Curie points defined

On the other hand, the minimum particles size to keep the ferroelectric property (critical size) differs depending upon the kind and composition of the materials. Summarizing the data of various kinds of materials, it varies from 7 nm for PbTiQs to 317 nm for Ba-Pb-Ti compounds. The Curie point defined as the point changing from the ferroelectric material to the paraelectric phase of PbTiCb reduces drastically with the decreasing particle size below 20-30 nm as shown in fig 2. As for the Curie point, some equations have been proposed for its estimation [K. Ishikawa, 2001]. 

Analytical pyrolysis is defined as the characterization of a material or a chemical process by the instrumental analysis of its pyrolysis products (Ericsson and Lattimer, 1989). The most important analytical pyrolysis methods widely applied to environmental samples are Curie-point (flash) pyrolysis combined with electron impact (El) ionization gas chromatography/mass spectrometry (Cp Py-GC/MS) and pyrolysis-field ionization mass spectrometry (Py-FIMS). In contrast to the fragmenting El ionization, soft ionization methods, such as field ionization (FI) and field desorption (FD) each in combination with MS, result in the formation of molecule ions either without, or with only very low, fragmentation (Lehmann and Schulten, 1976 Schulten, 1987 Schulten and Leinweber, 1996 Schulten et al., 1998). The molecule ions are potentially similar to the original sample, which makes these methods particularly suitable to the investigation of complex environmental samples of unknown composition. 

Thermal analysis systems require calibration prior to routine use. In TGA, calibration for mass is carried out by calibrating the microbalance using a set of standard weights, as for any balance system. Temperature calibration is effected by measuring the Curie point temperatures of a suite of International Confederation for Thermal Analysis and Calorimetry (ICTAC) Certified Reference Materials, which have well-defined Curie points. ... 

In this analysis the transition is defined as a step change in the heat capacity of the sample as a function of temperature. By far the most important transition that is generally considered to be second order is the glass transition, Tg. However, for completeness, other examples of second-order transitions include Curie point transitions where a ferromagnetic material becomes paramagnetic, the transition from an electrical superconductor to a normal conductor, and the transition in helium from being a normal liquid to being a superfluid at 2.2 K. 

Figure 16.5 The TGA Curie point method records for each standard an apparent sharp weight change at a well-defined temperature, which corresponds to a known transformation in the standard s ferromagnetic properties at that temperature. The figure shows the relative temperature precision from three replicate calibration runs using alumel and nickel Curie point standards. (Courtesy of TA Instruments, New Castle, DE, www.tainst.com.)...

For ferromagnetic substances the most important thermal characteristic is that the specific magnetization drops to zero above a well-defined temperature known as the Curie point, e.g., for metallic nickel the Curie point is about 358°C. 

If the specific magnetization is plotted against temperature, it will be found that above a certain, fairly well-defined temperature, the induction drops nearly to zero. In other words, the substance loses its ferromagnetism and becomes merely paramagnetic. This temperature is known as the Curie point, Tc. The Curie point is a critical point, not unlike the melting point of an organic compound, and subject to somewhat the same changes in mixtures and solutions. 

It is generally difficult to fix the Curie point within a few degrees. The reason for this is the rather peculiar shape of the plot of versus T. The point of inflection, as indicated in Fig. 34 may often be sufficiently clearly defined as to be determinable within a degree. This may then be taken as the Curie point. 

Curve II shows the same sample after having been heated to 800° in a stream of nitrogen. The Curie point is no less well defined, but has fallen to about 205° which is near the Curie point for cementite. The Hagg carbide has clearly undergone a reaction owing to its instability at high temperatures. 

Figure 44-1 shows the thermomagnetic curve for the Hagg carbide, care having been taken not to raise the temperature above 300°. The reproductibility of this curve, with falling or rising temperature, is excellent, and the Curie point is clearly defined as 247° 3°. 

The thermogravimetric curve is measured with a heating rate of 20 Km" and the Curie point of the specimen is defined as the onset point on the curve. 

The specimen is heated at a heating rate of 40 K min" . The curve shows a sharp peak, as shown in Figure 4.83. The Curie point is defined as the peak. 

Curie point pyrolysis involves coating of the sample on a ferromagnetic conductor (wire or capillary tube). The conductor is inductively heated to a specific temperature when exposed to a radiofrequency field. The composition of the conductor determines the Curie temperature (300- 10(X)°C). The major advantage of the Curie point PGC is the ability to heat samples reproducibly to accurately defined temperatures in milliseconds. The major disadvantage is the inability to vary temperature since a different rod is needed for each point. 

Tan et al. (1991) extended the temperature range studied by Fairburn et al. (1990) to 1000 °C. They showed the importance of the Temperature Rise Time (TRT) on the n-hexadecane conversion at these high temperatures (Figure 9), reporting conversions of 8% and 10%, corresponding to TRTs of 100 and 125 ms, at 900°C and 1000°C, respectively. Based on these results, they defined the Effective Residence Time (ERT) in the microreactor as the time period at which the exact Curie Point Temperature would yield the same conversion as the actual reaction for the Total reaction Time (TT). According to this definition... 

Curie point method The materials selected for temperature calibration are metals and alloys which are ferromagnetic at low temperature but which lose their ferromagnetism at well-defined Curie points. If the magnetic material is placed in the sample holder with a magnet placed below the sample, as shown in Figure 15.9c, the magnetic force on the sample will cause an apparent increase in mass. In a TG rim, the force will disappear fairly abruptly at the Curie point as shown in Figure 15.10. The mid-temperature... 

The existence of a critical point in the pressure-volume-temperature (PVT) diagram (actually, a point in the planar PV projection, but a critical line in a three-dimensional representation), a critical point (Curie temperature) in ferromagnetism, a critical point (Neel point) in antiferromagnetism, a critical temperature in superconductivity, and a critical point (lambda point) in liquid 2He4 are physical descriptions of the onset of a sudden macroscopic collective transition. If one approaches the critical point very closely, dimensionless parameters, defined to describe this approach, are common to all these disparate phenomena the approach to criticality, or to a phase transition, are really the same. 

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