This thesis presents the design, fabrication and characterisation of a thermal micro-sensor capable of operating on small scale samples. It was initially characterised by measuring the thermal conductivity of 100 μl liquid droplets and thin materials (sheets of paper) before being applied to the measurement of the water content of plant leaves. The device consists of a micro-heater and two thin-film thermocouples formed on a flexible polyimide substrate and used microfabrication techniques to precisely pattern the sensing elements. The heater was used to induce a temperature gradient within a sample in contact with the device, which was recorded as a difference in temperature (ΔT) between the two thermocouples. Various experiments have demonstrated that ∆T was dependent on the total thermal resistance (consisting of the bulk thermal conductivity and thermal contact resistance) of the sample under inspection. The device’s sensitivity to bulk thermal conductivity was shown by recording the variation in ∆T for 100 μl droplets of glycerine/water mixtures. Different compositions of the mixture served as thermal conductivity standards. The measurement of mixtures of propanol/water further demonstrated that the device could be used to indirectly monitor the composition of small volume binary solutions by measurement of the thermal conductivity. By monitoring the thermal conductivity of wetted paper, it was shown that the device was sensitive to the increase in the total thermal resistance as the paper dried out. Furthermore, theoretical and experimental drying times were in agreement and exhibited a similar dependence on the air temperature. This provided clear evidence of the device’s ability to monitor thermal conductivity of small samples. The final element of this work was the real-time monitoring of the water content of plant leaves. The device was clamped to an abscised leaf which was allowed to dry over a period of 6 hours. A comparison between the weight of the leaf and ∆T measurements showed a linear dependence. It was found that the changes in thermal properties were dominated by the water content of the leaf. The device was subsequently shown to be sensitive to changes in the water content of the leaves of plants subjected to water stress conditions, demonstrating its ability to monitor the real-time water content in-situ.

No description available.

No description available.

No description available.

No description available.

The problem of background radiation in homogeneous isotropic world models due to radiation emitted by extragalactic nebulae is considered in more detail and greater generality than seems to have been attempted previously. In chapter I a brief historical survey is made of former theoretical and empirical attempts to determine the cosmical background radiation in both the optical and radio regions of the spectrum.

The object of this thesis is to investigate the possibility of a total radiation detector for use in High Temperature Physics Research. The criterion taken being a detector which would handle a radiation pulse of length 100p sec with a time constant of better than lp sec and accuracy of about 1%. (Compared with 50p sec which is the best at present available), Total radiation measurements require a uniform spectral response as obtained with a black—body absorption i,e. with a surface black, The conduction equation for a radiation flux incident on a black on a material body is solved, by the Method of Laplace transforms, to give the temperature distribution in the body in terms of the incident flux. Improvement of standard resistive bolometers in terms of the above criterion is considered. A pyroelectric type of detector, in which a voltage output is obtained due to change on heating of spontaneous polarisation in a ferroelectric, is examined theoretically in great detail. The theoretical examination includes different types of electrode mounting on the unit (generally single crystal or ceramic barium titanate), the effect of piezoelectric output due to thermal expansion exciting resonances (a sort of thermo—mechanical coupling), the tertiary piezoelectric effect, and various possible and effects. Experimental work on the pyroelectric detector using the linear discharge equipment at Imperial College indicates that theory is closely obeyed and the actual tine constant is limited by the frequency of mechanical resonances (giving a piezoelectric output). The units manufactured have a tine constant of about 5p sec and a highly reproducible output, but these can easily be developed to give 1p secs tine constant and are capable of handling the high energy fluxes obtained in thermonuclear research. An appendix has been added showing that the pyroelectric detector can also be used as a very sensitive detector when used with a long time constant.

Experimental studies have been made of counter-current water to water heat transfer in the nonsteady state. A paraflow plate heat exchanger was used and the non-steady conditions were produced by changes in the inlet temperature of the hotter of the two water streams. The design and construction of the apparatus is described. It was found that a diffusion mechanism had a significant effect upon non-steady heat transfer. Previous theories in this field were reviewed and extended to allow for this effect. The analysis of the results utilised a Laplace transformation with respect to the time variable and the calculations were carried out with the aid of the University of London's MERCURY digital computer. The diffusion coefficients were calculated to be between 1,000 and 5,500 BTU/hr. per °F./ft. for water flow rates of between 200 and 400 g.p.h. These results suggest that the diffusion effect is caused by a longitudinal mixing mechanism.

No description available.

No description available.