Capacitance temperature coefficient

The Hg/dimethyl formamide (DMF) interface has been studied by capacitance measurements10,120,294,301,310 in the presence of various tetraalkylammonium and alkali metal perchlorates in the range of temperatures -15 to 40°C. The specific adsorption of (C2H5)4NC104 was found to be negligible.108,109 The properties of the inner layer were analyzed on the basis of a three-state model. The temperature coefficient of the inner-layer potential drop has been found to be negative at Easo, with a minimum at -5.5 fiC cm-2. Thus the entropy of formation of the interface has a maximum at this charge. These data cannot be described... 

Power supply designers are usually aware that the most stable ceramic capacitance comes from materials dubbed COG material, also called NPO (for negative positive zero, referring to its near perfect temperature coefficient). But this is a low dielectric constant material, and unsuitable for modern miniaturization. So the common materials in use today are called X7R, X5R, and so on. There are others, starting with a Y or Z prefix, which no power supply designer worth his or her salt will ever use. 

The temperature coefficient of conductance is approximately 1-2 % per °C in aqueous 2> as well as nonaqueous solutions 27). This is due mainly to thetemper-ature coefficient of change in the solvent viscosity. Therefore temperature variations must be held well within 0.005 °C for precise data. In addition, the absolute temperature of the bath should be known to better than 0.01 °C by measurement with an accurate thermometer such as a calibrated platinum resistance thermometer. The thermostat bath medium should consist of a low dielectric constant material such as light paraffin oil. It has been shown 4) that errors of up to 0.5 % can be caused by use of water as a bath medium, probably because of capacitative leakage of current. 

In case of a homogeneous temperature distribution in the heated area, h corresponds to the temperature coefficient of the heater material, otherwise h includes the effects of temperature gradients on the hotplate. As a consequence of the aheady mentioned self-heating, the applied power is not constant over time, and the hotplate cannot be simply modelled using a thermal resistance and capacitance. Replacing the right-hand term in Eq. (3.28) by Eq. (3.35) leads to a new dynamic equation ... 

Two general approaches have been used in low-temperature studies. In the first, the uncompensated resistance, electrode capacitance, diffusion coefficient, and kinetic and thermodynamic parameters describing the electrode reaction are incorporated in a master model, which is treated (usually by some form of digital simulation) to calculate the expected voltammetric response for comparison with experiment [7,49]. 

Special features of mica capacitors are long-term stability (for example, AC/C 0.03% over three years), a low temperature coefficient of capacitance (TCC) (+ 10-80MK-1) and a low tan<5. 

A parallel-plate capacitor at 25 °C comprises a slab of dielectric of area 10 4 m2 and thickness 1 mm carrying metal electrodes over the two major surfaces. If the relative permittivity, temperature coefficient of permittivity and linear expansion coefficient of the dielectric are respectively 2000, — 12MK 1 and 8MK 1, estimate the change in capacitance which accompanies a temperature change of + 5 °C around 25 °C. [Answer — 0.035 pF]... 

There are two ferrite material properties which were not discussed in Section 9.3.1 but which are important in the inductor context they are the temperature and time stabilities of the permeability which, of course, determine the stability of the inductance. The temperature coefficient of permeability must be low, and this has been achieved for certain MnZn ferrite formulations as indicated in Fig. 9.18. A small residual temperature coefficient of inductance can be compensated by a suitable coefficient of opposite sign in the capacitance of the resonant combination. 

A quartz crystal thermometer sustains a capacitance if the frequency of the RLC circuit is precisely tuned to 14 or 20 MHz (depending on the exposed crystal faces). The quartz crystal will then transmit a very precise frequency, which has a temperature coefficient (typically 1 kHz per degree centigrade). If the temperature fluctuations are precisely compensated by a feedback heater circuit, then a quartz crystal oscillator is precise to about 1 part in 1.4 x 108. 

Comparisons of published results suggest that capacitive sensing is as good or better than alternative techniques, including tunnelling, piezoresistive, and piezoelectric interfaces [2], Other attractive features of capacitive interfaces include compatibility with many sensor fabrication processes, low temperature coefficient, and low power dissipation. 

In addition to the desired dependence on AC, it has a matching-dependent offset and gain that depends on parasitics. Any deviation of the reference capacitor Cref from the nominal value of the sense capacitance Cs appears as offset. Since in many applications AC is much smaller than C0, this offset often exceeds the signal. Offset cancellation should therefore occur early to minimize the dynamic range of the readout electronics. Care should also be taken for the trimming not to introduce a poor temperature coefficient. One solution fabricates the reference with the same process and in close proximity to the sense capacitor. The z axis accelerometer shown in Fig. 6.1.3 [7] utilizes two rnicromachined structures for the sense and reference. The suspension of the reference structure has been made intentionally stiff. 

Bukun and Ukshe calculated the integral capacitance from an infinite series, for a multilayer interface by treating the net-charged ionic layers as the plates of a parallel multiplate capacitor. In practice, it was only necessary to consider three layers to approximate the series. They were able to define a set of parameters, derived from the experimentally determined temperature coefficient of capacitance, which enabled them to obtain reasonable verification of their theory as indicated by the agreement between calculated and observed capacitances. 

The temperature coefficient of capacitance, denoted a or TCC and expressed in K" , is determined accurately by measurement of the capacitance change at various temperatures from a reference point usually set at room temperature (Tj) up to a required higher temperature (Tj) by means of an environmental chamber ... 

In the electrical industry, the temperature coefficient of capacitance is usually expressed as the percent change in capacitance, or in parts per million per degree Celsius (ppm/°C). Moreover, for industrial dielectrics, it is usually plotted in the temperature range -55°C to +125°C. 

In electrical engineering, it is common to classify dielectrics in three main classes. Actually, dielectric materials are identified and classified in the electrical industry according to the temperature coefficient of the capacitance. Two basic groups (Class I and Class II) are used in the manufacture of ceramic chip capacitors, while a third group (Class III) identifies the barium-titanate solid-structure-type barrier-layer formulations used in the production of disc capacitors. 

Capacitor values vary with temperature due to the change in the dielectric constant with temperature change. The temperature coefficient of capacitance (TCC) is expressed as this change in capacitance with a change in temperature. 

Trimmers or trimmer condensers employing TiB2 bodies are used for minute adjustments of capacitance. Normally the rotor consists of a Ti02 body. Parts are made with extreme accuracy, and are usually supplied in one of three temperature coefficient types. The base is a low loss ceramic composition. 

---