摘要:

Conventional ac susceptibility measurements and the conditions used in thermal analysis methods have been combined for normal‐superconductor transition studies. Accordingly, during the ac susceptibility measurements, the Y‐based sample is exposed to a temperature gradient. By controlling this gradient, the boundary between normal and superconducting region can be swept slowly at will across the specimen. The extended thermal scanning assures a high resolution to the new method. The observed fine details (even discontinuities) of the susceptibility spectra demonstrate its performance and basis.

ISSN:0034-6748    

DOI:10.1063/1.1144384

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A device meant as a supplement to a standard electron paramagnetic resonance spectrometer, permitting easy and accurate determination of theTc‐onset critical temperature superconductors, has been designed and constructed. The method of tracing microwave absorption changes permits detecting even very small amounts of the superconducting phase and determining theTc‐onset critical temperature with an accuracy of an order of 0.1 K.

ISSN:0034-6748    

DOI:10.1063/1.1144385

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The transverse magneto‐optical Kerr effect is employed for the measurement of in‐plane surface magnetization. An automatic and computerized system is used for the detection of the field that acts on the surface in samples that do not have a negligible demagnetizing factor. The demagnetizing field due to the bulk is corrected, obtaining the true magnetization process of the surface.

ISSN:0034-6748    

DOI:10.1063/1.1144386

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We report on a sensitive and reliable ac technique to measure the Seebeck coefficientSof materials, in particular of highTcsuperconductors. The small temperature difference between the ends of the sample allows structure in theS(T) curve to be observed. This technique avoids the creation of a component proportional todS/dTin the measured signal. Such a component has in the past led to erroneous conclusions regarding the high‐temperature superconductor YBCO. A mathematical analysis shows the origin of this component and how it can be avoided. These predictions are confirmed by experiments on a YBCO single crystal.

ISSN:0034-6748    

DOI:10.1063/1.1144387

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We describe measurements of the quality factors of resonances in fibers made of tungsten, silicon, sapphire, and fused quartz. The silicon and sapphire samples exhibited losses consistent with thermoelastic damping. Our fused quartz samples showed excess loss at some frequencies. The tungsten fibers had quality factors much lower than the thermoelastic prediction at all frequencies that we measured. We calculate the expected Brownian motion of pendulums made with similar fibers.

ISSN:0034-6748    

DOI:10.1063/1.1144388

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A fast‐response chromel‐alumel (and chromel‐constantan) microthermocouple is described and its dynamic characteristics are measured. The microthermocouple features a microdisk junction 0.08 mm in diameter and 2.5 &mgr;m thick. Its time constant in turbulent flow of liquid Refrigerant‐113 (a poor heat conductor) is measured to be ∼4.6 ms. This time constant was reduced to ∼3.4 ms with a phase‐lead compensation circuit. It was possible to distinguish between vapor and liquid phase temperatures in turbulent subcooled boiling flow of Refrigerant‐113 with the compensated microthermocouple.

ISSN:0034-6748    

DOI:10.1063/1.1144389

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We present an apparatus for controlling the deposition of thin films. The apparatus is based on a quartz resonator whose frequency is controlled digitally by a computer. Temperature variations of the quartz during the evaporation process, attributed to the exothermic heat of condensation and radiation heating from the evaporation source, are compensated. A very stable deposition rate and a good film thickness evaluation are obtained.

ISSN:0034-6748    

DOI:10.1063/1.1144339

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

An experimental technique to measure simultaneously the thermal diffusivity and thermal conductivity of small specimens of thermal‐insulator materials is described. Experimental conditions are developed which approach the mathematical idealization of a one‐dimensional, infinite, two‐component, composite solid whose planar interface is subjected to a step‐function heat flux. The heat flux is generated by resistive heating of a thin metallic foil. Although this principle has been exploited previously, the present technique permits specimens with volumes two orders of magnitude smaller than prior work based on the same idealization. A number of difficulties related to small specimen volume are identified and overcome. Another improvement over past efforts is the use of the nonlinear least‐squares analysis to determine the optimum values for diffusivity and conductivity. A comparison of measurements of these properties for acrylic and Pyrex glass with literature values suggests that the technique affords accuracies of about 5% for materials in this range.

ISSN:0034-6748    

DOI:10.1063/1.1144340

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

This article discusses recent developments in high‐temperature electrostatic levitation technology for containerless processing of metals and alloys. Presented is the first demonstration of an electrostatic levitation technology which can levitate metals and alloys (2–4 mm diam spheres) in vacuum and of superheating‐undercooling‐recalescence cycles which can be repeated while maintaining good positioning stability. The electrostatic levitator (ESL) has several important advantages over the electromagnetic levitator. Most important is the wide range of sample temperature which can be achieved without affecting levitation. This article also describes the general architecture of the levitator, electrode design, position control hardware and software, sample heating, charging, and preparation methods, and operational procedures. Particular emphasis is given to sample charging by photoelectric and thermionic emission. While this ESL is more oriented toward ground‐based operation, an extension to microgravity applications is also addressed briefly. The system performance was demonstrated by showing multiple superheating‐undercooling‐recalescence cycles in a zirconium sample (Tm=2128 K). This levitator, when fully matured, will be a valuable tool both in Earth‐based and space‐based laboratories for the study of thermophysical properties of undercooled liquids, nucleation kinetics, the creation of metastable phases, and access to a wide range of materials with novel properties.

ISSN:0034-6748    

DOI:10.1063/1.1144475

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

We describe a simple microwave‐produced plasma source that has been constructed for plasma‐assisted chemical vapor deposition of diamond thin films. Microwave power from a 700 W domestic microwave oven magnetron is fed into a water‐cooled cylindrical stainless steel vacuum vessel. A methane/hydrogen gas mixture introduced into the vessel is excited by the microwaves to produce a well‐defined plasma ball which does not interact with the walls of the vessel. The position of the plasma ball in the vessel can be predicted by cold cavity calculations of the axial electric field profile. The vessel has several diagnostic ports which do not alter its resonant condition. Diamond has been deposited on various substrates, placed beneath the plasma ball on a graphite‐capped quartz pedestal. Some of these results are presented and discussed.

ISSN:0034-6748    

DOI:10.1063/1.1144341

出版商:AIP    

年代:1993

数据来源: AIP