Longqing Qiu

Liquid state nuclear magnetic resonance at low fields using a nitrogen cooled superconducting quantum interference device

Nuclear magnetic resonance (NMR) spectra of liquids were studied at fields from 470nTto65μT using a nitrogen cooled radio frequency superconducting quantum interference device. The authors demonstrated that low field NMR measurements with this device are feasible and may yield useful information. In particular, they determined the natural linewidth of distilled water to be 0.17±0.06Hz. In addition, they recorded J-coupled spectra of 2,2,2-trifluoroethanol in a measurement field regime that was determined to provide the best signal-to-noise ratio. Four peaks with frequency differences of about 2Hz were well separated.

Nuclear magnetic resonance in the earth’s magnetic field using a nitrogen-cooled superconducting quantum interference device

The authors recorded nuclear magnetic resonance (NMR) spectra of water, benzene, fluorobenzene, and 2,2,2-trifluoroethanol in the earth’s magnetic field (EMF) using a nitrogen-cooled superconducting quantum interference device (SQUID). In trifluoroethanol, the broadband detection characteristics of the SQUID with a noise floor of about 70fT∕√Hz enabled authors to simultaneously observe fluorine and proton spectra at 1940 and 2060Hz Larmor frequency, reflecting their heteronuclear J coupling in the high-field limit without showing a measurable chemical shift. To reduce the noise in EMF-NMR, the authors suggest the use of frequency-adjusted averaging, which compensates line broadening due to EMF fluctuations.

The freestanding sensor-based 3ω technique for measuring thermal conductivity of solids: Principle and examination

In recent two decades, the 3ω technique has been proven to be valuable for characterizing thermophysical properties of materials from nanoscale to bulk, but some inherent deficiencies in this technique such as laborious and repeated four-pad micro strip heater/sensor deposition process and flimsiness of the micro heater/sensor limit its practical applications. Here, the authors report a novel 3ω technique, based on a freestanding sensor replacing the conventional 3ω heater/sensor adjacent to the specimen surface. A zigzag temperature response curve of the new sensor instead of the classical straight line was observed and used to extract the specimen thermal conductivity. Experimental results which excellently agree with calculation values show that the new technique is of great application value to thermal properties characterization of amorphous bulks and hundreds of microns thick wafers.

Low-field NMR measurement procedure when SQUID detection is used.

In reported low-field nuclear magnetic resonance (NMR) measurements using Superconducting Quantum Interference Device (SQUID) detection, the pre-polarizing magnetic field has been usually oriented orthogonal to the measuring field, B(p) perpendicular B(m). Melton et al. systematically analyzed the consequences of B(p) decay in time after turnoff and showed that this decay should be nonadiabatic. We evaluated our measuring procedure in the light of that analysis, and found good quantitative agreement. It was showed that, when the decay time constant is comparable to the precession period of the magnetization of the sample, M, the optimum procedure is to orient B(p) parallel to B(m) and to apply a pi/2 pulse to flip M, similar as in the case of conventional NMR.

High-temperature superconducting quantum interference device with cooled LC resonant circuit for measuring alternating magnetic fields with improved signal-to-noise ratio

Certain applications of superconducting quantum interference devices (SQUIDs) require a magnetic field measurement only in a very narrow frequency range. In order to selectively improve the alternating-current (ac) magnetic field sensitivity of a high-temperature superconductor SQUID for a distinct frequency, a single-coil LC resonant circuit has been used. Within the liquid nitrogen bath, the coil surrounds the SQUID and couples to it inductively. Copper coils with different numbers of windings were used to cover the frequency range from <1 to nearly 100 kHz. A superconducting coil made of YBa(2)Cu(3)O(7-delta) tape conductor was also tested. With the LC circuit, the signal-to-noise ratio of measurements could be improved typically by one order of magnitude or more in a narrow frequency band around the resonance frequency exceeding a few kilohertz. The best attained equivalent magnetic field resolution was 2.5 fT/radicalHz at 88 kHz. The experimental findings are in good agreement with mathematical analysis of the circuit with copper coil.

Note: Non-destructive measurement of thermal effusivity of a solid and liquid using a freestanding serpentine sensor-based 3ω technique

A non-destructive thermal effusivity characterization method described as a freestanding serpentine sensor-based 3ω technique was reported. This freestanding serpentine sensor was fabricated by the mature flexible printed circuit production technique. Expression for the temperature response of the freestanding serpentine sensor with respect to the thermal effusivity of the test sample was presented. The technique was further verified by measuring four kinds of standard samples at room temperature. Experimental results which well agree with reference values demonstrate the new technique is of great application value to thermal effusivity characterization of solids, liquids, and structures to which the conventional 3ω technique is not applicable, e.g., solids with porous surfaces.

Tuned HTS SQUID-Detected Low Field MRI Using a Permanent Magnet for Pre-polarization With Automatic Transportation

We utilize a permanent magnet (PM) pair for sample prepolarization to improve the signal-to-noise ratio of low-field nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) measurements with a tuned high- Tc rf superconducting quantum interference device as a signal detector. The sample is first prepolarized in the gap of the PM pair and then automatically transported to the measuring position underneath the tuned superconducting quantum interference device in a magnetically shielded room. The transportation can be repeated in order to perform signal averaging. Because of the strong magnetic polarization field of about 1 T and the highly sensitive detector with a noise floor of about 7 fT/√Hz, the nuclear magnetic resonance spectrum of 0.5 ml fluorobenzene reaches a signal-to-noise ratio of about 13 in a single measurement, and 55 with 30 times averaging. The MRI images are acquired by using filtered back projection reconstruction. Twenty-four projections are obtained by recording free induction decay or spin echo signals with a gradient field applied at angular steps of 7.5° each. Two-dimensional low-field MRI images of pepper and carrot pieces were acquired with different numbers of averages. Here, a spatial resolution of about 0.3 mm × 0.3 mm is achieved.

The Effect of Low Frequency Disturbance to SQUID Based Low Field NMR

The influence of low-frequency magnetic field disturbances on SQUID based low field (LF) nuclear magnetic resonance (NMR) measurements is investigated. Two types of sinusoidal fields, a homogenous field and a linear gradient field, were applied as artificial disturbance sources. The influences on the free induction decay (FID) signals as well as on the spectra are discussed. The homogeneous disturbance field caused a frequency modulation of the FID signal. The measured spectra were found to be in good agreement with calculated traces obtained from a solution to the Bloch equation. The gradient disturbance field yielded an amplitude-modulated FID signal. In both cases, frequency mixing lines were observed. It is shown that for disturbances at the power line frequency and harmonics, the influence on the NMR spectra is negligible.

Multichannel ULF-MRI Study in Magnetic Unshielded Urban Laboratory Environment

Ultralow field (ULF) magnetic resonance imaging (MRI), which obtains images in the static magnetic field typically on the orders of tens to hundreds of microteslas, exhibits some potential advantages. However, the ULF-MRI system faces the challenge of poor signal-to-noise ratio (SNR). In addition to the introduction of a prepolarization technique and the usage of extremely sensitive superconducting quantum interference devices (SQUIDs), the multisensor parallel imaging technique can also improve the SNR and allow accelerated image acquisition. In this paper, a four-channel ULF-MRI system was built with low-Tc SQUID-based second-order axial gradiometer in an urban laboratory environment without magnetic shield. To suppress the temporal field fluctuation and to balance the environment gradient field tensor, an active compensation and a gradient field shimming system were developed. Using the Fourier imaging method and weighted image superposition, the 2-D and 3-D four-channel MRI images of pepper pieces were obtained at a measurement field of 129 μT, i.e., a Larmor frequency of 5.5 kHz with an in-plane resolution of about 2 mm × 2 mm. These results demonstrated the feasibility of performing 2-D and 3-D multisensor parallel ULF MRI in the unshielded urban environment.

SQUID based LF-NMR measurements in an urban laboratory environment

Nuclear magnetic resonance measurements were performed in an urban laboratory environment using a second-order low-Tc superconducting quantum interference device (SQUID) gradiometer. A static-active combined compensation system, consisting of three pairs of 2×2 m2 orthogonal squared Helmholtz coils, a 3 axis fluxgate, and home-made feedback electronics were constructed, and the low frequency variation of the environment was reduced by more than one order of magnitude; And its application in LF-NMR measurement was also discussed. A simple but efficient pre-polarization (Bp) coil was fabricated, and Bp exceeding 100 mT was achieved. NMR signals of proton were studied in both time and frequency domain, during both midnight and daytime. In the case of daytime measurement, the authors suggest the use of selective average, which can partly overcome the disadvantage of fluctuations of the ambient magnetic field.