Curie, point

There is no discontinuity in volume, among other variables, at the Curie point, but there is a change in temperature coefficient of V, as evidenced by a change in slope. To understand why this is called a second-order transition, we begin by recalling the definitions of some basic physical properties of matter ... 

The dielectric stiffness can be expressed as a linear temperature dependence based on the Curie-Weiss law at above the Curie point T. 

Lead zirconate [12060-01 -4] PbZrO, mol wt 346.41, has two colorless crystal stmctures a cubic perovskite form above 230°C (Curie point) and a pseudotetragonal or orthorhombic form below 230°C. It is insoluble in water and aqueous alkaUes, but soluble in strong mineral acids. Lead zirconate is usually prepared by heating together the oxides of lead and zirconium in the proper proportion. It readily forms soHd solutions with other compounds with the ABO stmcture, such as barium zirconate or lead titanate. Mixed lead titanate-zirconates have particularly high piezoelectric properties. They are used in high power acoustic-radiating transducers, hydrophones, and specialty instmments (146). 

Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Historically, materials based on doped barium titanate were used to achieve dielectric constants as high as 2,000 to 10,000. The high dielectric constants result from ionic polarization and the stress enhancement of k associated with the fine-grain size of the material. The specific dielectric properties are obtained through compositional modifications, ie, the inclusion of various additives at different doping levels. For example, additions of strontium titanate to barium titanate shift the Curie point, the temperature at which the ferroelectric to paraelectric phase transition occurs and the maximum dielectric constant is typically observed, to lower temperature as shown in Figure 1 (2). 

A typical characteristic curve of a PTC thermistor having a Curie point of 120°C is shown in Figure 12.37. 

Industrial Power Engineering and Applications Handbook Table 12.1 Curie points lor a lew PTC thermistors... 

Recommended reference temperature (Curie point) for thermistors (°C) ... 

Table 5.2. Thermodynamic properties of the shock-compression induced Curie point transition (after Graham et al. [67G01]).

Figure 3.8 Anomalous temperature dependence of relative dielectric constant of ferroelectric crystals at the transition temperature (Curie point).

Nickel is ferromagnetic, but less markedly so that either iron or cobalt and its Curie point (375°C) is also lower. 

If a paramagnetic salt is cooled, eventually a temperature is reached where the magnetic moments of the electrons line up. This temperature is known as the Curie point or the Neel point, depending upon whether the spins line up parallel below this temperature as in (b), with the moments pointing in the same direction to reinforce one another and produce ferromagnetism, or the moments line up in opposite directions as in (c) so that they cancel and antiferromagnetism occurs. 

For isothermal measurements, it is advisable to use a furnace of low thermal capacity unless suitable arrangements can be made to transport the sample into a preheated zone. The Curie point method [132] of temperature calibration is ideally suited for microbalance studies with a small furnace. A unijunction transistor relaxation oscillator, with a thermistor as the resistive part with completion of the circuit through the balance suspension, has been suggested for temperature measurements within the limited range 298—433 K [133]. 

Compound Band gap (eV) Ref. Magnetic susceptibility ref Curie point (°C) Ref... 

Figure 8.45 Apparatus for pyrolysis gas chromatography. A, filament or ribbon-type pyrolyzer and B, Curie-point pyrolyzer. (Reproduced with perm.i ion from ref. 848. Copyright American Chemical society).

Structure of NaN02 below and above the CURIE point. Bottom domains in a ferroelectric crystal of NaN02... 

DeLuca, S. Sarver, E. W. Harrington, P. d. B. Voorhees, K. J. Direct analysis of bacterial fatty acids by Curie-point pyrolysis tandem mass spectrometry. Anal. Chem. 1990, 62,1465-1472. 

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