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Superconductors provide a wide spectrum of possibilities for sensing different physical effects and actual values or changes of physical quantities, because their fundamental properties are strongly influenced by external conditions. The development of high- T c ceramic superconductors in the form of bulk and film samples has broadened the applicability of these materials for various kinds of sensors and opened a way to design a new class of detectors. Based on some early measurements showing the strong dependence of the microwave properties of high- T c compounds on an external magnetic field, our first device application has been designed to control a thick-film microwave ring resonator by low magnetic field H . In this paper a new and sensitive magnetic-field detector is presented, which is developed from the previous work mentioned above. The quality factor Q of a high- T c superconducting ring resonator is observed to increase and then decrease with the increase of the field strength H in the low magnetic-field range. Q shows a huge change in the field interval 0–3 mT. The value of d Q /d H is 1 dB/18 μT. By using an HP Network Anlyzer of 1 V/dB for this investigation, the accuracy of the measurement of output voltage guarantees to detect the magnetic field with a sensitivity of 1–10 nT.

Different thermodynamic states can develop during the operation of sensors, namely, static equilibrium, stationary non-equilibrium and time-dependent ones, which can be caused by reversible or irreversible processes, namely, spontaneous, generated and fast ones. From the point of view of stability these states can be divided into three categories, stable, unstable and indifferent. (The strong instabilities, such as dissipation, singularities, catastrophes, bifurcation and chaos, may also be investigated by specific apparatus originating from the fundamental thermodynamics.) Certain stability problems can be solved by the method of negative or sometimes positive feedback. The disturbance of an equilibrium state can be described unambiguously by designating new parameters corresponding to the disturbed state. It is shown that the new state is characterized by series of inequalities, the terms of which can be extensive and intensive quantities, non-equilibrium deviations and thermodynamic forces or forces and currents, depending on the character of the given problem. The genuine dynamic stabilization mechanism is connected to the equalization process. It is shown that the actual decay of an external disturbance is not necessarily a monotonic process in time. Some characteristic parameters decrease, while others increase, producing the so-called oscillatory cross effects. Starting from the derivation of the entropy and conductivity matrixes, their analogous properties and role in the conservation of stability are determined. The stability of the stationary or general non-equilibrium states is investigated by variational principles, leading directly to the stability criteria. The possible applicability of the methods presented is demonstrated by various types of examples.

A novel type of thermoelectromotive hybrid sensor has been developed to detect the position of a liquid level. This sensor utilizes the high thermoelectromotive force (t.e.m.f.) of semiconductors combined with the switching property of new high-Tc materials. The recent highest Tc of about 125 K offers the possibility of detecting the level of a liquid with a boiling point below this value. A markedly enhanced sensitivity of these semiconductor-superconductor hybrid sensors, i.e., up to a few mV/K, compared to those based merely on the t.e.m.f. of a high-Tc material (10−3 times the t.e.m.f. of a semiconductor) is reached by utilizing the high t.e.m.f. of the Si with proper carrier concentration. The prototype hybrid sensor is constructed by combining a high-Tc thick-film conducting line and Si sheet and connecting them electrically in parallel. Sensor output signals even up to volts have been measured. The liquid-level measuring accuracy is about 1 mm.

The real states, processes and stability relations of the operation of sensors have been described by irreversible thermodynamic methods. It has been realized with the help of extensive and intensive parameters, using fundamental properties of the entropy function. General form of thermodynamic motion equations, reasons of irreversibility and non-linearity, authentic non-linear source density functions and variational principles have been given. The causes of ceasing stability and arising unstable states have also been shown. Conditions for strongly non-linear and unstable phenomena (singularity, catastrophe and chaos) have been analysed too. The validity of the theory presented has been demonstrated by specific examples.

Using extensive and intensive parameters for the characterization of the actual state of measuring sensors, spontaneous, generated and fast process thermodynamic motion equations for the operation of the sensors are derived. The general expression for the entropy balance, containing the terms originating from the inhomogeneous distribution of intensive parameters, convective currents, external generations and internal sources, is also given. Some examples are shown to demonstrate the validity of this theory.

The results of earlier experiments obtained during the investigation of the thermoelectromotive force (t.e.m.f.) of different high- Tc superconductors have led to the development of a system for cryogenic liquid level indicators. The sensors can be made from high-temperature ceramic and thick-film superconductors, such as Y Ba Cu O and Bi (Pb) Ca Sr Cu O compounds, which are stable over time and also on repeated thermal cycling. The measuring equipment designed in the study is suitable for the examination of different materials to be used as cryogenic liquid level indicators. An example demonstrates the method of selecting the proper material, the sensitivity of the material and the simplicity of application.