摘要:

A comparison between the conventional photothermal frequency scan method and the recently developed photothermal lock‐in rate window technique for thermal diffusivity measurements of materials, is presented. In this comparison, a completely noncontact experimental configuration has been utilized based on infrared photothermal radiometry. This work shows that for thick materials with long thermal transport times across the sample where low‐frequency measurements are required, the frequency scan method may be more appropriate due to its simplicity. The rate window method, however, gives superior signal‐to‐noise ratio (SNR) for materials with very short thermal transport times such as metal foils, which otherwise require high frequency, low SNR measurements. A further advantage of the pulse duration‐scanned rate‐window mode is that it does not require knowledge of the instrumental transfer function as an input.

ISSN:0034-6748    

DOI:10.1063/1.1144686

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

The highly sensitive field sensor with a Pockels crystal which was devised by us has been modified by using a light reflection technique. By replacing a metallic mirror with a dielectric mirror which is sputtered on one end face of the Pockels crystal, the new field measuring system has an advantage of precisely measuring the field distribution in the long gap discharge. This is the case when the field sensing part consisting of the Pockels crystal is isolated from the main field measuring system and both are connected to each other with a long light guide cable. Based on our experiments, we found that the measurable minimum field strength in the long gap discharge is 2 V/cm with the accuracy of 16 dB and 0.2 V/cm with the critical accuracy of 0 dB. Furthermore, we point out that the size of the field sensing part can be reduced down to 1 mm in diameter by using a dielectric mirror and a light guide cable.

ISSN:0034-6748    

DOI:10.1063/1.1144687

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

During recent years, much work has focused on breaking urinary and biliary stones using extracorporeal shock wave devices. Up to now, in terms of fragmentation efficacy, the ‘‘electrohydraulic’’ principle is the gold standard. In an electrohydraulic shock wave generator, a plasma is created by a high‐voltage electrical discharge between two underwater electrodes. This kind of shock wave generator has a significant drawback because of the large variations of pressure pulses. Moreover, the wear of the electrodes is considerable, leading to nonconstant electrical discharge conditions all along the treatment. A new method to canalize the plasma between the electrodes, using highly conductive liquid, was investigated. Based on this principle a new spark plug was designed. This paper deals with the description and the characterization of this new type of spark plug. The electrical and geometrical parameters are presented. Measurements of the focal point pressure amplitude, of the spatial pressure distribution using different voltage settings, are reported. The acoustic energy in the focal zone is computed. We show that using this principle, the wear of the electrodes has been tremendously reduced. A comparison ofinvitroresults with other shock wave technologies shows that this new spark plug represents a significant breakthrough in terms of stone fragmentation efficacy. This spark plug is not only useful in the medical field, but it should also be of great interest to laboratories.

ISSN:0034-6748    

DOI:10.1063/1.1144688

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A nonaerodynamic sabot stripper has been designed and implemented for use with a 40.00‐mm smooth bore research gas gun. The stripper consists of several metal parts to stop and contain the sabot while allowing the carried object to pass unhindered through it. The single‐piece sabot is stopped by impacting replaceable layers of 19‐mm‐thick aluminum and steel plates and 3.2‐mm‐thick rubber sheets. The metal plates and rubber sheets have 25.4‐ and 31.8‐mm diam holes, respectively, for passage of the carried object. The sabot stripper is located 230 mm from the muzzle of the gas gun and is aligned before each shot using a special metal fixture that is inserted into the gun muzzle. Cubes measuring 12.7 mm have been launched in a flat‐faced orientation. Targets consisting of 76.2 mm×76.2 mm plates of various thicknesses are positioned in an assembly that attaches to the sabot stripper and is located 57 mm behind it. The velocity range for the experiments was from 0.61 to 0.94 km/s.

ISSN:0034-6748    

DOI:10.1063/1.1144689

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An instrument has been developed to monitor the equivalence ratio in combustion systems. It is based on a novel oxygen consumption measuring method. The instrument does not require knowledge of the chemical composition of the process being monitored. Both gaseous and solid (soot) products of incomplete combustion are successfully handled. The device is relatively compact and can be developed for moderate cost and portable use.

ISSN:0034-6748    

DOI:10.1063/1.1144690

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Two single element hydrophones and an eight‐element hydrophone array with side‐looking polyvinylidene fluoride (PVDF) sensing elements are described. They were designed for measurements in which the ultrasonic beam insonifies the hydrophone from a direction perpendicular to the hydrophone axis. These hydrophones and the array have found various industrial and medical applications, especially for determining the ultrasound exposure of a patientinvivo. The construction and characterization of the hydrophones and the hydrophone array are reported and the directivities of the elements are compared with modeling results.

ISSN:0034-6748    

DOI:10.1063/1.1145209

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A simple PC‐based system has been developed for studying the current‐controlled electrical switching in solids, in particular amorphous semiconductors. The system has provisions for investigating the switching behavior in the continuous current mode or in the pulse mode and for resetting memory‐switched samples. It also has facilities for studying the switching behavior under controlled heating.

ISSN:0034-6748    

DOI:10.1063/1.1144691

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A simple experimental apparatus has been used to produce individual mercury and sodium amalgam drops in the size range of 0.3–2.5 mm diam. A high resistance to the flow of liquid was provided by a capillary tube. The cross section consisted of a 0.20 mm o.d. tungsten wire placed in a 0.25 mm capillary bore. By this method, flow rates of 1 &mgr;l/s and lower were obtained, which enabled the formation of drops as small as 1 mm diam in a time interval of 20 s. The drops were formed at a plastic tip, and on reaching the desired size, were stripped off by a stream of argon. An optical sensor was used to detect the growing drop and to switch on the argon stream at the desired time. The growth of the drops at the tip was observed using an image analysis system which showed good agreement between the observed and calculated drop sizes at various instants. The consistency in size from drop to drop, for any particular setting of the apparatus, was evaluated by analyzing magnified pictures of the drops as well as by measuring their weights. The variation in the drop size was found to be within ±2% of the mean value.

ISSN:0034-6748    

DOI:10.1063/1.1144692

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A rheometer which uses a piezoelectric ultrasonic composite oscillator has been developed to measure the viscoelastic properties,G’(&ohgr;) andG‘(&ohgr;), of polymer melts at ultrasonic frequencies. The storage and loss moduli of an ethylene–tetrafluoroethylene copolymer melt were measured at six frequencies in the range 0.25×106–1.28×106rad/s (between 40 and 200 kHz nominal frequency) and at a temperature of 280 °C. This ultrasonic instrument operates at strain amplitudes ranging from about 10−8to 10−3and is therefore ideally suited for linear viscoelastic property measurements.

ISSN:0034-6748    

DOI:10.1063/1.1144693

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A torque converter and an electronic driver circuit are described that will generate a compensating applied torque that is equal and opposite to the frictional torque of a bearing over a range of angular velocities. The compensated bearing is effectively frictionless. The frictional torque is assumed to have a constant component and one that is proportional to the angular velocity.  

ISSN:0034-6748    

DOI:10.1063/1.1144694

出版商:AIP    

年代:1994

数据来源: AIP