Temperature modulation

Heat Capacity = Heat Flow/Heating Rate or for MTDSC experiments 

Heat Capacity = Modulated Heat Flow/Modulated Heating Rate 

The heat capacity component can then be removed from the averaged modulated heat flow to produce a kinetic heat flow signal. The heat capacity and the kinetic heat flow signals are often described (respectively) as the reversing and nonreversing contributions to the traditional heat flow signal, as measured in traditional DSC experiments. 

An additional calibration constant is required for accurate MTDSC experiments this is the heat capacity calibration. The heat capacity constant is calculated as the ratio of the theoretical heat capacity of a known standard to the measured heat capacity of the material. The heat capacity constant is sensitive to changes in the modulation conditions, especially the frequency of modulation. 

Better heat transfer is achieved if helium is used as a purge gas. The heat capacity constant is very sensitive to small changes in the flow rate of helium and so helium should only be used as a purge gas if mass flow controllers are used to control the flow rate. 

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For applied work, an optical characterization technique should be as simple, rapid, and informative as possible. Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature. Modulation Spectroscopy is one of the most usehil techniques for studying the optical proponents of the bulk (semiconductors or metals) and surface (semiconductors) of technologically important materials. It is relatively simple, inexpensive, compact, and easy to use. Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors. Modulation Spectroscopy is gainii in popularity as new applications are found and the database is increased. There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization. 

In temperature modulation, the sample may be mounted on a small heater attached to a heat sink and the temperature varied cyclically by passing current pulses through the heater. If the sample is properly conducting, the current can be passed through the sample directly. Generally, for this method must be kept below 10—20 Hz, and hence there are often problems with the 1//"noise of the detector. 

Globus MY, Busto R, Lin B, Schnippering H, Ginsberg MD. Detection of free radical activity during transient global ischemia and recirculation effects of intraischemic brain temperature modulation. J Neurochem 1995 65 1250-1256. 

B. Wunderlich. Temperature Modulated Calorimetry in the 21st Century. Ther-mochim. Acta 2000, 355, 43-57. 

Pyda and co-workers [49, 60] measured the reversible and irreversible PTT heat capacity, Cp, using adiabatic calorimetry, DSC and temperature-modulated DSC (TMDSC), and compared the experimental Cp values to those calculated from the Tarasov equation by using polymer chain skeletal vibration contributions (Figure 11.7). The measured and calculated heat capacities agreed with each other to within < 3 % standard deviation. The A Cp values for fully crystalline and amorphous PTT are 88.8 and 94J/Kmol, respectively. 

Androsch, R., Moon, I., Kreitmeier, S., and Wunderlich, B. (2000). Determination of heat capacity with a sawtooth-t rpe, power compensated temperature-modulated DSC. Thermochimica acta. 357-358,267-278. 

During the last years, so-called microhotplates (pHP) have been developed in order to shrink the overall dimensions and to reduce the thermal mass of metal-oxide gas sensors [7,9,15]. Microhotplates consist of a thermally isolated stage with a heater structure, a temperature sensor and a set of contact electrodes for the sensitive layer. By using such microstructures, high operation temperatures can be reached at comparably low power consumption (< 100 mW). Moreover, small time constants on the order of 10 ms enable applying temperature modulation techniques with the aim to improve sensor selectivity and sensitivity. 

The third block in Fig. 2.1 shows the various possible sensing modes. The basic operation mode of a micromachined metal-oxide sensor is the measurement of the resistance or impedance [69] of the sensitive layer at constant temperature. A well-known problem of metal-oxide-based sensors is their lack of selectivity. Additional information on the interaction of analyte and sensitive layer may lead to better gas discrimination. Micromachined sensors exhibit a low thermal time constant, which can be used to advantage by applying temperature-modulation techniques. The gas/oxide interaction characteristics and dynamics are observable in the measured sensor resistance. Various temperature modulation methods have been explored. The first method relies on a train of rectangular temperature pulses at variable temperature step heights [70-72]. This method was further developed to find optimized modulation curves [73]. Sinusoidal temperature modulation also has been applied, and the data were evaluated by Fourier transformation [75]. Another idea included the simultaneous measurement of the resistive and calorimetric microhotplate response by additionally monitoring the change in the heater resistance upon gas exposure [74-76]. 

A micrograph of the single-ended hotplate-based microsystem is shown in Fig. 6.2 and features a die size of 5.0 x 2.9 mm. This system is a minimal implementation of a temperature-controlled microhotplate system. Temperature modulation is facilitated by an direct access to the input voltage A modulation of the input voltage is translated into a modulation of the microhotplate temperature. Another interesting application of the system includes its use as a microcalorimeter or as a material research platform [145]. The schematic of the temperature-control loop is shown in Fig. 6.3. 

The features of the monoHthic integrated sensor systems have not yet been fully exploited. The almost linear relationship between input reference voltage and microhotplate temperature renders the systems suitable for applying any temperature modulation protocol. Due their compatibility with other CMOS-based chemical sensors the microhotplates can be also combined with, e.g., polymer-based mass sensitive, calorimetric or capacitive sensors. The co-integration with such sensors can help to alleviate problems resulting from cross-sensitivities of tin-oxide based sensors to, e.g., volatile compounds such as hydrocarbons. A well-known problem is the crosssensitivity of tin oxide to humidity or ethanol. The co-integration of a capacitive sensor, which does not show any sensitivity to CO, could help to independently assess humidity changes. 

The last and most advanced system presented in this book includes an array of three MOS-transistor-heated microhotplates (Sect. 6.3). The system relies almost exclusively on digital electronics, which entailed a significant reduction of the overall power consumption. The integrated C interface reduces the number of required wire bond connections to only ten, which allows to realize a low-prize and reliable packaging solution. The temperature controllers that were operated in the pulse-density mode showed a temperature resolution of 1 °C. An excellent thermal decoupling of each of the microhotplates from the rest of the array was demonstrated, and individual temperature modulation on the microhotplates was performed. The three microhotplates were coated with three different metal-oxide materials and characterized upon exposure to various concentrations of CO and CH4. 

R. Aigner, M. Dietl, R. Katterloher, and V. Klee. Si-planar-pellistor designs for temperature modulated operation . Sensors and Actuators B33 (1996), 151-155. 

Okano, T., Yamada, N., Okuhara, M., Sakai, H., and Sakurai, Y. Mechanism of cell detachment from temperature-modulated, hydrophilic-hydrophobic polymer surfaces, Biomaterials, 1995, 16, 297-303. 

Takei, Y. G., Aoki, T., Sanui, K., Ogata, N., Okano, T., and Sakurai, Y. Temperature-responsive bioconjugates. 2. Molecular design for temperature-modulated bioseparations, Bioconjugated Chem., 1993, 4, 341-346. 

Linn, C.E., Campbell, M.G., and Roelofs, W.L. (1988). Temperature modulation of behavioural thresholds controlling male moth sex pheromone response specificity. Physiological Entomology 13 59-67. 

It is also possible to measure conductivity with DSC apparatus50 52. Khanna et al51 describe different procedures, whilst Simon and McKenna52 used temperature modulated DSC. A procedure using modulated DSC has been standardized in ASTM E195253. 

Okano, T., N. Yamada, M. Okuhara, H. Sakai, and Y. Sakurai. 1995. Mechanism of cell detachment from temperature-modulated hydrophilie-hydrophobie polymer surffitematerialsl6 297-303. 

The contrast factors have been measured interferometrically [87] and with an Abbe refractometer, respectively. The sample is contained in a fused silica spectroscopic cell with 200 pm thickness (Hellma). The sample holder is thermostated with a circulating water thermostat and the temperature is measured close to the sample with a PtlOO resistor. The amplitude of the temperature modulation of the grating is well below 100 pK and the overall temperature increase within the sample is limited to approximately 70 mK in a typical experiment [91], which is sufficiently small to allow for measurements close to the critical point. 

A number of interesting effects occur in spatially periodically forced pattern forming systems with a nonconserved order parameter, which have been investigated during recent years [60-73, 120], Here we focus on nearly unexplored effects of spatially periodic forcing in system with a conserved order parameter, as they occur in phase separating systems which are forced by spatial temperature modulations and where thermodiffusion plays a crucial role. 

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