Introduction to Nanoscience and Nanotechnology. Chris BinnsЧитать онлайн книгу.
reverse the materials and produce films of Co nanoparticles in an Fe matrix as shown in Figure 1.11b.
A few years later this idea was confirmed and films with a magnetization exceeding the Slater–Pauling limit were produced [11]. The data are shown in Figure 1.12, which plots the magnetic moment per atom for Fe nanoparticles in Co matrices (red dots) and Co nanoparticles in Fe matrices (blue dots) compared to the Slater–Pauling curve for conventional Fe–Co alloys. Note that due to an accident of the units and the densities of Fe and Co, the conversion factor from magnetic moment per atom in Bohr magnetons (μB) to magnetic field produced in Tesla is very close to 1.0 so the two measures are often interchanged. The factors are 0.99 for Fe and 1.06 for Co so that the Co‐rich end corresponds to a slightly higher magnetic field than the value indicated in μB/atom.
Figure 1.12 High‐moment films produced by cluster beam deposition. Magnetic moment/atom in nanoparticle‐assembled films compared to the Slater–Pauling curve.
What the data clearly show is that the magnetization in the films of Fe nanoparticles embedded in Co matrices exceeds that of the Slater–Pauling curve till the density of nanoparticles reaches the percolation threshold at 25%. Then, the magnetization reduces to a value that is the weighted average of the Fe and Co bulk magnetic moments. This is due to the nanoparticles coming into contact and the Fe–Co interface reducing so that essentially there is a phase‐separated mixture. It is evident from the data for the Co nanoparticles in an Fe matrix, however, that a saturation magnetization of about 3 T can be achieved, which is significantly higher than the Slater–Pauling limit. The data in Figure 1.12 are from a material rather than isolated nanoparticles, but it is still in the form of a very thin film approximately 50 nm thick, that is, a long way from a bulk material. With big improvements in the flux from nanoparticle sources, however, there are now patented ideas to produce bulk quantities of this nanostructured alloy, which the author's group is working on at the Universidad de Castilla‐La Mancha in Spain. It is envisaged that this material will find its way into motors for all‐electric transport within eight years where it will produce at least a 20% improvement in the power to weight ratio of the motor.
This development has been labored as it is an excellent example of incremental nanotechnology moving from the lab into applications. This is just a single example and the method of producing materials shown in Figure 1.11 is a supreme way of making nanostructured materials in general. One can control the grain size independently of the volume fraction, there is free choice of the material in the grains or the matrix, and it is possible, using methods shown in Chapter 5, to make the nanoparticles out of more than one material in a range of motifs. These include alloy, core‐shell, and dumbbell nanoparticles, also called Janus particles as they have two different materials exposed at the surface. The method is likely to find its way into a range of applications in advanced materials.
1.3 The Mechanical Properties of Nanostructured Materials
Mechanical properties such as the strength of metals can also be greatly improved by making them with nanoscale grains. Several basic attributes of materials are involved in defining their mechanical properties. One is strength, which includes characteristics with more precise definitions but basically determines how much a material deforms in response to a force. Others are hardness, which is given by the amount another body such as a ball bearing or diamond is able to penetrate a material, and wear resistance, which is determined by the rate at which a material erodes when in contact with another. These properties are dominated by the grain structure found in metals produced by normal processing. An example of the grains structure of a “normal” piece of metal is shown in Figure 1.13a, which is an electron microscope image showing the grain structure of tin. Each grain is a single‐crystal with a typical size of about 20 μm (20 000 nm). The mechanical properties listed above are due to grains slipping past each other or deforming, so clearly what happens at the grain boundaries is very important in determining properties such as strength, etc. It is possible by various techniques including nanoparticle deposition (Figure 1.11), electro‐deposition, and special low‐temperature milling methods to produce metal samples in which the grains are a few nanometers across. An example is shown in Figure 1.13b, where, on the same scale as Figure 1.13a, the grain structure disappears to show a homogenous material. Blowing up the magnification a further 15 000×, however, (Figure 1.13c) reveals the new nanoscale grain structure. In this image, the individual planes of atoms are indicated by the sets of parallel lines and the boundaries are where the lines suddenly change direction as highlighted for one of the grains. Whereas in the coarse‐grained metal shown in Figure 1.13a, about one atom in 100 000 is at a grain boundary, in the nano‐grained equivalent, about a quarter of the atoms are at a grain boundary. Clearly, this change is going to have a marked effect on the mechanical properties of the material. Changes in mechanical properties with grain size were quantified over 50 years ago by Hall and Petch [12, 13], but the modern ability to control the grain size right down to the nanometer scale can produce significant increases in performance.
Figure 1.13 Grain size in nanostructured materials. Electron microscope images showing a comparison of the grain structure in conventional and nanostructured materials. (a) Conventionally processed material (tin) showing a typical grain size of about 20 μm. (b) Nanovate™ nanostructured Ni‐based coating produced by Integran Technologies Inc. On the same scale as (a) the material appears homogenous. (c) Increasing the magnification by a factor of 15 000 reveals the nano‐sized grains. The lines in the picture are atomic planes and the edges of the grains are revealed by changes in the direction of the planes as indicated for one of the grains.
Source: Reproduced with permission from Integran Technologies Inc. (http://www.integran.com).
Figure 1.14 Yield strength of aluminum alloys. Comparison of Deformation (Strain) vs. Load (Stress) for aluminum alloys with different grain sizes. A is a normal aluminum alloy (coarse‐grained). B – D Nanostructured aluminum alloy containing grains of size ~30 nm produced by various processes. The plastic limit occurs at the point where the slope changes and the nanostructured materials have a value that is up to four times higher than the conventional alloy.
Source: Reproduced with the permission of Elsevier Science from K. M. Youssef et al. [14].
The most dramatic improvement is seen in the “yield strength,” which quantifies the load a material can tolerate before it becomes permanently deformed. All metals are elastic under a small load, that is, when the load is removed they return to their original shape, while beyond a certain load they deform plastically and remain permanently changed. Figure