Cobalt layers

A composite material used for rock-drilling bits consists of an assemblage of tungsten carbide cubes (each 2 fcm in size) stuck together with a thin layer of cobalt. The material is required to withstand compressive stresses of 4000 MNm in service. Use the above equation to estimate an upper limit for the thickness of the cobalt layer. You may assume that the compressive yield stress of tungsten carbide is well above 4000 MN m , and that the cobalt yields in shear at k = 175 MN m . What assumptions made in the analysis are likely to make your estimate inaccurate ... 

Figure 1 shows the majority conductivity, C (I,J), and Figure 2, the minority conductivity, (j l,J), for the case in which the moments in the two cobalt layers are aligned. The total conductivity for parallel cobalt moments will be the sum of the conductivities shown in Figures 1 and 2. 

Figure 3 shows the calculated conductivity for one of the channels when the cobalt moments on either side of the copper layer are aligned anti-parallel. The spin channel for which the conductivity is shown in Figure 3 is locally the majority channel in the cobalt layer to the left of the copper (spin parallel to the Co moment) and locally minority to the right of the copper (electron spin anti-parallel to the local Co moments). The non-local conductivity for the other spin channel for the case in which the cobalt moments are antiparallel is the mirror image of the conductivity shown in Figure 3. 

Note that majority electrons that are accelerated by the electric field in one of the cobalt layers contribute to the current, not only in that layer (I = J) but in other layers as well, including the copper layers and the cobalt layers on the other side of the copper. On the other hand, minority electrons that are accelerated by a field in one of the cobalt layers contribute very little to the conductivity in the copper or in the cobalt on the other side of the copper. For anti-parallel alignment of the moments, electrons that are accelerated by the field in one cobalt layer contribute to the current in that layer and in the cobalt, but not in the other cobalt layer. The difference in the lolal current due to both channels between parallel and anti-parallel alignment is almost entirely non-local. It comes from those electrons that are accelerated by the applied electric field in one cobalt layer and propagate across the copper to the other cobalt layer where they contribute to the current. It is clear from Figures 1-4 that this process occurs primarily for majority electrons and for the case of parallel alignment. 

Figure 4 Giant Magnetoconductance. The change in the non-local layer dependent conductivity between parallel and anti-parallel alignment of the cobalt moments. For this case of strong scattering in both the cobalt and copper, contributions to the GMR come from electrons that are accelerated in one cobalt layer and contribute to the current in the other.

Figure 9 Majority Fermi surfaces for Co, Cu, and Co5Cu4. For values of kj greater than approximately 0.6 there are no allowed values of in Co. An electron in a copper layer with a value of k. greater than this value cannot scatter into the cobalt layers while conserving k,. The Fermi surface of Co5Cu4 shows two modes that are localized on the copper layers.

Figure 17.9. Maximum (saturation) value of the giant longitudinal magnetoresistance (GMR) in electrochemically grown Co/Cu multilayers as a function of Cu layer thickness. Cobalt layer thickness is held constant at 20 A per layer. The continuous curve is the corresponding RKKY function. (From Ref. 6b, with permission from the Electrochemical Society.)...

Figure 4.17 Regular layers Inside a cobalt nanoparticle larger than 20 nm in diameter, which are observed after the particle has been exposed to CO at the pressure of 1 bar and temperature 700 K. The light regions are the fine (approximately five atoms in thickness) hexagonal cobalt layers, dark region are the cubic cobalt layers [6].

When the CO disproportionation is catalyzed by cobalt, some ordered metastable structures are detected inside the active metal nanoparticles after the reaction. These structures are regular thin (approximately 5 atoms in thickness) alternating cobalt layers of different crystallographic modifications (Figure 4.17). Note that the appearance of such structures at thermodynamically equilibrium states of the catalyst substance is contrary to the Gibbs phase rule for the phase equilibria in solids. Thus, the metastable layered structures may be considered an analogue of spatial dissipative structures. 

To produce FGM with cobalt varied concentration a mixture was prepared of the following composition 64 % Ti -l- 16 % C -H 20 % Co. The mixture weighing 56 g was placed into a mold. Then cobalt powder was added. Three pellets were obtained with the diameter of 48 mm with various mass ratio of the mixture and cobalt layers 13/56 (0.23) 20/56 (0.36) 28/56 (0.5) correspondently. The relative density of the mixture layer was 0.58 of cobalt layer - 0.65. The SHS-densification was carried out in the reactional mold with the values of the delay time ti = 2 4- 5 sec pressure Pk = 30 MPa and time of exposure t2= 5 4-10 sec. 

Concentration profile of cobalt distribution throughout the sample thickness were constructed by means of micro-X-ray-spectral analysis (MXSA). The regime with the optimal correlation of the parameters mco/m(Ti-c) Pk ti t2 was determined. A complex of parameters was considered as the optimal one when the cobalt layer was melted at the expense of the heat of chemical reaction Ti + C -l- Co -> TiC + Co and... 

A detailed discussion of the symmetry properties of 0/Co(0001) is difficult since oxygen adsorbs in a disordered state on this surface. In the case of an ordered structure (O/Fe(001)) Huang and Hermanson [66] calculated an induced magnetic moment of oxygen to be about 0.24 Moreover, the photoelectron intensity differences (i.e. the MCDAD asymmetry) in the Co 3d band near the Fermi level decrease after oxygen exposure due to the chemical interaction of oxygen with the topmost cobalt layer. The reduced asymmetry displays a reduced magnetic moment of cobalt. 

With PPy as the substrate, Yan ef al. noticed interesting GMR behavior with a value of 4% in cobalt and copper multilayers, as shown in Figure 12.14 [65]. The copper layer serves as a spacer for cobalt layers, similar to a conventional metal electrode system. However, there is no report regarding conductive polymer layers as ferromagnetic layer spacers. The reported value is lower than that of the pure metal system. There were three proposed reasons for the difference the quality of the cobalt layer (with more copper in the ferromagnetic cobalt layers with a PPy substrate) the roughness of the PPy thin film (rougher compared than conventional metal) and the low conductivity of the PPy film used. 

However, Fig. 4 summarizes measurements by a number of investigators of the in situ hardness of the cobalt binder [9]. The hardness is plotted versus A being the thiekness of the cobalt layer where the hardness was measured, usually called the cobalt mean free path . The results in Fig. 4 satisfy the following Hall-Petch type relationship... 

The zinc, calcium and cobalt layered monoglycerolates act efficiently at loadings as low as 200 ppm. They show predominantly selective a-nucleation with nucleation occurring epitaxially in multiple directions, mainly from platelet edges. ... 

Boe] Boehmann, G., Klaus, E., Fritsche, G., Wagnre, W., Phase Structure of Boride Layers on Iron Substrates with Eleetroplated Cobalt Layers , Crysi Res. Techn., 22(7), 961-967 (1987) (ExperimentaL Morphology, 14)... 

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