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.
Slide Scanner for Optimal Histological Examinations
With its unprecedented scanning speed and top-quality on-screen imaging, the new Leica SCN400 Slide Scanner offers an alternative to the microscope for the examination of histological samples in pathology, research, and teaching. The Leica custom tailored lens for a digital sensor, specially designed for high-res scans, ensures that the resolution and color fidelity of the image on the screen are just as good as that of the microscope image. Thanks to the Dynamic Focus principle, which keeps the sample in focus for the full duration of the scan, even difficult samples can be effortlessly digitised with the Leica SCN400. The Leica SCN400 is able to load and scan up to four specimens at a time. With a scanning rate of 100 secs per 15 x 15mm at 20x magnification, sample throughput is substantially increased. The matching Autoloader Leica SL801 is capable of scanning up to 384 samples at the same time, overnight if required, offering completely new options for automated operation. The user can keep loading new samples or remove finished scans without interrupting the process. Once a sample has been digitised, it can be easily retrieved, processed and made available to a defined group of users in a database. Thanks to digitisation, there are no more problems with bleaching specimens. The Leica SCN400 provides a quick and inexpensive way of sending digitised samples to associates and colleagues all over the world for mutual discussion, enabling users to obtain second opinions and meet the growing quality requirements in medical diagnosis. Besides saving time and money sending the valuable samples, the risk of broken slides is also eliminated. The Leica SCN400 opens up new avenues for interactive teaching. Students are able to watch a sample being examined in real time on the monitor. Interesting, rare or classic case studies can be easily and safely stored together with annotations for teaching purposes and retrieved by students via the Internet as needed.
ANALYTICA NEWS
Circle no. 417
Automated Nanoparticle Characterisation System in use at the University of Oxford
NanoSight announced the release of the NS500 system. The NS500 incorporates new hardware and software to deliver NanoSight’s growing capability in particle-by-particle characterisation, in an automated package. The first system is in use at the University of Oxford in the Nuffield Department of Obstetrics and Gynaecology of the John Radcliffe Hospital as part of a programme supported by the Wellcome Foundation.
NanoSight’s technology, known as Nanoparticle Tracking Analysis (NTA), provides a high-resolution particle size distribution, and not by DLS (dynamic light scattering). NTA detects individual; particles as small as 20nm and, in real time, simultaneously tracks and sizes whole populations. The result is a particle size distribution that provides researchers with the over view of their samples showing everything in the whole range 20 - 1,000nm. NTA also provides count and concentration, together with a unique view that validates these results.
The NS500 adds fluorescence capability, enabling the user to tune into individual particles, with sensitivity to detect individual quantum dots whilst eliminating background interference of other particles and media. Standard beads may be used to bind to single particles for optimum study.
The fluid handling capability of NS500 provides the user with auto sample presentation, optimal dilution and in-situ cleaning. It is now a routine process to clean the cell with the ability to purge, flush and load samples through user-customisable software. Dilution may also be controlled in this way. Ease of use is enhanced with an autofocus function for readily homing in on the particles in the laser beam. This is combined with two software-controlled motorised stages to ease use, a direct response to user market research. The temperature control of the cell offers a broader range (15˚C to 55˚C) arriving at the set point rapidly for faster sample measurement and turnaround time.
PITTCON NEWS
Circle no. 418
Microtechnology Focus
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