Figure 2. XRD data of niobium-carbonitride (a) sintered material (b) milled material (c) HIPed material. The broadened peaks in (b) signify a reduction in grain size from microcrystalline to the nanocrystalline scale length. The widths of the peaks in (c) have significantly reduced, indicating grain growth caused by hot isostatic pressing.
OPTIMISING THE NORMAL STATE RESISTIVITY
Increasing disorder in the structure of a material increases the normal state resistivity. This can be achieved by doping the material, which introduces impurities that increase electron scattering. However, it can also be achieved by decreasing long-range structural order. This latter method, pioneered in Durham University, employs a fairly well developed technology used for decreasing grain sizes of powdered materials, namely, mechanical milling.
MECHANICAL MILLING
Mechanical milling is a vast subject that entails fine- tuning a number of process variables to achieve decreased powder sizes, the formation of alloys or induced chemical reactions. The process requires a powder mix and a number of milling balls to be sealed within a milling pot. The choice of ball and pot material can be critical and in the case of niobium-carbonitride, niobium or tungsten-carbide milling media is used to reduce contamination. The pot is then spun in a planetary motion at 300rpm for varying time periods during which the powder is subjected to sustained bombardments from the balls and the sidewall of the pot. If the process variables have been chosen sufficiently carefully, the long- range order of the powder grains is completely destroyed and an amorphous form of the material is produced. Decreases in grain size can be detected using x-ray
MATERIAL PRODUCTION
Processing nanocrystalline niobium-carbonitride in the above ways is of course only part of the story. It is first necessary to fabricate the parent microcrystalline material itself. Furthermore, it is of paramount importance to be able to produce the very best material and insure that its high quality can be reproduced consistently and in adequate quantities. The superconductivity group in Durham University’s physics department have fabricated niobium-carbonitride with a transition temperature of ~ 17.6 K by mixing niobium-nitride and niobium-carbide together in the required proportions, pressing the mix into pellets and then sintering them at 1650o
C for 114
hours. They are now in the process of improving the procedure to achieve better homogeneity and the ability to fabricate increased quantities.
powder diffraction (XRD). The XRD data, which produces peaks at angles for which Bragg-diffraction occurs from the planes that constitute the structure of the material, can be seen to be broadened in Figure 2b. Figure 2a is an example of niobium-carbonitride in bulk form; sharp peaks are prominently visible. In Figure 2b the XRD peak widths have been broadened in comparison. This indicates that milling has caused grain-size reduction, though in this case not necessarily to a complete amorphous state since the presence of some peaks is indicative of some remaining crystalline structures. Once the milling is complete it is necessary to further process the material using a hot isostatic press to bring back some short-range, nanocrystalline, order. In this way, the resistivity of the material can be fine-tuned.
HOT ISOSTATIC PRESSING
A hot isostatic press (HIP) subjects the milled material to a pressure of 2000 bar and temperatures that vary from 400o
C to 1200o C. Different samples are processed in
this way at different temperatures and then measured to determine which temperature produces the optimised material. During HIP’ing the large pressure densifies the sample and the elevated temperature promotes grain growth, transforming the sample from its milled powder form into a solidified bulk with a nanocrystalline structure. This increase in grain growth and the return of short-range order is visible in the XRD data shown in Figure 2c. On comparison of Figure 2b and 2c it can be seen that the XRD peak widths have decreased after HIP’ing, which is indicative of grain growth and the return to a crystalline structure.
CONCLUSION
Niobium-carbonitride’s transition temperature and resistance to radiation damage make it a viable contender for consideration in superconducting magnet designs as long as its upper critical magnetic field can be substantially improved. It is believed that the processes discussed here will lead to that improvement.
The material, in milled powder form, would then be loaded into tubes and drawn into long coiled wires, which would be HIP’ed to complete the material’s optimisation.
The main prospective applications for such a conductor are in fusion reactors, where superconducting magnets are the enabling technology, and MRI scanners, where increased field strength leads to increased resolution.
REFERENCES
[1] Poole, C.P., H.A. Farach, and R.J. Creswick, Type II Superconductivity, in Superconductivity. 2007, Academic Press Inc: San Diego, California. p. 344.
[2] Troitskiy, V.N., et al., Synthesis and characteristics of ultra-fine superconducting powders in the Nb-N, Nb-N-C, Nb-Ti-N-C systems. Journal of Nanoparticle Research, 2003. 5(5-6): p. 521-528.
[3] Dewhughes, D. and R. Jones, The Effect of Neutron-Irradiation Upon the Superconducting Critical-Temperature of Some Transition-Metal Carbides, Nitrides, and Carbonitrides. Applied Physics Letters, 1980. 36(10): p. 856-859.
[4] Niu, H.J. and D.P. Hampshire, Disordered Nanocrystalline Superconducting PbMo6S8 with Very Large Upper Critical Field. Physical Review Letters, 2003. 91(2): p. 027002.
[5] Taylor, D.M.J., M. Al-Jawad, and D.P. Hampshire, A new paradigm for fabricating bulk high-field superconductors. Superconductor Science & Technology, 2008. 21: p. 125006.
World’s Fastest Desktop Scanning Electron Microscope
Phenom-World BV announced the launch of a new collection of sample holders and inserts for the Phenom™ desktop scanning electron microscope (SEM). The new holders increase the range of possible samples while maintaining the Phenom’s market-leading time to image. Quick and easy sample loading ensures faster time to data. Most industrial and research applications require imaging of non- or poorly-conducting samples. Imaging these samples with the charge reduction sample holder eliminates additional sample preparation and so reduces the critical time to data. Imaging samples such as paper, polymers, organic materials, ceramics, glass, and coatings is fast and trouble-free with this sample holder.
Imaging long, axial-shaped samples is a challenge in any SEM, but required in many industrial quality and failure applications. With the micro-tool sample holder, it is possible to make high-resolution images from samples such as drills, end-mills, routers, boring bars, engraving tools, needles, fibers, (fuel) injectors and pencils. Unique on the market, this holder enables top-down imaging of samples up to 100mm long. Samples are loaded into the holder without the need for tools or other preparation. The new micro-electronics insert enables non-destructive imaging of micro-electronics, solar cells and other wafer-based samples. The unique clamping mechanism makes glue or other adhesives obsolete, allowing the sample to be mounted quickly and then returned to the production process, or to be used in other quality and failure analysis machinery.
The X-view insert enables cross-sectional imaging of coatings, multi-layer semiconductors and fracture surfaces. Preparing the sample is fast and easy compared to costly and time- consuming resin mounting. The X-view insert eliminates the need for screws and tools to clamp the sample. The introduction of the new holders is the second stage in series of market- focused solutions following the successful launch of the Fibermetric™ System in 2009.
Circle no. 255 Temperature Controlled Microscopy Systems used for Geological Applications
Heating and freezing stages are being used in thousands of laboratories worldwide. Applications may be found in just about all scientific disciplines from materials to foods, from chemistry to physics and biology. One area that continues to grow is the use in geology to study properties of the Earth. One example may be found at Kingston University where Professor of Applied Geology, Andrew Rankin, and his research group use heating/freezing stages in the temperature range from -196˚C to +1500˚C to investigate fluid inclusion in rocks.
Fluid inclusions are small droplets of fluid that have been trapped within crystals either during primary growth from solution or at some later stage, usually as a result of recrystallisation along healed microfractures. They are ubiquitous in both naturally occurring minerals and in laboratory-grown crystals. To the chemist or materials scientist, these gross defects cause endless obstacles in their quest to grow near perfect crystals. However, to the geologist, they provide a unique fossil record of the various fluids responsible for the formation and evolution of rocks and minerals throughout the history of the Earth. Linkam systems have enabled the routine study of geological fluids as well as many other samples. Accurate temperature control of a lab-based experiment is vital. Setting up the experiments is straightforward too. The optical microscope may be used to visually record sample changes as a function of temperature in conjunction with the positioning capability of the temperature stage.
Circle no. 256
Microtechnology Focus
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