Laser-Heated High-Temperature Levitation NMR Probe
Please note: Due to the safety requirements involved in operating the high powered laser, used to heat samples in this probe, only qualified and trained personnel are permitted to perform experiments using this probe.
The laser-heated high-temperature levitation NMR probe is custom design and constructed for use in studies of molten refractory ceramics. The probe provides a unique levitation system allowing molten sample temperatures well in excess of 2000 oC, a temperature requirement far above capabilities of standard container based NMR heating systems. The design of this probe is focused primarily on the study of the properties of 27Al, therefore many of the materials and design considerations are specific to this operational requirement. In addition this probe has been outfitted with a gradient coil system, which allows for pulsed-gradient NMR diffusivity measurements.
Probe Overview:
To overcome the limitations of current modern NMR heating systems it was necessary to adopt a relatively new system of containerless heating for our NMR probe. In order to accomplish this an apparatus using a gas flow levitator was designed and constructed from boron nitride1 (figure 1). The objective of this system is to allow a spherical sample to be levitated in a gas flow (Ar used to date). Once the sample is suspended in a stable manner, within the gas flow, heating is performed using a CO2 laser. With a power level between 60 and 120W, the laser generated sufficient energy to heat the sample to a molten state, while maintaining a stable levitation, with no contact to the levitator walls2,3.
After completing design and construction of this levitator it was placed into a standard Saddle NMR rf coil (figure 2), commonly used for both solids and liquids NMR. The design of the levitator and rf coil were both set to high tolerances, in order to assure that the center of the levitated samples would be located in the center of the rf coil. Adapting a standard single channel NMR coil circuit (figure 5 ) into our design, we now have a fully functioning NMR probe for measurements on molten refractory ceramics (figure 6).
The Levitator:
The levitator design is that of a 10mm inner diameter tube made of boron nitride (BN), threaded at its lower end for mounting into the NMR probe (figure 1). The wall thickness of the tube is 1.5mm. After being installed into the probe a gas inlet base, also constructed of BN, is installed onto the lower portion of the threaded end. The gas inlet base provides entry for the levitation gas into the convergent-divergent nozzle constriction in the BN tube. The gas flows up from the bottom of the levitator tube to the inlet side of the nozzle, the sample is placed in the gas stream on the outlet side. The nozzle's beveled edges make angles of approximately 30 degrees from vertical in both the up and down directions. This results in an inlet and outlet cone apex of approximately 60 degrees. The nozzle is designed with a 2mm hole at its narrowest constriction, through which the levitating gas reaches its peak speed. The flow causes the sample to sit in a stable position for levitation, with an estimated 0.1mm (~100 micron) clearance around the samples outer surface. With properly adjusted gas flow the sample will maintain a stable levitator. Our levitation is monitored through use of a digital video camera, allowing us to see the sample within the NMR setup while molten. In addition to the video image a narrow27Al line also provides indication that levitation is stable2,3.
Figure 1: Boron Nitride levitator tube with gas inlet base and nozzle installed. Gray Sphere represents sample. Click image to enlarge.
The rf coil:
In NMR several different styles of rf coils can be adopted. In this situation a saddle coil design (figure 2) has been used. This type of coil is commonly used in both solids and liquids NMR. The coil used has a 19mm diameter and has windings of two loops per side (figure 3). Both windings are made from wrapping five continuous strands of 30 gauge magnet wire around the form in the desired configuration, then attaching them with a high quality, high temperature epoxy. With proper air flow through the probe, and a sample temperature around 2000 oC, the epoxy used must be able to withstand temperatures of around 125 oC.
Figure 2: Standard design of NMR saddle style rf coil, single loop. Click image to enlarge.
Figure 3: Winding pattern for two loop rf saddle coil. Pattern is of flattened coil form. Design: Varian/Chemagnetics. Click image to enlarge
Gradient Coil:
The gradient coil used in this probe is an add in unit designed to produce a magnetic field gradient across the 3mm diameter of the sample. The coil design is that of a Maxwell Pair (figure 4) placed outside of the rf saddle coil. Outside of this Maxwell Pair is a shielding coil, consisting of a counter wound Maxwell Pair, which provides shielding from the effects of the Maxwell Pair gradient field, outside of the coil region of the probe 4. This coil formation provides gradient fields on the order of ~0.25 T/m.
Figure 4: Winding and current directions for Maxwell Pair coil. R is radius of the windings, d is spacing between the centers of the upper and lower coils.
Circuit and Final Probe:
Once a properly designed and functioning levitator and rf saddle coil have been developed the NMR probe circuitry needs to be added and the probe constructed into the final from for use in the superconducting magnet. The saddle coil is incorporated into a standard NMR single channel probe circuit (figure 5). The rf signal generated by a transmitter is the input to this circuit. For tuning and matching capacitors adjustable trimming capacitors are used, in order to provide easy tuning to multiple frequencies. For tuning to the 27Al frequency of 78.134MHz, 1-16pF glass capacitors were incorporated. glass capacitors are required, as most non-glass tuning capacitors contain Al2O3, which will result in a substantial background signal. An additional inductor, L, is added in parallel to the rf coil to adjust the frequency tuning range as needed. As with all single-coil NMR circuits, when the rf coil is placed around the sample it becomes both a source of introduced rf radiation and a detector for the NMR signal generated by the precessing magnetization within the sample.
Figure 5: Single channel NMR coil circuit with capacitance ratings for current probe used in experiments involving 27Al and 31P. Click image to enlarge
With a completed NMR circuit and sample levitation system the electronics are mounted into a mounting system designed to hold the probe in its canister while placed in the superconducting NMR magnet (figure 6 ). In the design of this mounting base there were many things that had to be taken into consideration. The first of these is the proper cooling of the probe itself. Incorporated in the probe are two air flow vents, on located between the levitator tube and rf coil and another outside of the rf coil. These vents allow several hundred cubic feet per hour of air to flow through the probe and direct heat away from the probe and magnet. This is essential in preventing the probe and magnet from suffering damage as a result of the extreme temperatures at the sample. These temperature considerations also require that the probe be made out of materials that do not retain excessive amounts of heat, and are resistant to temperatures over 300 oC.
Also of critical importance is the location of electrical components within the probe. In the general design of an NMR probe the electrical components are places in positions designed to limit interference and maintain close proximity to the rf coil. In a high temperature design such as this the operating temperatures and placement of materials is also of equal importance. Special consideration had to be paid to positioning of solder joints, as well as items such as the tuning capacitors. A combination of proper placement and sufficient air flow prevent premature failure of the electrical components as a result of heat exposure.
Figure 6: Complete probe configuration incorporating levitator, rf coil and single channel NMR circuitry into mounting system. Click diagram to enlarge.
© 2002
