Heat-flow sensors

Other instruments include the Calvet microcalorimeters [113], some of which can also run in the scanning mode as a DSC. These are available commercially from SETARAM. The calorimeters exist in several configurations. Each consists of sample and reference vessels placed in an isothermally controlled and insulated block. The side walls are in intimate contact with heat-flow sensors. Typical volumes of sample/reference vessels are 0.1 to 100 cm3, The instruments can be operated from below ambient temperatures up to 300°C (some high temperature instruments can operate up to 1000°C). The sensitivity of these instruments is better than 1 pW, which translates to a detection limit of 1 x 10-3 W/kg with a sample mass of 1 g. 

The general conclusion to be drawn from these studies is that the use of small pyroelectric elements as heat flow sensors in chemical investigations holds some promise. The early stage of the studies makes it difficult to assess the extent of their utility. New adsorber materials are an essential requirement if these structures are to fulfil their promise. 

In all energy systems, including renewable, it is important to measure both the rate at which heat is flowing and also the energy flow that a fuel provides. The detection of the calorific values by calorimeters have already been discussed in Section 3.2.4. Here, first the heat flow sensors will be discussed. 

Since we are attempting to reconcile measurements made by different techniques, the artifacts, limits and constraints that accompany each technique must be identified. These include the challenges inherent in the methods of sample-sensor coupling. For example, fhermal femperafure or heat flow sensors are influenced by factors such as the thermal conductivity of the cell-sensor construct, the thermal resistance of fhe sample-cell interface, and the internal thermal properties of the sample. The geometry of the heat flow pathways is also important. Mechanical sensors (force or... 

In order for both mass and heat-flow sensors to operate, the thin-film sample must adhere to the top surface of the QCM and be of uniform thickness. The mechanical behaviour of films on the quartz microbalance has been modeled by Kanazawa(12), who examined the amplitude of the shear displacement in the quartz crystal and in the overlying film for several cases. For a 1 volt peak RF applied voltage typical of the Stanford Research Systems oscillator driver, the amplitude of the shear wave of a bare crystal is 132 nm. Mecca [29] has calculated the inertial acceleration at the centre of a similar quartz resonator, and finds that it is roughly 10 g, where g is the gravitational constant. At these extremely high accelerations, powder or polycrystalline samples do not follow the transverse motion of the QCM surface and cannot be used without being physically bound to the surface with a thin adhesive layer. 

By June of 1997 there was a working mass/heat-flow sensor in the Lund laboratory based on the principles described above, and by 2001 the first of three U.S. patents was issued on this technology. 

Upon returning to Drexel in the summer of 1997, the author worked with graduate student Hamid Shirazi to develop a quartz crystal microbalance/heat conduction calorimeter based on this mass/heat-flow sensor. The development, testing, and initial uses of the QCM/HCC are documented in Shirazi s PhD thesis, available online [51]. Shira performed die following experiments with this new technology ... 

All three of these PhD theses contain sections on the theory of operation of each component of the mass/heat-flow sensor and experimental details such as block diagrams of the apparatus, sample preparation, data acquisition and control, calibration, and data analysis. 

A Mass/Heat Flow Sensor Combining Shear Mode Resonators with Thermoelectrics Principles and Applications, IEEE Frequency Control Symposium. IEEE, Tampa, FL, 2003,1062-1065... 

The principle of an automated instrumentation relies on the application of a direct ultra thin (0.15 mm) heat flow sensor, which is attached to a metal block of constant temperature which differs from the sample temperature. When the... 

There yawns a large gap between commercial (micro)calorimeters with maximum vessel volumes of 25, 30 or 100 mL and instruments of many litres for smaller domestic animals. The only exception known to the authors is the Seta-ram GF 108 1-L instrument used in the Leyden group [72]. A low-price solution for an intermediate size calorimeter was found in cooling/warming boxes sold as picnic equipment for less than US 200 [73], They are equipped with a Peltier battery as a heat pump between the inner volume of the box and the environment. In the same way the heat pump can work as a Seebeck heat flow sensor to determine heat production rates inside the box. The inner walls of the box may be additionally covered by copper foil of high thermal conductivity to facilitate heat flow to the sensor. 

Isothermal and semiadiabatic calorimeters both quantify cement hydration kinetics, but they do so in different ways. This is illustrated in Figure 2.1. In isothermal (heat conduction) calorimetry the heat production rate ( ) in a small sample (S) is measured by a heat flow sensor as heat is conducted to a heat sink that is placed in a thermostated environment. It is also necessary to have a reference sample (R) with the same properties (especially the same heat capacity) as the sample but without any heat production. This arrangement significantly reduces the noise in the measurements. The output from the calorimeter is the difference between the sample signal and the reference signal. 

In a semiadiabatic calorimeter the sample (S) is insulated and the temperature (T) increase during hydration is assessed as a measure of the rate of the hydration. Corrections must be made for the heat losses to the surroundings. The heat loss rate can be found by calculations using a measured heat loss coefficient or it can be directly measured with heat flow sensors placed in the insulation. 

To use a calorimeter for quantitative measurements, it has to be calibrated i.e. parameters that make it possible to evaluate the results in terms of standard units need to be determined. This involves measuring calibration coefficients, baselines and (sometimes) the time constants. The calibration coefficient is the parameter that transforms the voltage from the heat flow sensor in the calorimeter to thermal power. It is often called e (unit watts/volt) and is usually measured by electrical calibrations. The baseline is the signal from the calorimeter when no heat is produced in the sample. The baseline Uq (unit volts) is measured without any heat production in the sample ampoule. For applications where an accurate value of the baseline is important - such as determinations of 7 days heat of hydration - a baseline measurement should be made for 1-2 days with inert material in the sample vial. 

When voltage U (the output from the heat flow sensors) has been measured, thermal power P is calculated by the following equation ... 

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