Reference holder

Experimental Methods Measurements of specific heat and enthalpies of transition are now usually carried out on quite small samples in a Differential scanning calorimeter (DSC). DSC is applied to two different moles of analysis, of these the one is more closely related to traditional calorimetry and is described here. In DSC an average-temperature circuit measures and controls the temperature of sample and reference holders to conform to a Organisation and Qualities... 

The power compensation DSC instrument was first described by Watson et al.3) and by O Neill4) and it was developed into a commercial instrument by the Perkin-Elmer Corporation. It utilises separate sample and reference holders of low thermal mass, with individual heaters and platinum thermometers, as shown schematically in Fig. 1. In addidion to controlling the average temperature the instrument employs a... 

The procedure for obtaining a UV-VIS spectrum begins with the preparation of a solution of the species under study. A standard solution should be prepared in an appropriate solvent. An aliquot of the solution is transferred to a cuvette and placed in the sample chamber of a spectrophotometer. A cuvette containing solvent is placed in the reference holder. The spectrum is scanned over the desired wavelength range and an absorption coefficient is calculated for each major A. ... 

Hydrate Isobutane Reference Holder and Godbole (1982) Phases I-H-V ... 

Hydrate Methane + ethane Reference Holder and Grigoriou (1980) Phases L y-H-V... 

Hydrate Ethane + propane Reference Holder and Hand (1982)... 

Hydrate Natural gases, hexane, decane, and crude oils Reference Holder (1976)... 

PC instruments are preferable to HF instruments for isothermal studies, and in studies in which the temperature scanning rate is high, because the very small sample and reference holders in the PC type have much smaller thermal inertia than the relatively large heating block in the furnace of the HF type (Hatakeyama and Quinn, 1994). 

Three kinds of sample holders are available for DTA and DSC (Fig. 4.8.6). Type 1 holders are commonly used for a DTA apparatus. In this type, the sample and reference holders are placed on the same metal block and heated by the same heater the temperature difference between the two holders is indicated by a thermocouple. Type 2 holders are generally used in a quantitative DTA (heat-flux DSC) apparatus. Both sample and reference holders are maintained at the same temperature by two individual internal heaters, which, in turn, are heated by the same main heater. The temperature difference between the sample and the reference material is measured by a thermocouple placed outside of the holders. Type 3 holders are customarily used for a power-compensated DSC apparatus. This apparatus has separate heaters for heating the sample and reference holders thus maintaining the sample and the reference... 

Fig. 4.8.6. Arrangements of sample holder (5) and reference holder (R) in the heater of a DTA or DSC apparatus 1 DTA 2 heat-flux DSC 3 power-compensated DSC...

Adjust the sensitivity, heating rate, baseline, and other parameters of the DSC apparatus. Place the empty sample pans in the sample and reference holders. The initial and end temperatures (T, and Tc) (Fig. 4.8.13) for scanning are set and the sample and reference holders are maintained at T, for a suitable time to obtain the straight line I. Scanning at a heating rate of 5 or lOK/min is conducted from T, to Tc to obtain DSC curve II. The sample and reference holders are maintained at Te for some time to obtain the straight line, III. 

The reference material (e.g., sapphire or A1203) is placed in the reference pan which is placed in the reference holder, and scanning from T, to Tc is carried out to obtain curve IV. 

In thermogravimetry (TG or TGA) the change in sample mass is determined as a function of temperature and/or time. The instrument is a thermobalance that permits the continuous weighing of a sample as a function of time. The sample holder and a reference holder are bounded to each side of a microbalance. The sample holder is in a furnace, without direct contact with the sample, the temperature of which is controlled by a temperature programer. The balance part is maintained at a constant temperature. The instrument is able to record the mass loss or gain of the sample as a function of temperature and time [m = /( )]. Most instruments also record the DTG curve, which is the rate of the mass change dm/dt = f(T). 

A typical light-initiated polymerization Involved weighing 7-9 mg of acrylate with initiator into the standard DSC aluminum cup. The cup was placed in the DSC sample holder and an empty aluminum cup was placed in the reference holder. The usual nitrogen flow within the cell enclosure was established with the UV lamp, filter, and shutter assembly positioned over the holders. 

Figure 1. Schematic of modified DSC-IB apparatus (A) sample holder, (B) reference holder, (C) sample holder cover with quartz wirvdows, readily moved superstructure with (D) neutral density filters, (E) guillotine shutter, (F) circulating water heat filter, and (G)G.E. H3T7 medium pressure mercury lamp in a reflector enclosure...

A DTA instrument is designed to measures temperature differences between sample and reference as illustrated in Figure 10.2. A sample and reference are symmetrically placed in a furnace. The temperature difference between the sample and reference are measured by two thermocouples one is in contact with the underside of the sample holder (also called the crucible), the other is in contact with the underside of the reference holder. The reference should be made from a material that satisfies the following conditions it does not undergo thermal events over the operation temperature range, does not react with any component in the instrument, and has similar thermal conductivity and heat capacity to the sample being examined. 

Figure 10.4 Differential scanning calorimetry (DSC) instrumentation design (a) heat flux DSC and (b) power compensation DSC. A, furnace B, separate heaters and C, sample and reference holders. (Reproduced with permission from E.L. Charsley and S.B. Warrington, Thermal Analysis Techniques and Applications, Royal Society of Chemistry, Cambridge, UK. 1992 Royal Society of Chemistry.)...

DTA is usually operated in a high temperature range. Platinum and gold crucibles are commonly used as sample and reference holders. DSC is usually operated in a low temperature range (<500 °C), and thus aluminum pans are commonly used as sample and reference holders. The pans often need to be sealed to avoid sample mass change due to evaporation. A special press can be used to mechanically weld a lid and a pan together. 

Empty cells are placed in the sample and reference holders. An isothermal baseline is recorded at the lower temperature, and the temperature is then programmed to increase over a range. An isothermal baseline is then recorded at the higher temperature as indicated in the lower part of Figure 4.5. The two baselines are used to interpolate a baseline over the scanning section, as shown in the upper part of Figure 4.5. The procedure is repeated with a known amount of sample in the sample cell, and a d/Z/dT versus time trace is recorded. The deviation from the baseline is due to the absorption of heat by the sample. We may then write... 

A simple sealed-tube sample holder is shown in Figure 6.2(g). Other sealed-tube sample and reference holders are shown in Figure 6.16 (59). In one case, (a), the thermocouples were sealed directly into the tubes, and this presented problems with metal-to-glass seals. Heat transfer from the sample to the sensing thermocouple is fairly low in most of the examples given. The system in (e) is much better than most of the others because the... 

Figure 6.23. High-pressure DTA system described by Williams and Wendlandt (131. l ) Schematic diagram of apparatus. A. high-pressure DTA cell B. T T Controls Company Model No. TPC-2000 temperature programmer C Du Pont Model 900 recording module D, relief valve E, valve F. pressure gauge G. gas pressure regulator H. gas cylinder, b) Schematic diagram of DTA cell. A. furnace chamber B. high-pressure connectors for furnace wires and thermocouples. C. furnace D. DTA sample and reference holders E. gas outlet tube F, Buna-N O-ring G. base plate H. Conax connector for thermocouple wires 1. gas inlet-outlet connector.

A comparison (24) of the sample holders used in the DSC-1, DSC-lB, and DSC-2 instruments is shown in Figure 6.37. In the DSC-1 cell, the sample and reference holder consisted of a stainless-steel cup and support, a platinum-wire sensor, an etched Nichrome heater, and other thermal parts. All these components were mechanically crimped together in a very tight sandwich. This sample holder operated well over the temperature range —125 to 500°C. In the DSC-2 sample holder, the materials of construction used are a platinum-iridium alloy for the body and structured members of the holder, a platinum wire for both the heater and sensor, and x-alumina for electrical insulation. All parts of the holder are spot-welded together. 

The instrument contains two control loops. one for the average-temperature control and the other for the differential-temperature control. In the former, a programmer provides an electricai output signal proportional to the desired temperaiure of the sample and reference holders. The pro-... 

CZ) electrode connection D ceramic tube 5) adjustable screw <) Pt-wire electrode 15) sample or reference holder. 1 cylindrical sample or reference chamber... 

The reference holder is the container or support for the reference material. 

The power-compensating DSC has two nearly identical (in terms of heat losses) measuring cells, one for the sample and one reference holder. Both cells are heated with separate heaters, their temperatures are measured with separate sensors. The temperature of both cells can be linearly varied as a function of time being controlled by an average-temperature control loop. A second-differential-control loop adjusts the power input as soon as a temperature difference starts to occur due to some exothermic or endothermic process in the sample. The differential power signal is recorded as a function of the actual sample temperature. 

---