摘要:

For accurate determination of the activation energy of a deep‐level trap by deep‐level transient spectroscopy (DLTS), it is desirable to measure the thermal emission rate over at least two orders of magnitude. Thus, the least time required to acquire DLTS capacitance data over this range will be directly proportional to 100 times the minimum response time of the capacitance meter employed. When an unmodified Boonton 72BD capacitance meter is overloaded by a strong stimulating pulse, erroneous signals caused by the recovery of its overload detection circuitry and 1‐MHz amplifier will be superimposed on the first 200 ms of a transient waveform. Therefore, accurate data would require that each transient be captured for times as long as 20 s. In this paper, simple modifications to this instrument are described which significantly improve data‐acquisition speed and data quality. This is accomplished by eliminating overload recovery delays and by minimizing noise produced by digital circuitry. Specifically, overload recovery delays are avoided by installing a fast relay which grounds the input of the 1‐MHz amplifier during application of the stimulating pulse and by deactivating the internal overload detection circuitry. The noise produced by digital circuitry is reduced by stopping the multiplexing of the light‐emitting diode display.

ISSN:0034-6748    

DOI:10.1063/1.1137716

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A microcomputer‐controlled data‐acquisition system which is readily adapted to a wide variety of pulsed laser applications is described. Signals are sampled by commercially available gated integrators. Multichannel operation allows data to be normalized to a reference beam intensity, a necessity due to the pulse‐to‐pulse fluctuations of many pulsed lasers. Data are acquired and stored independently for each pulse. The advantages of postacquisition processing versus pulse‐to‐pulse averaging are demonstrated by an example of measurements on a nonlinear system.

ISSN:0034-6748    

DOI:10.1063/1.1137717

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

An apparatus for applying uniaxial stress >109Pa at temperatures of 20–300 K is described. The apparatus is used with a deep level transient spectroscopy (DLTS) experiment, but it has more general applications. We have used this apparatus to observe the splitting of a DLTS defect level atEc−0.17 eV in neutron irradiated silicon and have determined from the splitting, as a function of stress orientation, that the defect symmetry is consistent with the oxygen‐vacancyA‐center.

ISSN:0034-6748    

DOI:10.1063/1.1137725

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A cryogenic heat switch has been developed for use in low‐temperature physics experiments. In the ‘‘on’’ position, it relies on the thermal properties of liquid helium, achieving a thermal conductance comparable to that of the electrolytic copper of which the conduction cell is constructed. The switching action is achieved without moving parts: it is switched ‘‘off’’ by removing the helium from the conduction cell with a self‐contained cryopump and ‘‘on’’ by heating the cryopump to a temperature at which the gas is desorbed, eliminating the need for connecting tubing, external pumps, and gas handling accessories. The ratio of ‘‘on’’ to ‘‘off’’ conductance is greater than 104with switching times of several minutes or less.

ISSN:0034-6748    

DOI:10.1063/1.1137726

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A real‐time schlieren system is described that incorporates a video imaging system to permit quantitative data to be obtained as to relative sound pressure distribution and wave front shape of ultrasonic waves. For 2.5‐MHz sound, system jitter is less than a tenth of a wavelength, field of view is 125 mm, and position can be measured to 25 &mgr;m. To illustrate the capabilities of this system, a study of 1‐&mgr;s‐duration sound pulses in water reflecting off a metal surface at different angles of incidence was performed on the system. Rayleigh waves 3° removed from the Rayleigh angle were observed. At the Rayleigh angle, a null is observed in the reflected beam.

ISSN:0034-6748    

DOI:10.1063/1.1137727

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A closed‐loop tracking circuit incorporating high‐impedance metal–oxide–semiconductor (MOSFET) transistors has been developed for measuring high‐voltage electrostatic fields on the surfaces of conducting electrodes. One MOSFET is used as a field sensor, and the other as a tracking ‘‘dummy.’’ The feedback circuit yields a signal voltage output that is at the same time linearly dependent on the external surface field being measured, and virtually independent of the gain and the offset parameters of the specific MOSFET device used as a sensor.

ISSN:0034-6748    

DOI:10.1063/1.1137728

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

Experimental equipment for the precise oblique‐impact shock study of solids in several 10 s of GPa region was developed. A powder gun with a keyed launcher (keyed‐powder gun), which is capable of accelerating projectiles of 20–40 g in weight without rotation to velocities of even greater than 2 km/s, was produced for the first time. The gun has a double‐wall structure launcher with a key whose muzzle shape resembles an old moon (basic bore diameter is 27 mm). Measurement of projectile velocities in the launcher by the reflected‐light method, including a logic trigger circuit for shock‐wave measurements, is combined with this gun. It was confirmed that the parallel‐plate impact experiment (normal type or oblique type), in which the tilt angle of parallel plates is less than 0.18° could be performed, allowing precise measurements of impact velocity and shock‐wave data by electrical and optical methods. Further, the keyed‐powder gun should find other widespread applications, e.g., for hypervelocity‐aerodynamic simulation study.

ISSN:0034-6748    

DOI:10.1063/1.1137729

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A simple method is described that permits a consistent determination of thermally induced pressure variations in a piston‐cylinder, self‐clamping pressure cell at temperatures less than ambient. Significant pressure changes are found to be present even forT<75 K. It is also shown that the pressure coefficient of resistance of a manganin‐wire gauge is, to within experimental uncertainty, independent of temperature over the range 0<T≤300 K.

ISSN:0034-6748    

DOI:10.1063/1.1137730

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A simple and inexpensive thermal diffusivity apparatus is described for measurement up to1600 K. The novel features of the apparatus include a light pipe, a long furnace, and a differential thermocouple. A low heat‐load sample holder for clamping the sample in a vertical position is also described. The results of measurements on AXM‐5Q graphite are reported.

ISSN:0034-6748    

DOI:10.1063/1.1137731

出版商:AIP    

年代:1984

数据来源: AIP

摘要:

A novel reactor system is described which permits ultrahigh‐vacuum surface spectroscopic studies and atmospheric pressure reactions to be done without a dedicated apparatus. An application of this system to studies of metal single crystals is shown, although its use for kinetic studies of such samples is limited in the present design. The reactor is used to study the surface properties and reaction characteristics of platinum deposited on a sapphire surface. The major use of the system is to study such model‐supported catalysts without exposing the sample to atmospheric contamination.

ISSN:0034-6748    

DOI:10.1063/1.1137732

出版商:AIP    

年代:1984

数据来源: AIP