Temperature step mode

The previous described technique is very easy to use, but shows some drawback as soon as the accuracy of the Cp determination is concerned. With the temperature scanning mode, the sample is continuously heated and is never at the thermal equilibrium. However Cp is a thermodynamical parameter, defined at the thermal equilibrium. The temperature step mode has been developed to answer this limitation. A step of temperature is applied to the sample and the thermal equilibrium (characterized by the signal return to the baseline) is waited after each step (Fig. 2.16). 

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]. 

The performance of the temperature controller was measured in the tracking mode. Figure 6.18 shows a graph, where the temperature of one of the three microhotplates is kept at a constant temperature of 300 °C, the temperature of the second microhotplate is modulated using a sine wave of 10 mHz, while rectangular temperature steps of 150 °C, 200 °C, 250 °C, 300 °C, and 350 °C have been appHed to the third microhotplate. Temperature measurements on one of the hotplate that has been operated at constant temperature in the stabihzation mode showed a variation of less than 1 °C, even though the temperature of the neighboring hotplates was, at the same time, modulated dynamically (sine wave, ramp, steps). This is a consequence of the individual hotplate temperature control, without which thermal crosstalk between the hotplates would have been clearly detectable. The power dissipation of the chip is approximately 190 mW, when all three hotplates are simultaneously heated to 350 °C. In the power-down mode, the power consumption is reduced to 8.5 mW. 

Fig. 6.18. Temperature tracking in the PID control mode one hotplate is kept at constant temperature, the temperature of the second microhotplate is modulated in a sinusoidal way. Rectangular temperature steps are applied to the third microhotplate...

Heat, wait, and search the temperature at which the exothermal reaction is detectable is searched using a defined series of temperature steps. At each step, the system is stabilized for a defined time, then the controller is switched to the adiabatic mode. If the temperature increase rate surpasses a level (typically 0.02 Kmin-1), the oven temperature follows the sample temperature in the adiabatic mode. If the temperature increase rate remains below the level, the next temperature step is achieved (Figure 4.4). 

The modes of operation of a DMA are varied. Using a multifrequency mode, the viscoelastic properties of the sample are studied as a function of frequency, with the oscillation amplitude held constant. These tests can be run at single or multiple frequencies, in time sweep, temperature ramp, or temperature step/hold experiments. In multistress/strain mode, frequency and temperature are held constant and the viscoelastic properties are studied as the stress or strain is varied. This mode is used to identify the LVR of the material. With creep relaxation, the stress is held constant and deformation is monitored as a function of time. In stress relaxation, the strain is held constant and the stress is monitored versus time. In the controlled force/strain rate mode, the tanperature is held constant, while stress or strain is ramped at a constant rate. This mode is used to generate stress/ strain plots to obtain Young s modulus. In isostrain mode, strain is held constant during a tanpera-ture ramp to assess shrinkage force in films and fibers. 

Actual Sample Temperature. Any polymer sample will take a finite time to reach thermal equilibrium, and this is frequently commensurate with the timescale of the experiment. Most polymers have poor thermal conductivities. Thus, the most accurate measurements are made isothermaUy in the step mode, since a soak period can be arranged so that the sample reaches thermal equilibrium. Poorly designed ovens can exacerbate the problem. The guideline of 2 °C/min as the maximum rate for temperature scanning in the ramp heating mode is a useful one for a DMA instrument. This is the rate specified in the various ASTM standards (e.g., ASTM E1640-07, ASTM E1867-06 see Appendix). Samples such as thermoset adhesives coated onto wire mesh, which are thin and have... 

Continuous or pulse mode (flash pyrolysis) step mode (temperature-programmed pyrolysis)... 

The ARC system is often operated in a stepwise heat-wait-search modus. After heating to a certain temperature, the system is stabilized for a pre-defined time until the calorimeter starts seeking for a temperature increase caused by first decomposition processes. If the temperature increase surpasses a pre-defined threshold (e.g. 0.01 K/min) the furnace temperature follows the sample temperature in the adiabatic mode and the calorimeter tracks the adiabatic temperature rise due to the self-heating of the sample. If the threshold is not surpassed after a certain period of time, the calorimeter proceeds with the next temperature step. In comparison to DSC analysis ARC measurements are significantly more sensitive, usually by a factor of 100 or more. Sensitivity is as low as 0.5mW/g and self-heating rates of 0.01 K/min can be detected. 

Figure Bl.19.13. (a) Tliree STM images of a Pt(l 11) surface covered witli hydrocarbon species generated by exposure to propene. Images taken in constant-height mode. (A) after adsorption at room temperature. The propylidyne (=C-CH2-CH2) species that fomied was too mobile on the surface to be visible. The surface looks similar to that of the clean surface. Terraces ( 10 mn wide) and monatomic steps are the only visible features. (B) After heating the adsorbed propylidyne to 550 K, clusters fonn by polymerization of the C H...

The stress—relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps. (/) The specimen is subjected to a given stress at a temperature T for a time /, both chosen to allow complete orientation of the mobile units that one wishes to consider. (2) The temperature is then lowered to Tq T, where any molecular motion is completely hindered then the stress is removed. (3) The specimen is subsequendy heated at a controlled rate. The mobile units reorient according to the available relaxation modes. The strain, its time derivative, and the temperature are recorded versus time. By mnning a series of experiments at different orientation temperatures and plotting the time derivative of the strain rate observed on heating versus the temperature, various relaxational processes are revealed as peaks (243). 

Rheometric Scientific markets several devices designed for characterizing viscoelastic fluids. These instmments measure the response of a Hquid to sinusoidal oscillatory motion to determine dynamic viscosity as well as storage and loss moduH. The Rheometric Scientific line includes a fluids spectrometer (RFS-II), a dynamic spectrometer (RDS-7700 series II), and a mechanical spectrometer (RMS-800). The fluids spectrometer is designed for fairly low viscosity materials. The dynamic spectrometer can be used to test soHds, melts, and Hquids at frequencies from 10 to 500 rad/s and as a function of strain ampHtude and temperature. It is a stripped down version of the extremely versatile mechanical spectrometer, which is both a dynamic viscometer and a dynamic mechanical testing device. The RMS-800 can carry out measurements under rotational shear, oscillatory shear, torsional motion, and tension compression, as well as normal stress measurements. Step strain, creep, and creep recovery modes are also available. It is used on a wide range of materials, including adhesives, pastes, mbber, and plastics. 

---