Resistive SQUID calorimetry at low temperatures

A resistive SQUID may be used to measure heat current. We call an RSQUID used in such a way an RSqUID: It acts as a heat-current to frequency (q–f) converter of sensitivity =f/q. Two methods of heat capacity measurement by the use of an RSqUID are described. In the direct method the number of cycles of oscillation when the temperature of the RSqUID and specimen is changed by T is counted. The measured heat capacity C _m (to be corrected for the addenda) is given by C_m=n/(T). In the step method the specimen and heater together are separated from the RSqUID by a thermal resistance R. The RSqUID is biased to a frequency f ₁. The heater is switched on and a number of cycles is obtained which represents the amount of heat required to bring the specimen to equilibrium in the new steady state, in which f=f₂+f. By this method the quantity C_m=n/(Rf) is obtained. In both methods the heat current is integrated by counting to obtain an amount of heat Q=n/, which may thus be measured very precisely if the number of cycles is sufficiently large [and (T) calibrated accurately]. The first RSqUID constructed for heat capacity measurement is described. In this RSqUID the high thermal resistance of press contacts has been avoided and soft solder as well as niobium used for superconducting material. The sensitivity varied from 0.8 to 0.4 Hz nW_–1 between 2 and 7 K. The results of measurements made between 2 and 7K by the step method with this RSqUID in a rather unsophisticated cryostat achieved a relative accuracy of ~0.1% with samples of mass about 1 g. Results of measurements made on samples of pure copper and indium are described.     

Electric current injection NDE using a SQUID magnetometer

We built a portable low-cost superconducting quantum interference device (SQUID) magnetometer to be used as the sensing element of an electric current injection NDE system. Using this system, we measured the magnetic fields associated with aluminum plate with single flaws in the millimeter range (2 to 8 mm), at lift-off distances up to ten times the size of the flaw, with injection currents in the order of a few amps. Because the system is at a relatively large distance from the test plate, the signal generated by the flaw is masked by the sinal generated by the edges of the plate. This is independent of the amount of current injected. Due to this, visual inspection is difficult when the distance from the sensor to the flaw exceeds five times its size. To intensify the magnetic field disturbance due to the flaws, we used digital image-enhancement techniques. Applying the enhancement algorithm, magnetic field disturbances were visualized at distances up to ten times the size of the flaw.     

Comment on pulsed NMR thermometry at very low temperatures

At temperatures below 1 mK the rf power in pulsed NMR experiments is normally kept as low as possible in order to minimize eddy current heating in the metallic samples used for thermometry. However, for metallic samples with short Korringa constants a temperature dependence enters via the spin-lattice relaxation mechanism in addition to the paramagnetic dependence. It may cause systematic errors if the temperature of the conduction electrons is raised by eddy current heating due to the action of the rf pulse to a temperatureT _e which is not very small compared to ₁, with being the Korringa constant and ₁ the rotation frequency of the nuclear moments in the rf field.     

NMR in superfluid3He at very low temperatures

Just a few years ago it was generally believed that the NMR properties of superfluid³He far below T_c would not be qualitatively different from those found at higher temperatures. Surprisingly, the strange enhancement of relaxation processes at around 0.4 T_c named catastrophic relaxation, was then found. Recently an extremely long lived induction signal with new dynamical properties was discovered in the region of 100 K. Some of the newly discovered properties can be partly explained, although there are still many unsolved problems. The main aim of this article is to raise new questions for future investigations at the lowest temperatures presently possible.     

Measuring miniature eye movements by means of a SQUID magnetometer

A new technique to measure small eye movements is reported. The precise recording of human eye movements is necessary for research on visual fatigue induced by visual display units.¹ So far all methods used have disadvantages: especially those which are sensitive or are rather painful.^2,3 Our method is based on a transformation of mechanical vibrations into magnetic flux variations. In order to do this a small magnet is embedded in a close-fitting soft contact lens. The magnetic flux variations caused by eyeball movements during fixation are measured by means of a SQUID magnetometer. The recordings show the typical fixation pattern of a human eye. This pattern is composed of three kinds of movements: saccades, drift and microtremor. The last-mentioned type of movements are displacements in the order of 2 μm. It is possible to distinguish between movements which are perpendicular to each other.     

The use of a SQUID magnetometer for middle ear research

A new technique is described for the measurement of vibrations in the temporal bones of an isolated middle ear. The precise recording of vibrations in the middle ear is of importance for the construction and improvement of a middle ear prosthesis.¹ The method of measurement is based on a transformation of mechanical vibrations into magnetic flux variations. This is performed by attaching a small piece of permanent magnetic material to the eardrum or middle ear ossicles. The magnetic flux variations caused by vibrations of the eardrum or ossicles during application of sound can be measured by means of a SQUID magnetometer.Measurements showed that it is possible to measure vibratory displacement amplitudes of the eardrum down to about 10^?10 m in a frequency range between 200 Hz and 10 kHz, although the acoustical and magnetometer conditions were not optimal. The method offers several advantages compared to already existing methods.^2–5,8     

Imaging of small defects in nonmagnetic tubing using a SQUID magnetometer

Using a recently developed superconducting quantum interference device (SQUID) magnetometer system which offers higher resolution and greater sensitivity than previous designs, we have created high-resolution spatial maps of the magnetic anomalies created by small defects in thin-walled, nonmagnetic tubes. Images obtained using three different sensing methods (injected current, induced current, and ferromagnetic decoration) are presented, and compared in terms of sensitivity and signal-to-noise ratio (SNR). All three methods possess the sensitivity to detect holes on a tube's outer diameter as small as 0.37 mm diameter×0.54 mm deep, with varying SNR, as well as holes on the tube's inner diameter as small as 1.0 mm diameter×0.3 mm deep. The ferromagnetic decoration method produces the strongest signals and excellent SNRs. Using this approach, we have detected inner- and outer-diameter slots with dimensions 0.76×0.15×0.08 mm³. In addition, we examine results of numerical inversion calculations applied to the experimental images. With inversion techniques it is possible to determine the original current density distribution given the measured magnetic field, hence revealing the approximate shape and location of defects.     

Optimization of dc SQUID voltmeter and magnetometer circuits

We calculate the signal-to-noise ratio in a dc SQUID system as a function of source impedance, taking into account the effects of current and voltage noise sources in the SQUID. The optimization of both tuned and untuned voltmeters and magnetometers is discussed and typical sensitivities are predicted using calculated noise spectra. The calculations are based on an ideal symmetric dc SQUID with % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaacqaHYoGycqGH9a% qpcaaIYaacbaGaa8htaiaa-LeadaWgaaWcbaacbiGaa4hmaaqabaGc% caGGVaGaeuOPdy0aaSbaaSqaaiaa+bdaaeqaaOGaeyypa0JaaGymaa% aa!3D23!\[\beta = 2LI_0 /\Phi _0 = 1\] and moderate noise rounding % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4baFfea0dXde9vqpa0lb9% cq0dXdb9IqFHe9FjuP0-iq0dXdbba9pe0lb9hs0dXda91qaq-xfr-x% fj-hmeGabaqaciGacaGaaeqabaWaaeaaeaaakeaaiiGacqWFOaakcq% WFtoWrcqWF9aqpcqWFYaGmcqaHapaCcaWGRbWaaSbaaSqaaiaadkea% aeqaaGqaaOGaa4hvaiaac+cacaGFjbWaaSbaaSqaaerbbjxAHXgaiu% GacaqFWaaabeaakiab-z6agnaaBaaaleaacaqGGaacbiGaaWhmaaqa% baGccqGH9aqpcaaIWaGaaiOlaiaaicdacaaI1aGaaiykaaaa!471A!\[(\Gamma = 2\pi k_B T/I_0 \Phi _{{\rm{ }}0} = 0.05)\], where ₀ is the flux quantum, T is the temperature, L is the SQUID inductance, and I ₀ is the critical current of each junction. The optimum noise temperatures of tuned and untuned voltmeters are found to be 2.8(L/R)T and 8(L/R)T (1 + 1.5² + 0.7⁴)^1/2/² respectively, where /2 is the signal frequency, assumed to be much less than the Josephson frequency, and is the coupling coefficient between the SQUID and its input coil. It is found that tuned and untuned magnetometers can be characterized by optimum effective signal energies given by (16k _B TLE/² R)[1 + (1 + 1.5² + 0.7²)^1/2 + 0.75²] and 2k_B T _iR_iB/² L _p respectively, where B is the bandwidth, R _i is the resistance representing the losses in the tuned circuit at temperature T _i and L _p is the inductance of the pickup coil.This work was supported by the Division of Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, and by the U.S. Office of Naval Research.Guggenheim Fellow.     

Observation of higher order NMR larmor lines by SQUID in solids at low magnetic field

We describe an apparatus allowing the observation of the NMR static longitudinal component of nuclear magnetizationM _z by a SQUID at low magnetic fields and atT=4.2 K in Plexiglas, Teflon, and CaF₂. The higher order NMR Larmor lines at 2f ₀, 3f ₀, and 4f ₀, wheref ₀ is the Larmor frequency of the allowed NMR line, were measured. These lines correspond to a flip of two, three, and four spins, respectively, by one photon. The amplitudes of 2f ₀ and 3f ₀ lines and their field dependences show that the common assertion that the forbidden lines at 2f ₀ and 3f ₀ appear in the same order of perturbation theory and that their intensity decreases with static magnetic fieldB ₀ asB ₀ ^–2 is wrong. In fact, the third Larmor line 3f ₀ appears at higher order than the second Larmor line 2f ₀ and its intensity decreases asB ₀ ^–4 . This is also shown by a theoretical calculation of its intensity. For the first time the NMR SQUID technique allowed the detection of the 4f ₀ line, corresponding to the simultaneous flip of four spins by one photon.     

A nitrogen triple-point thermal storage unit for cooling a SQUID magnetometer

In order to achieve turnkey operation, we plan to use cryocoolers to cool a SQUID magnetometer system. To minimize the magnetical and mechanical interference from the coolers, we intend to switch them off during the actual measurements. Consequently, a thermal storage unit (TSU) is required with sufficient capacity at an appropriate temperature (<77 K). In a feasibility study, we consider a load of 0.5 W from the SQUID sensor unit and an operating time of 10 h. To account for an increased load caused by the TSU itself, an overall capacity of 15 Wh is aimed at. The nitrogen triple-point is chosen because of the large latent heat involved in the transition from solid to liquid and the corresponding well-suited temperature (63 K). Furthermore, any safety risks involved with the use of nitrogen are small compared to alternatives. To contain the nitrogen, highly porous alumina is used. A structure was made in which layers of copper and porous material alternate, thus establishing a good thermal contact between the nitrogen and the casing of the TSU. Experiments show an overall capacity of the system around 85% of the expected theoretical value. Suggestions for improvements are given so as to arrive at a TSU capacity of 15 Wh.     

SQUID NMR studies of TmPO4

The hyperfine-enhanced nuclear moment in TmPO₄ was studied by SQUID NMR. A resonance frequency _x /2 = 274 MHz per tesla was obtained, leading to an enhancement factor of 77.9. The temperature dependence of the spin-lattice relaxation time T ₁ of ¹⁶⁹Tm nuclear spin in TmPO₄ was measured at liquid helium temperatures. The temperature dependence of T ₁ can be understood in terms of nuclear Orbach and direct processes.     

Fibre-epoxy composites at low temperatures

The thermal and mechanical properties of carbon, glass and Kevlar fibre reinforced epoxy composites are discussed, with particular reference to the behaviour of these materials at cryogenic temperatures. The effects of production techniques and various fibre arrangements are determined.     

Ranque effect at low temperatures

Methods and results of experimental research on the Ranque effect at a gas temperature reduced to 80° K.     

Heat conductivity of a beryllia-based composition at low temperatures

The thermal conductivity of beryllium oxide with additions of Y₂O₃, MgO, Fe₂O₃, TiO₂, A1₂O₃, SiO₂, ZrO₂, and glass was measured at 120–260°K. The additions were made in amounts up to 5 wt. %.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 6, pp. 980–985, December, 1980.     

Oxidation of TiC at low temperatures

The isothermal oxidation of TiC powders was carried out at low temperatures of 350–500 °C at oxygen pressures of 3.9, 7.9 and 16 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electro-microbalance. The oxidation kinetics are described by the one-dimensional diffusion equation. It was found that oxidation consists of four steps, I (fast step), II (slow step), III (fast step) and IV (slow step), at all the pressures. Two activation energies were obtained in steps II–IV: 125–150 kJ mol^–1 below about 420 °C and 42–71 kJ mol^–1 above that temperature. The low- and high-temperature oxidation mechanisms are discussed in connection with the formation of oxycarbide/titanium suboxides and the crystallization of anatase, followed by the generation of cracks in the grains.     

Deformation of silicon at low temperatures

The dislocation arrangements produced around microhardness indentations made in silicon at room temperature have been studied by transmission electron microscopy. Loops consisting of 30°- and 60°-dislocations are produced and move on the {111} planes. It is suggested that, during indentation, the theoretical shear strength is exceeded locally and that the observed dislocations arise as a result of the accommodation of the displacements due to block slip. On annealing up to 1030° C the loops do not appear to be mobile, rather new loops consisting of edge and screw components are formed which can move large distances.     

Strength of adhesives at low temperatures

The paper reports tests on the shearing strength of a range of adhesives at 77K.