Ultra-low Field Research Articles

Facile synthesis of differently shaped, ultrathin, and aligned graphene flakes without a catalyst for highly efficient field emission

Aligned graphene flakes (AGFs) were prepared on different substrates without a catalyst by using radio frequency (rf) sputtering deposition. Their shapes can be readily controlled by adjusting substrate temperatures and rf powers. Ultrathin AGFs (less than 5 layers) can only be prepared with substrate temperatures higher than 1000K, and AGFs grown at 1100K are wrinkled graphenes. The rf power controls the AGF shapes by means of hydrogen plasma etching, and the growth rate of AGFs decreases with the increase of rf powers. The catalyst-free growth characteristic determines that the growth of AGFs is substrate independent, but their ultimate shapes greatly depend on the geometric configuration and surface topography of substrates due to the defect-guided nucleation of AGFs. The field emission properties of differently shaped AGFs and AGF composites were measured. Optimal field emission properties are obtained from AGF–Si nanowire composites. They have an ultralow turn-on electric field of 1.80V/μm, which for the as-grown Si nanowires is 7.33V/μm, and also have excellent field emission stability after being perfectly aged. We consider that both the nanosharp edges of AGFs and the high aspect ratios of Si nanowires are responsible for this excellent field emission performance.

Rotary scanning acquisition in ultra‐low‐field MRI

To develop a method of achieving large field of view (FOV) imaging with a smaller amount of data in ultra-low-field (ULF) MRI. In rotary scanning acquisition (RSA), data from the imaging object is acquired at multiple angles by rotating the object or the scanner. RSA is similar to radial-trajectory acquisition but simplifies the measurement and image reconstruction when concomitant fields are nonnegligible. RSA was implemented to achieve large FOV with only three localized superconductive quantum interference device (SQUID) sensors at the ULF-MRI field of 50 μT. Simulations suggest benefits of RSA, including reduced concomitant field artifacts, large FOV imaging, and SNR improvement. Experimental data demonstrate the feasibility of reconstructing large FOV images using only three SQUID sensors with 33% of the amount of data collected using a Cartesian trajectory. RSA can be useful in low-field, low-weight, or portable MRI to generate large FOV images with only a few sensors. Magn Reson Med 75:2255-2264, 2016. © 2015 Wiley Periodicals, Inc.

High <inline-formula> <tex-math notation="TeX">${\rm T}_{1}$</tex-math></inline-formula>- and <inline-formula> <tex-math notation="TeX">${\rm T}_{2}$</tex-math></inline-formula>-Contrast Ability of Ultra-Fine PEGylated Iron Oxide Nanoparticles in Ultra-Low Field MRI Measured By a High- <inline-formula> <tex-math notation="TeX">${\rm T}_{\rm c}$</tex-math></inline-formula> DC-SQUID

The effect of ultra-fine superparamagneticPEGylated iron oxide nanoparticles (SPIONs) on the 1 H nuclei spin relaxation time T 1 and T 2 were systematically investigated for the performance as T 1 or T 2 contrast agents in ultra-low field (ULF) nuclear magnetic resonance and resonance imaging (NMR&MRI). A high-T c dc-SQUID sensor was used to detect the resonance signal from water samples containing SPIONs of different sizes and concentrations. The relaxation rates (T 1,2 -1 ) was shown to depend linearly on the iron concentration (C, in mMol of Fe). The relaxivities (defined as d(T 1,2 -1 )/dC) r 1 and r 2 were fitted and compared with high field (3 T) NMR results. An impressive improvement of more than an order in r 1 was found for all SPIONs of different sizes, which leads to a much lowered relaxivity ratio r 2 /r 1 . For all SPIONs measured, r 2 /r 1 was around 1, which implies that SPIONs are high performance T1 contrast agent in ULF NMR&MRI. The highest r 1 and r 2 values were found in the sample containing 16.7 nm nanoparticles, both of them being around 200 mMol -1 s -1 . Furthermore, we also demonstrated T 1 -weighted contrast imaging using 16.7 nm nanoparticles. Clear intensity gradient were shown in the images, indicating a good contrast enhancement capability.

Superconductors in SQUID-Based Ultralow Field NMR—Flux-Trapping in Type-II Wires

Recent efforts in the development of ultralow field nuclear magnetic resonance (NMR) have focused on increasing prepolarizing field (B P ) strength. The strong B P , which is up to hundreds of milliteslas, is mandatory for high-quality NMR signals. However, B P needs to be completely removed within milliseconds so that spin relaxation signal measurement can take place before the sample magnetization wears off. In a previous study, where we compared three pick-up coils made of Nb, NbTi, and Pb, we found that only the pick-up coil made of Pb, which is a type-I superconductor, was unaffected by the strong magnetic field from the B P coil and produced a sharp NMR signal. The other coils made of type-II superconductors all began to suffer from a degraded NMR signal with B P above a certain threshold. Here, we show detailed measurement of magnetization loops of NbTi and Pb wires and argue that the counter pulse strategy suggested in our previous study can minimize the trapped flux and recover the spin relaxation signal. We argue that this counter pulse strategy can be applied to superconducting B P coils, so that the counter pulses can neutralize flux trapped inside the B P coil due to its strong internal field, thereby dramatically increasing the limit on the B P strength.

Effect of magnetic field fluctuation on ultra-low field MRI measurements in the unshielded laboratory environment

Magnetic field fluctuations in our unshielded urban laboratory can reach hundreds of nT in the noisy daytime and is only a few nT in the quiet midnight. The field fluctuation causes the Larmor frequency fL to drift randomly for several Hz during the unshielded ultra-low field (ULF) nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) measurements, thus seriously spoiling the averaging effect and causing imaging artifacts. By using an active compensation (AC) technique based on the spatial correlation of the low-frequency magnetic field fluctuation, the field fluctuation can be suppressed to tens of nT, which is a moderate situation between the noisy daytime and the quiet midnight. In this paper, the effect of the field fluctuation on ULF MRI measurements was investigated. The 1D and 2D MRI signals of a water phantom were measured using a second-order low-Tc superconducting quantum interference device (SQUID) in three fluctuation cases: severe fluctuation (noisy daytime), moderate fluctuation (daytime with AC) and minute fluctuation (quiet midnight) when different gradient fields were applied. When the active compensation is applied or when the frequency encoding gradient field Gx reaches a sufficiently strong value in our measurements, the image artifacts become invisible in all three fluctuation cases. Therefore it is feasible to perform ULF-MRI measurements in unshielded urban environment without imaging artifacts originating from magnetic fluctuations by using the active compensation technique and/or strong gradient fields.

Actively Shielded Bias Field Tuning Coil for Optically Pumped Atomic Magnetometer Toward Ultralow Field MRI

In this paper, we describe a method for directly detecting magnetic resonance (MR) signals using optically pumped atomic magnetometers (OPAMs) in ultra-low-field MR imaging (ULF-MRI). For direct detection with OPAMs in ULF-MRI, we propose the use of an actively shielded bias field tuning coil to match the resonant frequencies between MR signals and OPAM. Design of the tuning coil was based on both a target field approach and a stream function method. The results of field distribution analyses demonstrated that the resonant frequency of an OPAM could be tuned by the actively shielded bias field tuning coil. In addition, the actively shielded bias field tuning coil minimized the distortion of an external magnetic field in the sample region. These results indicate that actively shielded bias field tuning coil can be designed to match the resonant frequencies of MR signal and an OPAM with the inhomogeneity of sample region <;0.25%. However, the actively shielded bias field tuning coil must be fabricated with an error of 0.5 mm or less between the centers of the primary and shield coils.

Development of Ultra-low Field SQUID-MRI System with an LC Resonator

We are developing an Ultra-Low Field (ULF) magnetic resonance imaging (MRI) system using high temperature superconductor (HTS)-rf-superconducting quantum interference device (SQUID) for food inspection. The advantage of the ULF MRI system is that non-magnetic contaminants, which are difficult to be detected by a magnetic sensor, can be detected and localized. The system uses HTS-SQUID with high sensitivity that is independent of frequency, because the signal frequency is reduced in ULF. However the detection area of HTS-SQUID is difficult to be increased. Therefore, we studied to increase the detection area using an LC resonator. The LC resonator is composed of a coil (22.9 mH, Φ40mm inner diameter) and a capacitor (the setting resonance frequency of 1890Hz). The signal is detected by a copper wound coil of the resonator, and transferred to HTS-SQUID that inductively coupled to the coil immersed in liquid nitrogen at 77K. We combined the LC resonator with the ULF MRI system, and obtained the 2D-MR images. The signal detector, with the SQUID and the LC resonator, provided a 1.5 times larger detection area. The size of 2D-MR image was near the size of the actual sample. Then we obtained 2D-MR images by a filtered back projection (FBP) method and a 2D-fast fourier transform (FFT) method. In the 2D-FFT method, the pixel size of the image was smaller than that of image by FBP method. As a result, the quality of the 2D-MR image by 2D-FFT method has been improved. There results suggested that the system we are proposing is feasible.

T 1 relaxation measurement of ex-vivo breast cancer tissues at ultralow magnetic fields.

We investigated T 1 relaxations of ex-vivo cancer tissues at low magnetic fields in order to check the possibility of achieving a T 1 contrast higher than those obtained at high fields. The T 1 relaxations of fifteen pairs (normal and cancerous) of breast tissue samples were measured at three magnetic fields, 37, 62, and 122 μT, using our superconducting quantum interference device-based ultralow field nuclear magnetic resonance setup, optimally developed for ex-vivo tissue studies. A signal reconstruction based on Bayesian statistics for noise reduction was exploited to overcome the low signal-to-noise ratio. The ductal and lobular-type tissues did not exhibit meaningful T 1 contrast values between normal and cancerous tissues at the three different fields. On the other hand, an enhanced T 1 contrast was obtained for the mucinous cancer tissue.

Effect of synthesis temperature and N2/O2 flow on morphology and field emission property of SnO2 nanowires

A large amount of tin oxide (SnO2) nanowire arrays were synthesized on the flexible conductive carbon fiber substrate by thermal evaporation of tin powders in a tube furnace. The temperature, as well as the flow rate of the carrier N2 gas and the reaction O2 gas, plays an important role in defining the morphology of the SnO2 nanowires. Morphology and structure of the as-grown SnO2 samples are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). Results show that all the samples possess a typical rutile structure, and no other impurity phases are observed. The morphology changes from rod to wire with the increase of reaction temperature. Ratio of length to diameter of the nanowires increases first and then decreases with the flow ratio of N2/O2 gas. The optimum synthesis conditions of SnO2 nanowire are: reaction temperature 780 ℃, N2 and O2 flow rates being 300 sccm and 3 sccm respectively. In our growth process, the nanowire grows mainly due to the vapor-liquid-solid (VLS) growth process, but both the VLS process and surface diffusion combined with a preferential growth mechanism play the important role in morphology evolution of the SnO2.Field emission measurements for Samples 1-6 are carried out in a vacuum chamber and a diode plate configuration is used. Relationship between the growth orientation, aspect ratio, density and uniformity of the arrays and field emission performances will be investigated first. Results reveal that the field emission performance of SnO2 nanostructures depends on their morphologies and array density. The turn-on electric field (at the current density of 10 upA/cm2) decreases and the emission site density increases with tin oxide array density, and the turn-on electric field of Sample 5 (synthesized at 780 ℃, nitrogen and oxygen flow rates being 300 sccm and 3 sccm respectively) is about 1.03 V/m at a working distance of 500 m. By comparison, for the turn-on electric fields of the not well-aligned SnO2 nanowire arrays we have 1.58, 2.13, 2.42, 1.82, and 1.97 V/m at 500 m. These behaviors indicate that such an ultralow turn-on field emission and marked enhancement in (～ 4670) can be attributed to the better orientation, the good electric contact with the conducting fiber substrate where they grow, and the weaker field-screening effect. Our results demonstrate that well-aligned nanowire arrays, with excellent field-emission performance, grown on fiber substrate can provide the possibility of application in flexible vacuum electron sources.

Hyperpolarized Xenon Imaging with the SWIFT Approach in Ultra-low Field MRI: A Simulation Study

Hyperpolarized Xenon Imaging with the SWIFT Approach in Ultra-low Field MRI: A Simulation Study

Postmortem analysis of a failed liquid nitrogen-cooled prepolarization coil for SQUID sensor-based ultra-low field magnetic resonance

A liquid nitrogen-cooled prepolarization (<TEX>$B_p$</TEX>) coil made for ultra-low field nuclear magnetic resonance and magnetic resonance imaging (ULF-MR) designed to generate 7 mT/A was fabricated. However, with suspected internal insulation failure, the coil was investigated in order to find out the source of the failure. This paper reports detailed build of the failed <TEX>$B_p$</TEX> coil and a number of analysis methods utilized to figure out the source and the mode of failure. The analysis revealed that pyrolytic graphite sheet linings put on either sides of the coil for better thermal conduction acted as an electrical bridge between inner and outer layers of the coil to short out the coil whenever a moderately high voltage was applied across the coil. A simple model circuit simulation corroborated the analysis and further revealed that the failed insulation acted effectively as a damping resistor of <TEX>$R_{d,eff}=6{\Omega}$</TEX> across the coil. This damping resistance produced a 50 ms-long voltage tail after the coil current was ramped down, making the coil not suitable for use in ULF-MR, which requires complete removal of magnetic field from <TEX>$B_p$</TEX> coil within milliseconds.

Impact of pressure on high intensity colorless distributed combustion

In recent years, colorless distributed combustion (CDC) has been shown to provide ultra-low pollutants emission, enhanced stability, fuel flexibility and thermal field uniformity. To achieve CDC conditions, fuel–air mixture must be properly prepared prior to the mixture ignition. In this paper, the impact of moderate pressure increase on a test combustor is examined with emphasis on pollutants emission under near CDC conditions. Increase in pressure at constant mass flow rate resulted in a significant increase in NO emission and departure from CDC conditions. This is a result of reduction of the volume flow rate and injection velocity with pressure increase, as well as an increase in the apparent residence time of gases within the combustor. These conditions led to less than adequate mixing and more residence time of gases in the post flame zone to form NOx. CO emissions were reduced as longer residence time allowed for complete conversion of CO to CO2. Also, the increase in pressure enhanced chemical kinetics and suppressed dissociation. Increasing the combustor pressure with constant heat release intensity, where mass flow rates are increased to keep the apparent residence time constant, showed a slight increase (1PPM) of NO under premixed conditions and significant reduction of CO. OH∗ chemiluminescence showed that the reaction zone did not change with increase in pressure under premixed combustion. For non-premixed combustion, the reaction zone intensity increased and was more concentrated with increase in pressure to result in higher emissions as compared to the premixed case. In both cases, CO emission was significantly reduced due to the increased heat load and temperatures within the combustor. Relations describing change of NO and CO emissions with pressure were used to predict the combustor emissions at higher pressures. Difference between premixed and non-premixed modes revealed the need for more adequate mixing, necessary for CDC condition. Increase in pressure without considering flow velocities and flowfield can lead to departure from CDC conditions with simultaneous increase in emissions.

Ultralow field emission from thinned, open-ended, and defected carbon nanotubes by using microwave hydrogen plasma processing

Ultralow field emission is achieved from carbon nanotubes (CNTs) by using microwave hydrogen plasma processing. After the processing, typical capped CNT tips are removed, with thinned, open-ended, and defected CNTs left. Structural analyses indicate that the processed CNTs have more SP3-hybridized defects as compared to the pristine ones. The morphology of CNTs can be readily controlled by adjusting microwave powers, which change the shape of CNTs by means of hydrogen plasma etching. Processed CNTs with optimal morphology are found to have an ultralow turn-on field of 0.566V/μm and threshold field of 0.896V/μm, much better than 0.948 and 1.559V/μm of the as-grown CNTs, respectively. This improved FE performance is ascribed to the structural changes of CNTs after the processing. The thinned and open-ended shape of CNTs can facilitate electron tunneling through barriers and additionally, the increased defects at tube walls can serve as new active emission sites. Furthermore, our plasma processed CNTs exhibit excellent field emission stability at a large emission current density of 10.36mA/cm2 after being perfectly aged, showing promising prospects in applications as high-performance vacuum electron sources.

Multi-flux-transformer MRI detection with an atomic magnetometer

Recently, anatomical ultra-low field (ULF) MRI has been demonstrated with an atomic magnetometer (AM). A flux-transformer (FT) has been used for decoupling MRI fields and gradients to avoid their negative effects on AM performance. The field of view (FOV) was limited because of the need to compromise between the size of the FT input coil and MRI sensitivity per voxel. Multi-channel acquisition is a well-known solution to increase FOV without significantly reducing sensitivity. In this paper, we demonstrate twofold FOV increase with the use of three FT input coils. We also show that it is possible to use a single atomic magnetometer and single acquisition channel to acquire three independent MRI signals by applying a frequency-encoding gradient along the direction of the detection array span. The approach can be generalized to more channels and can be critical for imaging applications of non-cryogenic ULF MRI where FOV needs to be large, including head, hand, spine, and whole-body imaging.

Thermodynamics around magnetic phase transitions in alternating double-chain spin systems

The thermodynamics and quantum phase transitions of two typically alternating double-chain systems are investigated by Green's function theory. (i) For the completely antiferromagnetic (AFM) alternating double-chain, the low-temperature antiferromagnetism with gapped behavior is observed, which is in accordance with the experimental result. In a magnetic field, we unveil the ground state phase diagram with zero plateau, 1/2 plateau, and polarized ferromagnetic (FM) phases, as a result of the intra-cluster spin-singlet competition. Furthermore, the Grüneisen ratio is an excellent tool to identify the quantum criticality and testify various quantum phases. (ii) For the antiferromagnetically coupled FM alternating chains, the 1/2 magnetization plateau and double-peak structure of specific heat appear, which are also observed experimentally. Nevertheless, the M—h curve shows an anomalous behavior in an ultra-low field, which is ascribed to the effectively weak Haldane-like state, demonstrated by the two-site entanglement entropy explicitly.

Resonance magnetoplasticity in the EPR scheme under ultralow magnetic fields

Resonance relaxation displacements of dislocations in NaCl crystals exposed to crossed ultralow magnetic fields (static field B = 26–261 μT and radiofrequency field B ∼ 3 μT) in the electron paramagnetic resonance scheme are studied. The effect is also observed in the magnetic field of the Earth when crossed with a pulsed pump field with a resonance duration of ∼0.5 μs. Changes in the microhardness of crystals of ZnO, triglycine sulfate, and potassium hydrogen phthalate after their exposure to the magnetic field of the Earth and the orthogonally directed pump field are found out.

Design of Ring Core Fluxgate Magnetometer as Attitude Control Sensor for Low and High Orbit Satellites

We have designed a fluxgate magnetometer which utilizes the detection of ultra-low DC fields down to (even less) 3 nT. In addition to high sensitivity, it also has quite a large dynamical measurement range. The second harmonic voltage used as the sensor signal shows linear dependence on the applied field up to 1 Oe. We report some critical factors affecting the noise level and sensitivity of magnetometer. The core diameter and magnetic properties are some of them. We will report optimal core dimensions resulting in minimal electrical noise. During experiments, we have recognized that ring-shaped six-layered ribbon core must be well attached to the template, since otherwise, it may vibrate on its original position as a function of the excitation frequency. Thus, additional electrical noises become unavoidable.

A Novel Design of Fluxgate Magnetometer: An Insect like Configuration

In the present work, we propose a novel design of the fluxgate magnetometer which utilizes the detection of ultra-low DC fields down to (even less) 10 μOe. Measurement point has four eyes, like insects, with the diameter of each having 100 μm. It means that the magnetometer can be used for the magnetization analysis of local areas as low as 8,000 μ m 2. Sensing elements used in the sensor are in the form of metallic glass with high relative permeability (∼10,000) and low coercive field (<1 Oe). In addition to high sensitivity, it also has quite a large dynamical measurement range. The 2 nd harmonic voltage used as the sensor signal shows linear dependence on the applied field till almost 0.4 Oe. Such a large dynamical range facilitates its use in spacecraft applications, mine detection, earth skills, etc.

Kinetic inductance magnetometer.

Sensing ultra-low magnetic fields has various applications in the fields of science, medicine and industry. There is a growing need for a sensor that can be operated in ambient environments where magnetic shielding is limited or magnetic field manipulation is involved. To this end, here we demonstrate a new magnetometer with high sensitivity and wide dynamic range. The device is based on the current nonlinearity of superconducting material stemming from kinetic inductance. A further benefit of our approach is of extreme simplicity: the device is fabricated from a single layer of niobium nitride. Moreover, radio frequency multiplexing techniques can be applied, enabling the simultaneous readout of multiple sensors, for example, in biomagnetic measurements requiring data from large sensor arrays.

Intrinsic nanostructure in Zr2−xFe4Si16−y(x = 0.81, y = 6.06)

We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr2−xFe4Si16−y(x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric β-FeSi2-derived slabs are separated by Zr-Si planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these Zr-Si layers consist of 3.5 nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in x-ray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2μB or larger down to 1.5 K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat γ = C/T suggests that quasi-particle masses are not particularly strongly enhanced. The onset of superconductivity at Tc ≃ 6 K results in a partial resistive transition and a small Meissner signal, although a bulk-like transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nano-sized superconducting grains are separated by a non-superconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a potentially rich source of new bulk superconductors.