Differential thermal output

In differential thermal aned rsls, l.e.- DTA, we use one thermocouple Taucked" against the voltage output of another of the same composition to produce a "net" EMF. What this means Is that either the positive (or negative) legs of both thermocouples are electrically connected so that the net EMF at any given temperature of the two Is zero. Only If one thermocouple temperature differs from that of the other does one obtain an EMF response. 

Figure 9.5 shows the differential thermal analysis (DTA) output from the ceramic. The endotherms confirm formation of newberyite and liinebergite. Liinebergite is formed by the reaction... 

Differential thermal analysis output of MgKP04 6H20 ceramic with boric acid. 

The differential scanning calorimeter evolved from an older instrument known as a differential thermal analyzer, or DTA. The DTA, which is based on the work of Le Chatelier in 1887, was developed in 1899 for identification of specific types of clays, which are difficult to differentiate by more traditional methods. The concept of the DTA is quite simple. A differential thermocouple, which consists of two otherwise identical thermocouples connected in opposing polarities, is placed in a furnace in a position which allows the bead of one thermocouple to be inserted into an inert reference material, while the bead of the other thermocouple is inserted into the sample. The difference in temperature between the reference and sample materials is obtained directly as a function of temperature as the entire assembly is heated at a controlled, usually linear, rate. In the absence of any thermal difference between the sample and reference material, the output of the differential thermocouple will be zero. When a thermal event occurs, c.g., heat released during crystallization, the change in specific heat at the glass... 

The reflector is gradually lifted up to compensate for reactivity change due to fuel bum-up. Regular power operation is attained at a constant speed that is controlled according to the reflector differential reactivity worth. With no other reactivity control systems being used, the reactor thermal output drifts only by several percents during operation. 

The thermal decomposition characteristics used included the Arrhenius parameters obtained by means of the Russian manometric method ([9,11,151] and citations herein) or by means of methods that give comparable results (especially differential scanning calorimetry [9,151]). Another source of these characteristics was the simple non-isothermic differential thermal analysis (DTA) whose outputs were evaluated by means of the Kissinger method [152-154] in the sense of relationship [154] ... 

Principles and Characteristics Differential thermal analysis (DTA) is defined by ICTAC as A technique in which the temperature difference between a substance and a thermally inert reference material is measured as a function of temperature, while the substance and reference material are subjected to a controlled temperature programme . For the determination of the differential temperature AT temperature sensors, generally thermocouples, are used which are in direct contact with the materials or their containers. The output of the... 

A liquid flow microcalorimeter, the thermal activity monitor (TAM), is commercially available from ThermoMetric (formerly LKB/Bofors). This instrument consists of two glass or steel ampules with a volume of 3 to 4 cm3 (25 cm3 ampule available with a single detector), placed in a heat sink block. Recently, an injection-titration sample vessel was developed which acts as a microreactor. This vessel is provided with flow-in, flow-out, and titration lines, with a stirring device. The isothermal temperature around the heat sink is maintained by a controlled water bath. Each vessel holder, containing an ampoule, is in direct contact with a thermopile array, and the two arrays are joined in series so that their output voltages subtract. The two pairs of thermopile arrays are oppositely connected to obtain a differential output,... 

Differential scanning calorimetry is primarily used to determine changes in proteins as a function of temperature. The instrument used is a thermal analysis system, for example a Mettler DSC model 821e. The instrument coupled with a computer can quickly provide a thermal analysis of the protein solution and a control solution (no protein). The instrument contains two pans with separate heaters underneath each pan, one for the protein solution and one for the control solution that contains no protein. Each pan is heated at a predetermined equal rate. The pan with the protein will take more heat to keep the temperature of this pan increasing at the same rate of the control pan. The DSC instrument determines the amount of heat (energy) the sample pan heater has to put out to keep the rates equal. The computer graphs the temperature as a function of the difference in heat output from both pans. Through a series of equations, the heat capacity (Cp) can be determined (Freire 1995). 

The reference material is chosen to be one that exhibits no thermally induced transitions within the temperature range of interest. Thus, transitions that occur in the sample during the applied temperature programme appear as peaks or troughs, depending on whether they are exothermic or endothermic, on the plot of differential heat flow versus temperature (the thermogram) that is the output of the instrument (Figure 22.1). Commonly used reference materials are listed by Hatakeyama and Quinn (1994). 

Finally the proof-mass potential is raised to Vstep (Fig. 6.1.15c). Only a mismatch of the sense capacitors (or other mismatch in the circuit, e.g., between the integrating capacitors) results in a non-zero differential output voltage AV0. Note, however, that for identical sense capacitors the output is zero, as is required. Thermal noise sampled onto Cs and Qnt is effectively cancelled. 

The equipment used in DTA studies is shown schematically in Fig. 16.16. The sample is loaded into a crucible, which is then inserted into the sample well (marked S). A reference sample is made by placing a similar quantity of inert material (such as AI2O3) in a second crucible. This crucible is inserted in the reference well, marked R. The dimensions of the two crucibles and of the cell wells are as nearly identical as possible furthermore, the weights of the sample and the reference should be virtually equal. The sample and reference should be matched thermally and arranged symmetrically with the furnace so that they are both heated or cooled in an identical manner. The metal block surrounding the wells acts as a heat sink. The temperature of the heat sink is slowly increased using an internal heater. The sink in turn simultaneously heats the sample and reference material. A pair of matched thermocouples is used. One pair is in contact with the sample or the sample container (as shown) the other pair is in contact with the reference. The output of the differential thermocouple, or AT, is amplified and sent to the data... 

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