4 - Effects

Thermistor

The name thermistor was introduced for temperature sensitive resistors with large temperature dependency that was found at transition metal oxides. These simple devices are prepared by ceramic processing technology. Since a negative temperature coefficient of resistance is observed in semiconductor oxides, such as the precisely controlled mixtures of the oxides of Mn, Co, Ni, Cu, and Zn, these sensors were named NTC-thermistors. PTC-thermistors have opposite type characteristic, which is resulted in by the temperature dependent electrical properties of grain boundaries in doped piezoelectric (e.g., BaTiO₃) ceramic materials. The steep increases of resistance of the latter type make them particularly useful as self-regulating heating elements, current limiting devices, etc.

Resistance temperature detector

Resistance temperature detector (RTD) is the commonly used term for temperature sensors the operation of which are based on the positive temperature coefficient of metals. For many metals, the resistivity via temperature characteristic, r(T), is approximately linear within a limited range, i.e.:
where is the resistivity at a reference temperature T_o; is the temperature coefficient of resistance, also called TCR, defined as , and is the actual temperature difference related to T_o . The above phenomenon can be exploited in temperature sensors made of metal wires, as well as of thin- and thick-film materials. The largest and most reproducible TCR values can be found in materials that are free from impurities and defects. Platinum RTDs are the most known and well-standardized types, but other metals, such as copper and nickel, are also used in low-cost applications.

A thermoelement (also called thermocouple) is a junction of two conducting (metal or semiconductor) materials, A and B, electrically connected at a "hot" point of temperature, T1. The nonconnected ends of both legs are kept at another temperature T0 ("cold" point). In the open circuit case, the net current flow through the thermoelement is zero and a thermoelectric or Seebeck voltage can be observed between the thermocouple leads at the cold point. For small temperature differences, this can be approximated as: where and denote the Seebeck coefficients (thermopower or thermoelectric power) of materials A and B. They express specific transport properties, determined by the band structure and the carrier transport mechanisms of the materials. U_s is always created in an electrically conducting material when a temperature gradient is maintained along the sample, but it can not be observed with two legs of the same material for reasons of symmetry. Thermocouple sensor types are widely used in the practice. They are standardized and many types are available using both precious or base metal wire pairs.

When piezoelectric materials are under stress, the centers of gravity of the positive and negative charges are separated forming an electrostatic dipole and hence a polarization of the film. In electrets, the centers of gravity of the positive and negative charges are separated even without a stress being applied. These will exhibit spontaneous polarization, which means that there must be permanent electrostatic charge on the surfaces of the film, with one face positive and another negative, depending on the direction of the polarization vector. The spontaneous polarization will be a strong function of temperature, since the atomic dipole moments vary as the crystal expands or contracts. Heating the crystal will tend to desorb the surface neutralizing ions, as well as changing the polarization, so that a surface charge may then be detected. Thus, the crystal appears to have been charged by heating. This is called the pyroelectric effect. The electric field developed across a pyroelectric crystal can be remarkably large when it is subjected to a small change in temperature. We define a pyroelectric coefficient, p, as the change in flux density, D, in the crystal due to a change in temperature, T, i.e. . Using a capacitor, the pyroelectric voltage signal is , where is the permittivity, and d the thickness of the pyroelectric film. When the temperature changes, an excess of charge appears on one of the polar faces and a current will flow in the external circuit. After the initial surge, the current dies away exponentially with time and eventually falls to zero until another temperature change comes along; this is very similar to the time-dependent behavior of piezoelectric materials. Pyroelectric films can be used to detect any radiation that results in a change in temperature of the film but are generally used for infrared detection. Important parameters of the materials are the heat capacity per volume, c_th, and the heat conductivity, g_th. In high accuracy measurements and thermovision applications these parameters have a great importance; they influence the sensitivity and the resolution.