摘要:

We describe the construction of four different systems used for annealing samples in oxygen. The gas pressure can be controlled over nine decades (10−6–2×103bar). The maximum temperature, 1700 °C, is limited by the ceramic materials used for thermal insulation and for the crucibles. The low‐pressure furnaces (10−6–1 bar) have an external heating system and incorporate an active regulation of a small gas flow. The intermediate pressure furnaces (1–150 bar) are heated either by a special resistive wire or by rf heating with a Pt‐20%Rh susceptor, both in the pressurized gas chamber. Quenching is possible in the latter. For the kbar range, we designed a double‐chamber autoclave with separate heater and sample spaces, filled with Ar and O2. The gas pressures are balanced in a cold, separate vessel by means of a silicon hose. The temperature is measured with a thermocouple feeding into the hot, pressurized oxygen environment very close to the sample. The construction problems of this new autoclave have been solved with particular emphasis on the safety requirements. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145215

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

In order to conduct chemical kinetic investigations under piston engine like conditions, a combustion bomb which emulates the pressure rise and decay within a piston engine cylinder has been designed and built. This allows one to investigate chemical kinetics in a realistic pressure‐time burst, without the complication of turbulent mixing. While phenomena like flame speed and heat release depend much more strongly upon fluid mechanics, emissions formation and removal depend strongly upon chemical kinetics. Typical problems of interest include soot oxidation during the pressure decay, coupling of emissions‐related chemical kinetic pathways for various components of reformulated fuel mixtures, investigation of toxics formation, and development of reduced kinetic mechanisms for engine codes which include chemical submodels. Here, the device and the processes by which it works, experimental data and a modified two‐zone thermodynamic model by which one can infer temperatures, volumes, and mass fractions are described. Combined results from experiments and the model indicate that the BDCB produces an excellent imitation of an internal combustion (IC) engine pressure‐time burst. Moreover, this device exhibits better cycle‐to‐cycle reproducibility than research engines, and it allows for unimpeded optical access and rapid sampling. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145173

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An apparatus to measure the wettability parameter using the familiar capillary rise principle for a liquid metal/alloy in contact with a ceramic tube was constructed. The liquid levels outside and inside the capillary tube were measured using acoustic feedback and an electrical continuity technique, respectively. Measurements made by immersing silicon carbide, alumina, and silica tubes in various pure metals and alloys are presented and compared to previously published data in the literature. Reproducible measurements were obtained for nonwetting systems; however, the expected capillary rise was not observed for reaction wetting systems. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145174

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A radio frequency measurement technique for measuring the bulk resistivity of II–VI compound semiconductors is described. Wafers ofn‐type CdS are used to demonstrate the technique. An equivalent circuit model is introduced which predicts a frequency dependence for the CdS wafer impedance which agrees well with the experiment. The model assumes a broad distribution of relaxation times associated with the polarization. The radio frequency method gives values for the resistivity within 15% of four point probe measurements for the lower resistivity wafers, and within 5% for the higher resistivity wafers. ©1994 American Institute of Physics.  

ISSN:0034-6748    

DOI:10.1063/1.1145175

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

An analog interface has been developed in order to perform very accurate noise spectral analysis with currently available instrumentation. It employs the device under test as a front‐end element, whose input‐referred noise voltage is amplified by a feedback loop which adds an extremely small noise contribution, followed by two cascaded low‐noise amplifiers. The interface is suitable for spectral analysis in the 100 mHz–100 kHz frequency range. Its intrinsic noise contribution is estimated around 1 nV/&sqrt;Hz at 100 mHz and 50 pV/&sqrt;Hz at frequencies beyond 1 kHz. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145176

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A compact low‐power long‐life sealed‐off cw CO2laser has been developed by incorporating a catalyst‐coated outer jacket that helps to regenerate CO2from the dissociation products formed during discharge. Maintaining the catalyst at an optimized temperature prevented CO2adsorption in its bulk and resulted in the required level of CO oxidation activity. The laser system has been operated during the day for the past five months at a constant output level of about 4 W. The gas analysis performed at different catalyst temperatures suggests that although the presence of CO in large amounts is detrimental, an optimum concentration of carbon monoxide supports the CO2laser operation. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145177

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

Transient measurements of thermal conductivity, thermal diffusivity, and specific heat capacity have been performed with hot disk sensors in thin samples of metallic materials. With this new variation of the hot disk method the sample size can be reduced to a volume less than ten cubic centimeters for copper at room temperature. It is also shown that the specific heat capacity can be conveniently measured in transient recordings of slightly longer duration. On comparing with standard values the accuracy turns out to be better than 1% while the precision (standard deviation of the mean from six measurements) on the average is about 0.5% for all values recorded. ©1994 American Institute of Physics.  

ISSN:0034-6748    

DOI:10.1063/1.1145178

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

A new version of laser heated pedestal growth system is described which is amenable for the rapid growth of single crystal fibers. The system incorporates a Gaussian reflector to reshape the radial intensity profile of the CO2laser beam. When He is used as the growth atmosphere, high optical quality sapphire fibers can be produced at rates as high as 2 cm/min with this apparatus. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145179

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We report further measurements of the temperature dependence of the piezotube coefficient for material PZT‐5A. These new results support our earlier findings that the temperature dependence of the piezoresponse is linear to within 10% and is reduced by a factor of 5.5 on decreasing temperature from 300 to 4.2 K. Our new data imply that the piezocoefficient temperature dependence is reproducible for tubes of various sizes, even when the room temperature coefficients differ by more than 20% from their published values. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145180

出版商:AIP    

年代:1994

数据来源: AIP

摘要:

We experimentally examine the ripple and its spectra in the dc output of a symmetrical Cockcroft–Walton (CW) cascade rectifier circuit. Results show that the ripple due to the load consists of even harmonics and the ripple due to the circuit asymmetry consists of odd harmonics. In the latter, the ripple caused by the asymmetry of circuit elements cannot be compensated for completely through the asymmetry of the input voltage because of nonlinearity of a CW circuit. Approximately, the residual ripple is inversely proportional to the load resistance. ©1994 American Institute of Physics.

ISSN:0034-6748    

DOI:10.1063/1.1145181

出版商:AIP    

年代:1994

数据来源: AIP