooling rates are generally from lo t o 2 0 T / m i n ; the most eommon is lO"C/min. The amplifier is a low-noise, stable dc microvolt type (if thermocouples are used). Outpnt from t h e amplifier is recorded on a two-channel styin-chart no-
tion of time. A onechamel recorder may be osed for the AT signal with the temper* ture signal multiplexed on it, a t 25", 50', or 100°C intervals. Using a X-Y recorder, the AT signal is plot,ted directly 8s a Rmction of T,, T,, or T,, as mentioned earlier. For calorimetric work, the stripchart recorder is prderred, while for greater temperature accuracy, the X-Y recorder isrecommended. Both types of recording systems are generally available from the manufacturers. The IITA curve is very dependent on the type of furnace atmosphere present and hence, for reproducible results, the at,mosphere must he rigoro~islycontrolled. Furnace atmospheres may be static or dynamic, i.e., stationary or flowing. Gazes present are usually Ns,He, Ar, Oz: COz,etc. Some hlrnnces have p~.ovisions for maintaining an atmosphere of n corrosive gas or vapors of a low-boiling organic liquid. I t is also desirable t o be able t o control the ga. pressure in the range from l W 5 torr to200 atm.
Commercial DTA Instruments
A large number of DTA instruments are available commercially. Many manufactureis have camp1et.e modular thermal analysis systems which generally inchide DTA, thermogravimetry, evolved gas detection, dilabometry, differential scan-
Lt.'L , @-..~ . 0 28. parot"% Figure
A.D.A.M.E.L.
ATD-67
DTA
(Conlinued on page A630)
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Chemical Instrumentation ning calorimetry, and so on. The DTA module is normally the first one that is obtained with the others being added a t a later date. There is a wide choice of DTA modules, each differing in the temperature range, sample holders, type of furnace atmosphere control, and so on. I t should be noted that high-temperature svstems are somewhat more ex~ensive rhnn the lower trn,perature r a l w ~nrrrlrls, er n1.d or due to rlte rypc of I ~ c a ~ windings design.
I . A.D.A.M.E.L.; ATD-67 Differential Thermal Analyzer The ATD-67 Differential Thermal Analyzer, manufactured by A.D:A.M.E.L., consists of three units, as illustrated in Figure 28. The center unit contains the
recorder, two amplifiers (one for measuring the furnace or sample temperature, and the other for the differential temperature), and part of the furnace temperature prw g r a m e r . Located a t either side of the central unit are the two furnace and sample holder assemblies. These may be of two types: (a) the Model 1, cqmble of operation in an air or inert stmosphere; and (b) the Model 2, used for static controlled atmospheres or a t reduced pressures. Maximum temperatures are: Model 1, 1 0 5 0 T ; Model 2, 1200DC. The convenience of two furnaces and sample holders is utilized in that while one furnace is cooling, the other may he in the heating cycle. A somewhat more sophisticated apparatus is also available, the Model ATD63, in which a more elshorl~terecording system, sample holders, and furnaces are employed.
Figure 29.
Aminco Thermoandyrer.
2. AMINCO Thermoanalyzer The Aminco Thermoandyzer (Figure 29) consists of five basic components: (1) , , s a m ~ l eholder and thermocou~leassemhly; (2) furnace assembly; (3) flow control system; (4) differential temper* ture amplifier; and (5)furnace power programmer. The furnace and sample holder design are based on that previously described by Lodding and Hammel (16) in 1959; maximum temperature limits are 1000" or 1500°C. A dynamic atmosphere can be maintained in the sample and reference cavities, in that a gas stream moves upward through the furnace chamher and into the sample and reference cavities. The gas then passes through the sample and reference materials, down the bores of the ceramic support tubes, and emerges from the two outlets a t the hottom of the furnace assembly. These outlets feed to separate flow-meters and needle valves for the two gss streams.
3. Micro Differential Thermal Analyzer; Bureou de Liaison The Model M-4 DTA apparatus m5y he used t o study samples with amass range of several pg to 50 mg. Three interchangeable sample holders are provided: (a) microholder of 0 . 6 ~ ~volume; 1 ( b ) semimicro holder of 6-PI volume; and ( c ) semimicro holder with a. capacity of 25 pl. Each holder is fitted with three thermocouples, two for AT and one for T detection. The sample and reference containers are small platinum cups to which the thermocouple wires are welded.
. Figure 30.
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The Stone DTA system.
(Continued on page A6331
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Chemical Instrumentation The standard furnace, Model F-10, is Kanthal wire wound and has a maximum t,emperature of 1200°C. Other iurnnces inclnde high-temperatme (1600°C) and low-temperature (-160' to 3RODC)units. The sample holder chamber can be flushed with n dynamic gas atmosphere and may be used a t pressures to 5 X 10-s tow.
4. Columbia Scientific Insfrumenis A. Stone Thermal Instruments The Stone DTA system, as ilhistrated in Figure 30, employs a modolar approach to instnimentation. Components of the syst,em include s recorder-rontroller module, a furnace platform, fnmare, and
a sample holder. Several furnaces are available while mmerous sample holders can bc selected. A unique feature of the Stone instruments is the dynamic gw atmosphere control. Nol only can permanent bype gases be used but steam and law-boiling ol.gnnic compounds as well. Three ditferent types of sample holders can be chosen, differing msjnly in their sample container configuration. The ?ample may be contained in R. cylindrical cavity in a metal block, n shitllow dish resting on a ring thermocouple, or n cylindrical cup or crucible. These three types are ilhistrnted in Figure 31. In (A), the sample is placed in a cavit,y in a metal or ceramic block in direct contact with the thermocouple. I t features f d l dynamic gas flow and is used for soil, mineral, and ot,her samples which do not fuse or $inter on hest,ing. The other
LOFFERENTIAL TC REFERENCE TC
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Figure 31. Stone DTA sample holder(01 block type, (b) ring thermocouple type, (cl cylindrical sup type.
holders, ( B ) and (C), employ metal containers to retain the sample. The highlysensitive ring-thermocouple designs may he used for samples varying in mass from 0.1 t o 20 mg. Larger ~amples,from 20 t o 200 mg, may he atndied in the leas sensitive cylindrical cup type. Sample holders are designcd for operation t o maximum for,race temperatures of 1100' or 1600°C. A simolt,aneous DTA-EGD (effluent gas detection) accessory is also available for the furnacesample holder module. (Continued on page A654)
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5. Deltatherm (Technical Equipment C ~ r ~ o r o f i o nDeltotherm ); I1 ond Delfofherm 111
Chemicul Instrumentation Evolved gases are detected by an exterior mounted thermal conductivity cell. Also available is a hieh-oressure DTA samole holder for use u
