摘要:

We present the design and evaluation of a novel precision balance consisting of a thin silica rod suspended on a beryllium copper spring flexure. At opposing ends of the rod are a capacitance sensor and a soleniod/magnet force actuator. When an external force is applied to the silica rod, the resultant deflection of the springs is detected and fed back through a servo controller and current amplifier to the force actuator, thus maintaining a null signal. Consequently, the coil current, which, for relatively small fields, can be considered proportional to its applied force, is monitored as the balance output. Three class I dead weight masses of 2, 5, and 100 mg were used for calibration with six less accurate weights used to verify the linearity of the balance to better than 1%. Using standard vibration isolation techniques and an inexpensive environmental chamber, the present design will measure loading forces up to 1 mN (100 mg) and it has a resolution of better than 70 nN Hz1/2(7 &mgr;g Hz1/2). Presently measuring 40 mm in diameter and 40 mm height this compact design is constructed from low expansivity materials giving a thermal sensitivity of better than 0.03 &mgr;N K−1. Results from a calibration of an atomic force probe are also presented and discussed.

ISSN:0034-6748    

DOI:10.1063/1.1144919

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

This paper describes a highly controllable, short‐range translation and rotation mechanism. Two solenoid‐magnet force actuators are applied to a simple elastic mechanism resulting in separable translation and rotation drive axes. To provide design rules and assess potential error sources, a theoretical model is discussed and experimentally evaluated. Two such actuators using monolithic aluminum and single crystal silicon flexures have been constructed and measured using stylus‐based, optical and x‐ray interferometric techniques. They are used to demonstrate both longer range motions of 80 &mgr;m and 3.6 mrad and higher precision motions, with resolution of better than 0.6 nrad over ranges of around 10 &mgr;rad across 1 &mgr;m displacement. Performance of these devices was limited primarily by the available drive electronics and measurement techniques, so it is expected that even better precision can be achieved.  

ISSN:0034-6748    

DOI:10.1063/1.1144920

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

By modulating the scanning tunneling microscope junction bias voltage at microwave frequencies, imaging and spectroscopy of insulating surfaces have become possible. In order to explore the spectroscopic capabilities of this instrument, we have developed a tunable microwave frequency alternating current scanning tunneling microscope. We combine the reliable beetle‐style sample approach with coaxial sample and tip contacts. This provides us with a stable microwave‐frequency‐compatible scanning tunneling microscope. This alternating current scanning tunneling microscope design is compatible with ultrahigh vacuum and low‐temperature operation.

ISSN:0034-6748    

DOI:10.1063/1.1144921

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A reduced chi‐squared test has been used to evaluate factors that affect the spatial resolving power of large array biomagnetic sensors for the brain. Realistic array geometries are used and a classical description of spatial resolving power is applied to determine when two separate sources may be resolved. Array parameters such as sensor spacing, coil diameter, and gradiometer type are varied to determine their effect on spatial resolving power. The consequences of the number of sensors is considered and a comparison of existing systems is made. The effects of the vector nature of magnetic sources on spatial resolving power is also considered. It is shown that spatial resolving power is not strongly dependent upon individual sensor diameter, but that sensor spacing is important. It is also found that the instrumental spatial resolving power as a function of depth degrades much more quickly when planar gradiometers are used, as compared to axial gradiometers.

ISSN:0034-6748    

DOI:10.1063/1.1144922

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new type of end‐loss‐ion energy analyzer (ELA; a multigridded Faraday cup) is applied to measure plasma ion temperaturesTipand plasma potentials &Fgr;Pin the GAMMA 10 tandem‐mirror device. As compared with conventional‐type ELA, this new ELA has obliquely placed ion‐ and secondary‐electron‐repeller grids and a collector plate with respect to external tandem‐mirror magnetic fields. One of the most useful capabilities of this new ELA for precise ion measurements is to greatly reduce the current of high‐energy electrons onto the collector plate. Also, this compact‐sized ELA is scannable in the plasma to obtain radial profiles ofTipand &Fgr;Pwithout providing any perturbations of the tandem‐mirror magnetic fields because it neither adds its own magnetic field nor needs to shield against the ambient magnetic field. Ion trajectories in the new ELA are numerically calculated so as to make a further understanding of its physics principles and to optimize its design. The energy‐calibration experiments for the new ELA are carried out using ion beams; a fairly good agreement in the analyzed beam energies from the new ELA data and from conventional‐type ELA data is attained. Furthermore, the first application of the new ELA to plasma diagnostics is made; a substantial reduction in energetic‐electron influxes onto real ion signals makes a significant improvement in the analyses ofTipand &Fgr;P.

ISSN:0034-6748    

DOI:10.1063/1.1144923

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The fabrication and behavior of an all‐carbon cathode for the growth of diamond films by dc discharge plasma chemical vapor deposition is presented. A graphite rod with a diameter of 2.5 mm was passivated in a methane dc plasma. During this process, the graphite was coated with pyrolytic carbon. Simultaneously, a thorn shaped protuberance (hereafter referred to simply as the thorn) of the same material grew from the end face of the rod towards the anode. The pyrolytic carbon prevented excessive sputtering of the cathode during diamond film synthesis in the hydrogen–methane plasma and the thorn presented a pointlike electron emitter providing a stable discharge. Using such cathodes, polycrystalline diamond films with well faceted crystallites have been grown at rates of approximately 5 &mgr;m/h. Nucleation and growth of individual, well faceted diamond crystallites even occurred on the cathode. When producing diamond films at methane concentrations above 1 vol %, a tubular form grows on the end of the cathode thorn.

ISSN:0034-6748    

DOI:10.1063/1.1144924

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new microcalorimeter for measuring heat capacity of thin films in the range 1.5–800 K is described. Semiconductor processing techniques are used to create a device with an amorphous silicon nitride membrane as the sample substrate, a Pt thin film resistor for temperatures greater than 40 K, and either a thin film amorphous Nb–Si or a novel boron‐doped polycrystalline silicon thermometer for lower temperatures. The addenda of the device, including substrate, is 4×10−6J/K at room temperature and 2×10−9J/K at 4.3 K, approximately two orders of magnitude less than any existing calorimeter used for measuring thin films. The device is capable of measuring the heat capacity of thin film samples as small as a few micrograms.

ISSN:0034-6748    

DOI:10.1063/1.1144925

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An isothermal scanning calorimeter controlled by linear pressure variations is described for the pressure range 0.1–400 MPa at temperatures from 303 to 573 K. The rate of pressure variations can be as low as 0.002 MPa/s over the whole pressure range. The functioning of the instrument was tested by measuring the coefficient of thermal expansivity of liquidn‐hexane, with calibration performed with gaseous nitrogen and by measuring the coefficient of thermal expansivity of benzene with calibration performed with liquidn‐hexane. The results are compared with literature results obtained by the piezothermal technique and with results obtained in the present instrument by the use of a pressure stepwise scanning mode. An example of an investigation of an isothermal solid‐to‐liquid transition in benzene is also given.

ISSN:0034-6748    

DOI:10.1063/1.1144926

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new ac calorimetry for dielectric solids was developed. A long and thin metal strip directly evaporated onto the surface of the sample is used as a heater and thermometer. Temperature oscillation of the heater is measured from the third harmonic voltage at the heater. Both the heat capacity and thermal conductivity of the sample are obtained by the frequency dependent phase and amplitude of the third harmonic voltage. Since heat capacity addenda and heat leaks in the measurement are extremely small, prior calibration is not required. The calorimeter is fully automated and has been tested on two ferroelectrics, KH2PO4and triglycine sulfate, at temperatures above 77 K. The optimal frequency region for the calorimetry is also discussed.

ISSN:0034-6748    

DOI:10.1063/1.1144927

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new digital circuit for fast and stable frequency tripling was constructed. The frequency tripler covers the input frequency range from 0.1 Hz up to 5 kHz with a maximum phase shift less than 1 deg at the highest frequency. The tripler generates a stable signal synchronized to the input after just one input period. Since the circuit design is flexible, input frequency range and phase accuracy can be improved using faster integrated circuits.  

ISSN:0034-6748    

DOI:10.1063/1.1144928

出版商:AIP    

年代:1994

数据来源: AIP