Nickel, Curie point

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.)...

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

The Curie point of the alloy is determined by the iron-to-nickel ratio an Fe Ni ratio of 50 50 was used to give a Curie point of 530°C. This pyrolysis temperature was chosen because it has been shown22 to give a balance between fragmentation from polysaccharides and protein fractions. Foils with Curie points of 300°C to 1000°C are commercially available (Figure 15.2). How-... 

Cobalt s Curie point is 1,121°C, and its melting point is 1,495°C. About 25% of all cobalt mined in the world is used as an alloy with other metals. The most important is the alloy alnico, which consists of nickel, aluminum, and cobalt. Alnico is used to make powerful permanent magnets with many uses, such as CT, PET, and MRl medical instruments. It is also used for electroplating metals to give a fine surface that resists oxidation. 

N2O decomposition on nickel increases suddenly at the Curie point, owing to a smaller electronic interaction in accordance with the foregoing explanation. On the other hand, the electronic work function of nickel, according to Cardwell (71), above the Curie point is 0.2 volt higher than below it. The transfer of metal electrons to the N2O molecules, therefore, occurs less readily above the Curie point. Hence the bond between an 0 atom and N2 in the adsorbed N2O molecule is not weakened so much at temperatures above the Curie point. Thermal decomposition of N2O, therefore, requires a higher energy of activation. ... 

Fig. 7. Abrupt changes in the catalytic decomposition of NjO on passing the Curie point (360°) of pure nickel. The two curves refer to two experiments with different velocities of the gas currents. Ordinate yield in % of theoretical decomposition.

Dimethyl benzene Apiezon Preconcentration on Apiezon L coated nickel wire then Curie point pyrolysis-GC [210]... 

Since there is no contact between pan and furnace, the thermal lag is higher than in DSC. The standards recommended by ICTA and distributed by NBS are ferromagnetic standards exhibiting loss of ferromagnetism at their curie point temperature within a magnetic field Nickel (354°C), Permanorm 3 (266°C),... 

Lefebvre and LeClerc (36) carried out thermodynamic studies on catalysts of the Fischer-Tropsch synthesis. They drew attention to the significance of the specific Curie points, of various compounds, for their activity or inactivity as catalysts. They assumed that the active catalysts were cubic iron oxide and hexagonal nickel. Pichler and Merkel (37) found that the Curie point attributed by Lefebvre and LeClerc to cubic iron oxide is actually the Curie point of one special form of Fe2C. The hexagonal nickel seems to be actually a nickel carbide. 

Curie point temperatures can also be determined by DTA and DSC techniques. As illustrated in Figure 7.66, the specific heat of nickel, increases gradually up to the Curie point at 357°C, making a sudden change at this point. The sample size used was 75 mg. 

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. 

It has been shown by Marian and others (Marian, 57 Ross, 58) that for solid solutions of copper in nickel, both Curie point and specific magnetization are linear with respect to concentration. Marian s data for the Curie points are shown in Fig. 50. The thermomagnetic curve for a homogeneous solid solution has the same shape and abrupt Curie point shown by a pure ferromagnetic substance except that the specific magnetization (tr ) at any given temperature is lower, and the... 

Fig. 50. Curie point vs. concentration for nickel-copper alloys.

It appears also that this particular catalyst contained no unalloyed metallic nickel in particles equal to or larger than the critical ferromagnetic domain. This must be true because all ferromagnetism disappeared nearly 100° below the Curie point of nickel. It has been suggested (Emmett and Skau, 60) that activity may be related to nickel... 

It will, of course, be understood that this explanation of thermal deactivation is by no means the only deactivation process which can occur. Morris and Selwood found that nickel supported on magnesia could be thermally deactivated with no recession of the Curie point. Nevertheless, the method seems to be a powerful one, which might be applied to other ferromagnetic metals, and to ferromagnetic oxides and ferrites such as copper ferrite undergoing isomorphous replacement of copper by zinc, as described by Snoek (44). 

The intermetallic compounds MnAs, MnSb, and MnBi crystallize with a nickel arsenide-type structure, and they have many features in common regarding their structure and nature of physical properties. At room temperature, all these compounds are ferromagnetic. The Curie point of manganese arsenide is 45°C of MnSb and MnBi it is 314 and 360°C, respectively. Their ferromagnetism is due to the uncompensated spins of the 3d electron shells of the manganese atoms, located at the crystal lattice points with the coordinates 0, 0, 0 and 0, 0, i. 

Fe—Ni Alloys-. Samples of Fe and Ni in experiments can have nonmagnetic or magnetic soft ferro magnets. For the alloy compound of iron and nickel, they have a much lower saturation magnetization compared to pure samples of each metal. For example, when we have 37% of Ni, it has a low curie point and an FCC structure. Theoretical calculations estimate a more complicated magnetic structure for these types of alloys due to the different combinations. 

Curie-point pyrolysis involves placing the sample wire into a radio frequency field that induces eddy currents in the ferromagnetic material and causes a temperature rise. When the wire reaches the Curie-point temperature, it becomes paramagnetic and stops inducting power. The temperature at which the wire stabilizes (the Curie point) is a function of the type of metal. For example, the Curie points of cobalt, iron, and nickel are 1128, 770, and 358°C, respectively. Wires made from alloys of these metals produce intermediate temperatures. For example, the commonly used nickel-iron wire has a Curie point of 510°C. Differences between filament and Curie-point pyrolyzers depend on the pyrolysates examined and may be obscured by other instrumental differences, including the design of the transmission system to the detector. 

The Curie point measured for nickel is noticeably higher than the mean T3. This is because of the form of the loss of apparent weight. Figure 6 shows the several curves from a single participant in the ICTA test program. It is clear that the tailing off for nickel would yield a much greater difference between the Curie temperature and T3 than for any of the three alloys tested. 

In Curie-point pyrolysis the material is placed on an iron-nickel alloy foil which is heated to the Curie point of the foil (this is 530°C for 50 50 Fe-Ni foils). For a given type of foil, the Curie-point temperature is constant, therefore this type of pyrolysis is very reproducible. The foil holding the sample is rapidly heated to its Curie point by passing a radio-frequency current for 3 s (in the case of the Horizon 200-X instrument used at Aberystwyth) through a coil surrounding the foil. The foil takes around 0.5 s to reach this point at this temperature the material on the foil is thermally... 

Figure 5 illustrates temperature calibration curves for both DTA/DSC and TGA using high piuity nickel as a magnetic material (Curie point = 358.4°C). Unlike Tm, the Ciude point temperature is practically unaffected by the changing of the heating rate between 1 to 20°C/min. 

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