摘要:

The theory of bipolar pulse conductivity detection has been successfully applied to the pressure‐jump (P‐jump) method of relaxation kinetics. This method has distinct advantages over ac bridge detection, including the elimination of capacitance effects which make current balancing significantly easier, the ability to study low‐conductance solutions at higher signal‐to‐noise ratio, and reduction of power input into the cell solutions which decreases heating effects. TheP‐jump instrument is described, and the electronic circuits for both methods are given. The bipolar pulse circuit diagram is presented and described in detail. Example relaxation curves are shown for single phase and diphasic systems. A signal‐to‐noise improvement of a factor of 2 is observed for bipolar pulse detection over that observed for the optimal ac bridge detection.

ISSN:0034-6748    

DOI:10.1063/1.1144777

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

By replacing the anvils in a diamond cell with transparent gem materials, a gem anvil cell (GAC) has been constructed for high‐pressure generation. We reached pressures of 8.9 and 7.8 GPa with cubic zirconia and sapphire anvil cell, respectively. For high‐temperature studies the cell can be heated to temperatures as high as 2000 °C with resistive heating of the gasket. Simultaneously high temperature and pressure approaching 1700 °C and 6 GPa were obtained as indicated by the direct conversion of graphite to diamond in the high‐temperature GAC.

ISSN:0034-6748    

DOI:10.1063/1.1144779

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new technique to measure wear and profile of impregnated diamond drill bit for mineral exploration is presented. A three‐dimensional surface mapping of worn diamond drill bit surface is obtained by the laser triangulation method. With adequate algorithms all relevant data can be obtained. Consequently, the volume loss between two distances cored can easily be measured with a great precision. Moreover, computerized cross sections of the worn surface give relevant information about the profile resulting from the drilling process.

ISSN:0034-6748    

DOI:10.1063/1.1144787

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We report the experimental results of noise measurements performed on junction field effect transistors (2SK162) in the frequency range 1–100 kHz, where the internal correlation effects are not negligible. The experimental data have been analyzed to determine the input current noise spectrum and the spectral energy sensitivity of the device, expressed in terms of noise temperature. We obtainedVn&bartil;0.6 nV/&sqrt;Hz, andIn&bartil;4 fA/ &sqrt;Hz at 1 kHz,In&bartil;25 fA/ &sqrt;Hz at 100 kHz. We also report and discuss the results of measurements performed at the temperature of 200 K, which show that the reduction of the noise is not very significant for this specific device.  

ISSN:0034-6748    

DOI:10.1063/1.1145210

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new method was developed to prepare a small single crystal sample for electrical conductivity measurement. The method was based on the use of commercially availableXYZmechanical stages as micromanipulators, and its accuracy was high enough to attach successfully four gold wires for four‐probe measurement on a needlelike crystal 0.2 mm long.

ISSN:0034-6748    

DOI:10.1063/1.1144788

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Fully developed velocity profiles around the side of a needle falling at terminal velocity in a falling needle viscometer and the flow disturbance region near the ends of the needle were experimentally investigated. A flow visualization technique was used to obtain the main features of the flow field around the falling needle. Comparisons made between the experimental and predicted fully developed velocity profile showed good agreement. The regions where the flow field is disturbed by the needle ends were analyzed from photographs and found to be about 6% of the total needle length.

ISSN:0034-6748    

DOI:10.1063/1.1144789

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An innovative, Couette‐type viscometer/rheometer was developed, designed, and fabricated with the main objective being to measure viscosity and elastic properties of low‐viscous, non‐Newtonian, and visco‐elastic fluids, like dilute polymer solutions. The goal was to simplify and improve some existing drawbacks of commercial instruments employing several novel design solutions, particularly with regard to instrument precision and sensitivity. With a single pair of cylinders and a torsion bar stiffness of 0.116 Nm/rad, viscosities from 0.5 to 50 000 cP (centi‐Poise) in a shear‐rate range from 1 to 200 s−1, and oscillatory tests from 0.1 to 10 Hz, could be measured. The transducers’ electronic signals are handled by a software, developed in C language, and an IBM‐PC compatible computer with a data acquisition board. The innovative design solutions (use of a cruciform torsion bar, optoelectronics sensors, and a novel alignment procedure) have improved the critical instrument performance and reduced the number and cost of components, customarily used in commercial viscometers/rheometers. The overall steady shear‐rate calibration was performed using the standard‐viscosity fluids of 51 and 445 cP. Initial results show high precision (around 4%), particularly at higher velocities. The oscillatory testing was performed by measuring the phase shift between the stress and strain for viscoelastic fluids and for the limiting case of a standard Newtonian fluid. The novel design has proved efficient and with potential for further improvements.

ISSN:0034-6748    

DOI:10.1063/1.1145230

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A novel fiber‐optic cladding mode detector which uses the benefits of dark field detection in a fiber‐optic sensor configuration is described. This detector integrates the microbender plate, and the photodetector. It facilitates the detection of cladding or radiation modes produced due to the microbends at the site of the bend itself thereby enhancing the signal strength. The design of this integrated detector along with the calibration curves are given. The detector performance is evaluated by using it in a displacement sensor configuration. Comparison of performance with core mode detection is also given.

ISSN:0034-6748    

DOI:10.1063/1.1144790

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We derive, discuss, and test against experimental data an analytical model of the gain saturation in microchannel plate (MCP) devices. By introducing a simple recharging circuit for each dynode, we extend the well‐known, unsaturated gain model of Eberhardt to a microchannel operating in condition of gain saturation and show that the amplification of a current pulse and the voltage drop along the channel can be described by a pair of coupled differential equations. Solutions of these equations are given in various conditions, including an approximate solution, valid in the case of weak saturation and a general solution in implicit form. The behavior of a microchannel operating in current mode is studied by finding the transient and steady‐state solutions obtained with an input step current wave form. Exact solutions are given for the charge gain of pulses with a short duration, compared to the dynode recharging time, and for the gain recovery of a microchannel after the amplification of a short pulse. The single channel saturation model is then extended to multistage MCP assemblies by taking into account the statistical distribution of the photoelectrons at the input and the spread of the multiplied electron cloud in the interplate gaps. The expressions found in this way are used for the best fit of experimental data from aZ‐stack MCP photomultiplier operated in single and double pulse mode. Satisfactory agreement between the model and experimental data is obtained in the case of single pulse measurements, finding a reduced chi squared &khgr;2=4.67. Less satisfactory agreement is found for double pulse data, giving &khgr;2=7.46 and a clear indication that the model may be significantly improved by taking into account the charge redistribution among the dynodes during the recharging process, neglected in the present formulation.

ISSN:0034-6748    

DOI:10.1063/1.1144791

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Optically controlled, sequential breakdown of the spark‐gap switches is a simple method for triggering compact Marx generators. Techniques for reliably initiating breakdown of the first switch are discussed. A 300‐kV Marx generator for flash x‐ray applications is described.

ISSN:0034-6748    

DOI:10.1063/1.1144792

出版商:AIP    

年代:1994

数据来源: AIP