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Meas. Sci. Technol. 11 (2000) 1703–1707. Printed in the UK PII: S0957-0233(00)16157-0
A novel high temperature NMR probe
I J F Poplett et al
containerless levitation systems using split-gap resonators
have been used [16–18].
Over the last five ...
A novel high temperature NMR probe design
the jacket and the internal structure of the jacket ensures
water circulates alo...
I J F Poplett et al
Figure 3. 17
O static spin-echo NMR spectra of sol–gel-produced
ZrO2 as a function of temperature meas...
A novel high temperature NMR probe design
5. Conclusion
A novel high temperature NMR probe based on optical
(IR) heating w...
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Transcripts - PoplettMESmithStrangeMeasSciTechnol2000v11p1703HighTempNMRZrO2

  • 1. This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: This content was downloaded on 29/07/2014 at 12:30 Please note that terms and conditions apply. A novel high temperature NMR probe design: application to 17 O studies of gel formation of zirconia View the table of contents for this issue, or go to the journal homepage for more 2000 Meas. Sci. Technol. 11 1703 ( Home Search Collections Journals About Contact us My IOPscience
  • 2. Meas. Sci. Technol. 11 (2000) 1703–1707. Printed in the UK PII: S0957-0233(00)16157-0 A novel high temperature NMR probe design: application to 17 O studies of gel formation of zirconia I J F Poplett†‡, M E Smith†§ and J H Strange‡ † Department of Physics, University of Warwick, Coventry CV4 7AL, UK ‡ School of Physical Sciences, University of Kent, Canterbury, Kent CT2 7NR, UK E-mail: Received 7 August 2000, accepted for publication 19 September 2000 Abstract. A novel optically heated high temperature, water-cooled NMR probe design is described in detail. The sample temperature is easily and accurately controlled to at least 1300 K. The probe characteristics are evaluated and the relative merits of the design discussed. Its operation is illustrated by in situ measurement of the 17 O static NMR spectra and spin–lattice relaxation times (T1) of gel-formed zirconia (ZrO2). A characteristic doublet of crystalline monoclinic ZrO2 is clearly observed at ∼700 K. These data represent the first in situ 17 O solid state NMR observation of the structural development of a sol–gel-produced oxide at elevated temperatures. Keywords: NMR probe, high temperature, optical heating, 17 O, zirconia. 1. Introduction Nuclear magnetic resonance (NMR) spectroscopy and relaxation methods are well established as valuable means of studying structure and atomic motion in solids [1, 2]. Phase changes, chemical transformations and associated changes of atomic mobility are important factors in determining properties of technologically important materials, and these often occur at elevated temperatures. To provide detailed structural information from NMR spectra often demands maximum resolution, which is usually achieved with magic angle spinning (MAS) [3]. Since the combination of MAS and elevated temperatures (at least in excess of 600 K) is difficult, a common approach is to quench the system and carry out room temperature MAS NMR experiments (e.g. [4, 5]). High temperature structural detail is often preserved by quenching but dynamic processes require observation at elevated temperatures. The ability of NMR to follow these processes non-destructively in situ has been applied to a number of cases. Examples of high temperature NMR spectroscopic studies include silicate melts [6, 7], site exchange in nepheline [8] and intermetallic alloys [9]. High temperature measurements of the NMR relaxation times are more widespread and examples include measurement of ionic transport in fluorides [10], in alkali silicate glasses [11] and in solid oxygen ion conductors [12]. Despite the fundamental physical importance of the temperaturedependenceofstructureanddynamicssuchwork is limited to laboratories that can provide specialist high § Author for correspondence. temperature NMR probes. Most commercial probes use heated gas flow to increase the temperature but this is really only efficient up to ∼800 K, although MAS probe designs for temperatures up to 1000 K exist [13]. Ready access to temperatures above 1200 K requires careful probe design with a high temperature furnace that will ideally allow the sampletemperaturestobechangedrapidly(particularlywhen chemicalreactionorgasevolutionisofinterest)butwithgood sample temperature stability and uniformity. NMR signals from solids are notoriously weak, often exhibiting broad lines with long spin–lattice relaxation times. Add to this the decrease in signal with temperature due to the Boltzmann factor describing the nuclear magnetic susceptibility, and increased thermal noise from heating part of the tuned NMR circuit and the signal-to-noise (S/N) ratio becomes an important consideration. There are a number of published designs for high temperature furnace enclosures suitable for NMR (e.g. see reviews [1, 14]). Designs usually employ a resistive heater element that surrounds the NMR receiver coil. Such elements can introduce significant unwanted noise into the sensitive tuned rf receiver circuit, and even when wound ‘non-inductively’ can also change the value of, and degrade the homogeneity of, the magnetic field. More complex probe designs exist, for example a compact furnace that fits inside the NMR coil [15], allowing the NMR coil to be cooled. There has been significant recent interest in laser heating samples as an alternative to resistive heating. Several designs of such probes exist including modified MAS probes, and for the very highest temperatures in excess of 2300 K 0957-0233/00/121703+05$30.00 © 2000 IOP Publishing Ltd 1703
  • 3. I J F Poplett et al containerless levitation systems using split-gap resonators have been used [16–18]. Over the last five years there has been a considerable growth in the study of oxide-based materials using 17 O NMR. This increase has come about through the realization that 17 O has a large chemical shift range, which makes it very sensitive to structural changes, and despite being a quadrupole nucleus (nuclear spin I = 5/2) in many ionic systems the quadrupole interaction is relatively small. 17 O NMR relaxation measurements provide information about the motion of the oxygen [12]. The natural abundance of 17 O is unfortunately low (0.037%) so that enriched samples are usually required—again placing a premium on S/N optimization of the probe system. There is considerable interest in using zirconia as a solid electrolyte, for example in solid oxide fuel cell applications [19]. Developing 17 O solid state NMR to study oxide behaviour in situ at elevated temperatures (up to at least to 1300 K) stimulated us to adopt a novel approach using radiant heat focused directly onto the sample. This paper describes a high temperature probe in which the infra-red radiation produced by two small 150 W halogen light bulbs (such as used in slide projectors) is focused within an optical cavity on to the NMR sample. The objective was to heat the sample directly and concentrate the heating on the sample, not its surroundings. To achieve a high coil filling-factor close contact with the rf coil is desirable and this leads to heating of the coil. The probe design presented here could easily be adapted to maintain the coil at a low temperature, for example by cooling the coil as in [14]. With just a standard wire coil sufficient S/N was achieved for both the spectra and the relaxation time measurements. By not cooling the coil smaller temperature gradients over the sample result compared to if the coil had been cooled. Sample temperatures of up to 1300 K are readily achieved. 17 O static NMR spectra and spin–lattice relaxation time measurements in sol–gel prepared nanocrystalline zirconia (ZrO2) are reported as an illustration. Such measurements permit structural development and oxygen mobility to be observed in situ by NMR. 2. High temperature probe design A water-cooled enclosure to fit to the end of a conventional probe body has been designed to allow safe, extended use in the air gap of a (wide bore, 89 mm) superconducting solenoidal magnet. The total probe length is 510 mm and maximum diameter 72 mm, allowing it to fit into the room temperature shim set of the magnet. The enclosure is shown schematically in figure 1. It is cylindrical with an overall length of 132.5 mm and diameter of 50 mm. The main body was machined in four parts from copper and these are held firmly together by three stainless steel screw-threaded locating rods. The two end pieces are constructed to contain part of a 25 mm radius spherical mirror. These fit to the two centre sections, which themselves each form a conical reflector of angle 7◦ . Parabolic geometry would have been ideal but the geometry used was much simpler to machine accurately and produces efficient focusing. Figure 1. Schematic diagram showing the high temperature optically heated NMR probe and its water jacket. (Approximately to scale.) The sample, contained in a silica tube, is held in the rf transmitter/receiver coil located at the centre of the enclosure with its axis perpendicular to the enclosure axis (and main magnetic field B0). The 150 W/15 V halogen light bulbs (Philips type 6550) are located at a distance of 17 mm from each mirror with the filament on the optic axis. Light from both bulbs is then focused directly onto the sample in the coil. The light bulbs are each mounted in two brass bars fitted into a polyamide housing, thermally insulated from reflected radiation by a boron nitride plate. The bulb filaments are aligned along the optic axis, parallel to the magnetic field. The filaments are liable to distortion from the interaction of the filament current and magnetic field gradient, and the alignment minimizes this. The two bulbs are in series and connected to a 30 V/20 A DC power supply. The internal surfaces of the copper enclosure are highly polished. Copper has a reflectivity of 97% for infra-red radiation, and although this could be improved to 99.5% by gold plating the surfaces, plating proved unnecessary as sample temperatures in excess of 1300 K are achieved when full power (150 W) is applied to both light bulbs. The copper heating enclosure is surrounded, except for the 12 mm gap (see end projection) for power, thermocouple and rf connections, by a cylindrical brass water jacket, 132.5 mm in length and external diameter of 63.5 mm. The water inlet and outlet are located at the same end of 1704
  • 4. A novel high temperature NMR probe design the jacket and the internal structure of the jacket ensures water circulates along its full length. Sample temperatures are measured using a platinum/10% rhodium–90% platinum thermocouple placed close to the sample (but not within the coil as this introduced excessive noise). The sample temperature, relative to this position, was calibrated before making measurements using another thermocouple placed within the sample volume. A 12 turn coil was constructed from 1 mm diameter platinum wire, one end of which is earthed by a screw to the inside copper wall of the probe, the other being fed through the probe wall to a screw junction inside a copper shielding box from which a rigid coaxial lead emerged along the 12 mm slot in the water jacket. The coil’s dimensions are approximately 10 mm diameter and 25 mm in length. The coaxial lead terminated with a variable 25 pF parallel tuning capacitor and a series matching capacitor (Jackson Brothers, high voltage type 620) located in the probe body immediately under the water jacket. 3. Other experimental details The experiments employed conventional widebore 7 T superconducting magnets where the 17 O resonance frequency was ∼40.7 MHz. The rf performance of the probe at ambient temperature was evaluated by measuring 17 O in tap water, which also provided the chemical shift reference at 0 ppm. Chemagnetics Infinity and Bruker MSL spectrometers were used. A zirconia gel was prepared by taking Zr(OPri )4.Pri OH dissolved in propanol and adding water dropwise so that the final H2O:zirconium ratio was 3:5 (a slight excess of water). 46 at.% 17 O-enriched water was used in the hydrolysis. The mixture was left for one day and then vacuum dried. The resulting powder was then used for the in situ studies. In order to obtain good spectra the resonance was observed using an echo sequence 90◦ –τ–180◦ with extended phase cycling to suppress quadrature images, ringing effects and direct magnetization [20]. Typical echo times were 40 µs and the recycle period used was 1 s. T1 was measured using the saturation-recovery method, with a saturation comb consisting of fifty 90◦ pulses, separated by ∼1 ms (the T ∗ 2 for 17 O above 500 K) with the signal subsequently recorded by an Oldfield echo sequence [20]. Recovery delay times ranging from 10 µs to 60 s were used to check for multiple component exponential behaviour. Particularly at the higher temperatures the reduced S/N required 24 hours to complete a T1 determination. 4. Results and discussion 4.1. Probe performance The principal criteria in designing the probe were the highest temperature achievable and the ease of use. Putting the high temperature enclosure in a standard probe body meant that it could be located quickly and easily in the room temperature shims of the magnet. There was no need for any special atmosphere and the water jacket kept the outside of the probe close to ambient even at the highest operating temperatures. The power versus temperature Figure 2. The temperature measured in the NMR coil as a function of the power applied to each bulb. performance is shown in figure 2, the sample temperature reaching ∼1300 K with 150 W applied to each lamp. To calibrate the system two thermocouples were used, one in the normal operating position at the edge of the enclosure and the other located within the quartz tube embedded in MgO to simulate a sample. The temperature difference between the thermocouples depended on the operating temperature and was a maximum of 120 K at 1300 K. It was found that the temperature generated at a particular power was reproducible, varying by only ±8 K. The water flow rate had a negligible effect on the temperature generated in the sample space. The low thermal mass of the system meant that the thermal response was rapid (maximum temperature was reached and stabilized within ∼30 minutes). The temperature gradient was determined by moving the test thermocouple along the axis of the tube. For 1300 K (at the sample position) measuring along the axis of the coil compared to the centre of the coil ±6 mm was 14 K lower and at inner wall of the optical cavity it was 120 K less, which was regarded as an acceptable temperature gradient. This temperature difference was measured at 11 temperatures, decreasing with decreasing operating temperature, and at a given operating temperature was highly reproducible. Hence NMR measurements were always made without the thermocouple embedded in the sample reducing the level of rf noise introduced. The probe circuit had a quality factor of ∼120 and a tuning range with the coil described above of 32.7 to 52.8 MHz. The tuning range was easily extended by winding appropriate coils. The 90◦ pulse determined for 17 O in water was ∼18 µs, corresponding to an rf field of ∼10 mT. For a natural abundance water sample confined to the central ∼1 cm of the coil using a 90◦ pulse with 256 scans an S/N of 30:1 was achieved. For experiments on solid samples the pulse lengths were reduced by a factor of (I + 1 2 ) to take into account quadrupole effects. By monitoring the 17 O resonance from water when current was first passed through the bulbs there was no noticeable change in the linewidth or the resonance position. This indicates that the bulbs are not interfering with the magnetic field, which is very important for NMR applications, and can be a problem in resistively heated probes. 1705
  • 5. I J F Poplett et al Figure 3. 17 O static spin-echo NMR spectra of sol–gel-produced ZrO2 as a function of temperature measured in situ. 4.2. 17O NMR data for zirconia as a function of temperature Thestatic17 ONMRspectraobtainedinthetemperaturerange from ambient to 700 K are shown in figure 3. The S/N in figure 3 was obtained after averaging for ∼1 hour. At 300 K the spectrum is a composite of a 7.4 kHz line on top of one ∼30 kHz wide. At 395 K the prominent line is still over 7 kHz wide and there may be some residual underlying broad component but the S/N is not sufficient to confirm this. Progressively heating to 475, 575 and 600 K there is a continual narrowing to ∼4 kHz (table 1). Further heating causes a slight increase in linewidth until 700 K is reached when the line splits into a doublet consisting of two much narrower components each ∼1.7 kHz wide. Throughout this temperature range T1 decreases with increasing temperature (table 1). The S/N meant that the data could only reasonably be fitted to one component (i.e. on the basis of the data one was unable to unambiguously fit two components). At the higher temperatures the long averaging times necessary demonstrated the high long term stability of the probe. The static NMR spectra reveal some structural detail and comparison to ex situ MAS [21, 22] is very interesting. In the initially formed gel many protons and residual organic groups from the alkoxide precursor are known to exist [22]. The Table 1. Summary of in situ 17 O NMR measurements on sol–gel-produced zirconia. Temperature (K) Shift (ppm) Linewidth (kHz) T1 (s) (±5 K) (±5 ppm) (±0.2 kHz) (±0.05 s) 300 310 7.4 (30.6) N.D. 395 332 7.3 N.D. 475 345 6.2 0.31 575 368 4.7 0.29 600 381 4.0 0.29 620 380 4.4 0.20 660 390 4.2 0.17 700 413, 347 1.7, 1.8 0.11 N.D.—not determined. presence of these groups produces a high degree of structural disorder, and in static spectra also causes spectral broadening through unaveraged dipolar interactions. These effects probably cause the broad underlying resonance. The more prominent sharper resonance probably results from Zr–O– Zr-related groups in precursor ZrO2. Heat treatment removes hydroxyls and organic fragments, resulting in the loss of the broader component and the reduction in the width of the main component as the gel, although still amorphous, becomes less disordered. The line then broadens just before initial crystallization, probably related to structural rearrangement. MAS studies [21, 22] have shown that the gel before crystallization contains two peaks at ∼410 and 320 ppm, close to the position of crystalline monoclinic [23]. These peaks must be contained here within the one much broader resonance. Multiple magnetic field studies have shown that although 17 O is a quadrupolar nucleus in highly ionic materials such as ZrO2 quadrupolar broadening is negligible [24]. The static linewidth here is probably a combination of chemical shift anisotropy, chemical shift distribution (from the structural disorder) and dipolar coupling. Both x-ray diffraction using synchrotron radiation and 17 O MAS NMR show crystallization of identically prepared zirconia gels at 655 K [22]. 17 O MAS NMR shows formation of tetragonal zirconia (which gives an intermediate isotropic 17 O chemical shift at 378 ppm) and a large decrease in linewidth (a factor of 3–4) on crystallization [22]. The samples usually have complex structures with residual unconverted gel, and crystalline tetragonal and monoclinic regions. Here the static spectrum shows very little change at this temperature. Crystallization into a nanocrystalline sample is characterized by an increase in the degree of local order. The linewidths of MAS NMR spectra are often dominated by the distribution of isotropic chemical shifts. The implication of the relatively small change in the static NMR linewidth at the crystallization temperature is that the anisotropic interactions must make the dominant contributions to the linewidth and these do not change much. Also as these samples are often mixtures of tetragonal and monoclinic their peaks overlap in static NMR spectra. However the monoclinic component of the sample at this point can be observed from the two peaks at 413 and 347 (±5 ppm). This is the first in situ direct observation by 17 O NMR of conversion of a gel to a crystalline sample. 1706
  • 6. A novel high temperature NMR probe design 5. Conclusion A novel high temperature NMR probe based on optical (IR) heating within a reflecting optical cavity has proved to be an effective design, allowing measurement of the static spectrum and relaxation times of 17 O. In principle this method of sample heating could be adapted for use with other NMR probe designs since the sample is heated directly by radiation. The method is economical in heating power, with low heating loss, producing temperatures up to 1300 K and avoids additional magnetic fields. It is a significantly cheaper design than comparable laser heated probes. Its operation has been demonstrated by the first successful in situ measurements of 17 O NMR spectra and relaxation times of a gel-formed oxide over the temperature range 300 to 700 K. Formation of polycrystalline monoclinic ZrO2 was clearly observed by the technique. Acknowledgments The authors thank EPSRC for the award of grant GR/K74876 which provided most of the funds for this development. The authors also thank the Universities of Kent and Warwick for their support in purchasing NMR equipment and MES also thanks the EPSRC for support of NMR equipment. DrMGTuckeristhankedforhishelpinpreparingthesample. References [1] Strange J H 1987 Cryst Latt. Defect Amorph. Mater. 14 183 [2] Brinkmann D 1992 Prog. NMR Spectrosc. 24 527 [3] Andrew E R 1981 Int. Rev Phys. Chem. 1 195 [4] Dirken P J, Smith M E and Whitfield H J 1995 J. Phys. Chem. 99 395 [5] Eckert H 1992 Prog. NMR Spectrosc. 24 159 [6] Farnan I and Stebbins J F 1990 J. Am. Chem. Soc. 112 32 [7] Maekawa H and Yokokawa T 1997 Geochim. Cosmochim. Acta 61 2569 [8] Stebbins J F, Farnan I, Williams E H and Roux J 1989 Phys. Chem. Minerals 16 763 [9] Bastow T J, Smith M E and West G W 1997 J. Phys.: Condens. Matter 9 6085 [10] Figueroa D R, Chadwick A V and Strange J H 1978 J. Phys. C: Solid State Phys. 11 55 [11] Ali F, Chadwick A V, Greaves G N, Jermy M C, Ngai K L and Smith M E 1995 Solid State NMR 5 133 [12] Fuda K, Kishio K, Yamauchi S and Fueki K 1985 J. Phys. Chem. Solids 46 1141 [13] Doty F D 1990 Abstract Rocky Mountain Conf. on Analytical Chemistry (Washington) vol 32 [14] Stebbins J F 1991 Chem. Rev. 91 1353 [15] Adler S B, Michaels J N and Reimer J A 1990 Rev. Sci. Instrum. 61 3368 [16] Massiot D, Trumeau D, Touzo B, Farnan I, Rifflet J C, Douy A and Coutures J P 1995 J. Phys. Chem. 99 16 455 [17] Florian P, Massiot D, Poe B, Farnan I and Coutures J P 1995 Solid State NMR 5 233 [18] Robert E, Lacassagne V, Bessada C, Massiot D, Gilbert B and Coutures J P 1999 Inorg. Chem. 38 214 [19] Steele B C H 1995 Solid State Ionics 75 157 [20] Kunwar A C, Turner G L and Oldfield E 1986 J. Magn. Reson. 69 124 [21] Bastow T J, Smith M E and Whitfield H J 1992 J. Mater. Chem. 2 989 [22] Chadwick A V, Mountjoy G, Nield V M, Poplett I J F, Smith M E, Strange J H and Tucker M G Chem. Mater. submitted [23] Bastow T J and Stuart S N 1990 Chem. Phys. 143 459 [24] Bastow T J, Moodie A F, Smith M E and Whitfield H J 1993 J. Mater. Chem. 3 697 1707

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