摘要:

Ultrasonic pulse‐echo measurements are used to determine the changes inc11E,c33D,c44D,c44E, andc12Ethrough the reversible ferroelectric—antiferroelectric phase transition in a Pb(Ti, Zr, Sn)O3ceramic. The results indicate that after the transition is enforced by hydrostatic pressure the material loses both electrical and mechanical orientation and becomes elastically isotropic.

ISSN:0021-8979    

DOI:10.1063/1.1714304

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The mechanisms of ion formation in cesium plasmas at relatively low electron temperatures (3500°K) and pressures of a few Torr are investigated. It is concluded that most of the ions are molecular and that they are formed by collisions between cesium atoms in the first excited state. The excited atoms are predominantly produced by inelastic electron collisions.The excitation cross section is computed by means of the impact parameter method. It has a maximum value of 100×10−16cm2at 7 eV. The de‐excitation cross section is computed and found to be approximately constant at 50×10−16cm2. Radiation trapping and diffusion of excited atoms are also studied.The ionization cross section is calculated by means of statistical thermodynamics from the measured recombination coefficient for the inverse of the ionizing process. It is found to be 1450×10−16cm2. The plasma is uniform throughout its volume except in the immediate vicinity of the bounding electrodes.The proposed mechanism results in performance characteristics for cesium thermionic converters, operating in the ``ignited mode,'' which are in qualitative and quantitative agreement with experimental data.

ISSN:0021-8979    

DOI:10.1063/1.1714305

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

We wished to determine if an electrode polarity effect existed in vacuum breakdown conditioning. To investigate this conditioning effect, we used a three‐electrode experimental tube constructed in such a manner that one electrode (1) could be made to oppose either of the other two (2 or 3) without destroying the vacuum. High‐current arcs were drawn between both possible electrode pairs to bring them to a reproducible state of apparent deconditioning. These pairs (1 vs 2 and 1 vs 3) were then, in turn, conditioned by a series of high‐voltage pulses at a gap of 2.3 mm. The following conclusions were drawn: (1) within experimental error, all conditioning took place at the pulse cathode, (2) when the arc and pulse polarities were similar, the subsequent pulse breakdown voltage was lower than when the polarities were opposed, and (3) conditioning was not due to the removal of a gas layer from the electrode, but, rather, to removal of cathode deformations. Predictions concerning the polarity effect in vacuum breakdown conditioning were drawn from the major theories of vacuum breakdown, and compared with our experimental results. We found that our results agreed with the predictions of some vacuum breakdown theories, flatly contradicted the predictions of others, and tended to disagree with the predictions of a third group of theories.

ISSN:0021-8979    

DOI:10.1063/1.1714306

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The electrical properties of inert gas (Xe, Kr, Ar) plasmas have been studied in a nuclear reactor environment at gamma radiation dose rates up to 6×108rad/h. The measurements are made by inserting a gas‐filled thermionic diode into the core of a 5‐MW nuclear reactor. The space‐charge neutralization effects caused by the ion‐electron pairs generated in the gas are measured and interpreted in terms of plasma properties of the gas. The electrical conductivity of the plasma, at a constant radiation level, goes through a maximum as a function of pressure in the range of 1–300 Torr. If the radiation dosage is varied, keeping all other factors the same, one finds the electrical conductivity monotonically increases with dose rate in the range of 106−6×108rad/h.Experiments are described which indicate that electrons can be removed from a low work function ``cold'' electrode into the plasma by a mechanism different from either thermionic or field emission. A physical model is proposed to explain this phenomenon.

ISSN:0021-8979    

DOI:10.1063/1.1714307

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Kesaev's recent data on the stability of vacuum arcs for a wide variety of solid and liquid cathodes furnish a greatly expanded range of criteria for determining the mechanism of the cathode spot. The large effects of phase and crystalline texture of the cathode suggest that an essential process occurs within the cathode metal itself. This is thought to be secondary electron emission produced by positive ions through a diffusion‐controlled neutralization process. Each secondary initiates a short burst of thermal ionization in front of the cathode, which supplies practically all of the arc current. Survival of the arc depends on the emission of at least one new secondary within the period of the burst, and this depends on the current and on the rate of diffusion of ions into the cathode, the secondary emission being greater the larger the current and the slower the diffusion. Relative diffusion rates are estimated for most of the cathode metals used by Kesaev and it is found that the current required for a given arc lifetime increases regularly with the rate of diffusion of ions into the cathode metal. The analysis yields five new heats of diffusion—polycrystalline mercury and bismuth 16.2 and 18.3 kcal/mole, respectively; and liquid bismuth, thallium, and aluminum, 8.8, 3.2, and 3.4 kcal/mole, respectively.The diffusion rate is correlated with cathode temperature, dislocations, and vacancies, and the moving spot probes in all directions for areas of low diffusion rate. The arc is extinguished when all parts of the cathode line lie on micro‐areas of high diffusion. The arc lifetime data furnish estimates of the concentrations of dislocations in all the solid and liquid (transient existence) metals as prepared by Kesaev. The concentration of edge dislocations is found to be approximately inversely proportional to the fourth power of the heat of vaporization.The statistics of arc lifetime as a function of current may be correlated by the Poisson formula, with each burst of ionization counting as a single Poisson trial, which is successful if it produces one or more secondary electrons. The analysis for liquid indium indicates that the bursts have a lifetime of about 5×10−7sec.The random motion of the free spot on mercury is thought to be determined by random radial temperature‐fluctuation gradients at the edge of the spot. The velocity of the free spot on mercury is limited to about 104cm/sec because the spot must not outrun its secondary emission, which lags behind positive ion incidence on the cathode by about the 10−7sec required for the diffusion secondary emission process in mercury. Retrograde motion of the spot in a magnetic field is found, qualitatively and semiquantitatively, to be due to the temperature gradient set up across the spot by the Righi—Leduc thermomagnetic effect. The retrograde velocity on bulk liquid mercury is subject to the same velocity limit of about 104cm/sec as is the random velocity in zero field. A spot at a mercury—molybdenum boundary can jump to a substantially higher velocity when the Righi—Leduc gradient is high enough, and when, also, the increased velocity is great enough to permit the spot to operate on a mercury layer so thin that the diffusing ions have access to the molybdenum surface, where the more abundant supply of electrons accelerates the neutralization process, and thus decreases the time‐lag of secondary emission.It is suggested that more sophisticated observation and analysis of cathode‐spot statistics should furnish a valuable metallurgical tool for studying the texture and dislocation properties of metal surfaces. From the analysis of Kesaev's data some speculations are made about the origin and behavior of edge dislocations.

ISSN:0021-8979    

DOI:10.1063/1.1714308

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

The anomalous surface reflection of x rays reported recently by Yoneda has been investigated for several materials using MoK&agr;1, CuK&agr;1, and CrK&agr;1radiation. The experimental evidence speaks strongly in favor of the Warren and Clarke interpretation in terms of totally reflected small‐angle scattering. For clean surfaces the scattering appears to be due to surface irregularities of the bulk material itself. Because of its sensitivity to the roughness and chemical contamination of optically or near‐optically finished surfaces, the Yoneda effect has considerable promise as a new x‐ray tool for studying surfaces and thin films.

ISSN:0021-8979    

DOI:10.1063/1.1714309

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Vacuum distillation methods were demonstrated to be effective for producing high‐purity crystals of LiF. It was established from chemical analyses of condensate products that under the experimental conditions adopted, 995°C and 4 to 5×10−2Torr, the order of vaporization for the fluorides, NaF, MnF2, LiF, MgF2, and CaF2, corresponds to the order of their decreasing vapor pressures. Under similar experimental conditions LiF containing no more than 200 ppm of oxygen could be produced from mixtures of LiF and Li2O.

ISSN:0021-8979    

DOI:10.1063/1.1714310

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

Magnetic susceptibility of iron—tin alloys containing 0.45, 0.82, 1.33, 1.83, and 2.14 at.% tin have been studied as a function of temperature up to 1500°K. Tin influences the magnetic properties of iron in an unusual way. The mass magnetic susceptibility of &ggr; iron is increased by about 1% due to 0.45 at.% tin. The paramagnetic moments of the bcc phase, determined from the susceptibility data between 1300° and 1500°K, show a small increase with increasing tin content. These results are, in principle, consistent with the ferromagnetic properties studied by Fallot (1936) indicating that tin does not act as simple diluent in regard to the magnetic behavior of iron. Electrical resistivities of iron—tin alloys, determined at 4.2°, 77.2°, and 297.0°K, show that tin increases the resistivity of iron very considerably, i.e., about 8 &mgr;&OHgr; cm per 1 at.% at 4.2°K. Lattice parameters of iron—tin solid solutions up to 5 at.% tin (quenched from 1013°K) have been measured at 298°K. None of the present theories are capable of explaining satisfactorily the observed behavior.

ISSN:0021-8979    

DOI:10.1063/1.1714311

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

An analysis is made of the strain‐energy distribution in a slotted double‐cantilever cleavage specimen. In this type of specimen the crack is medially constrained by a pair of longitudinal slots, so that surface energies can be determined in nonmonocrystalline samples.The analysis treats the arms of the crack as end‐loaded cantilever beams, and the post crack‐tip region as a pair of elastically supported beams. The web produced by slotting is assumed to act as an elastic foundation for the remaining beam‐like portions of the post crack‐tip region. An expression for the specific surface energy of the material is then derived from conservation of energy at the instant the crack begins to propagate. Linear elastic behavior of the material is assumed and shearing strains are neglected.Extension of the analysis to conventional (unslotted) specimens in which medial constraint of the crack is obtained through a preferred direction of cleavage, is discussed semiquantitatively in terms of an effective foundation modulus.

ISSN:0021-8979    

DOI:10.1063/1.1714312

出版商:AIP    

年代:1965

数据来源: AIP

摘要:

A brief study is described of a simple method for producing plane shock waves in solids at pressures higher than those attainable by conventional explosive methods. Mach reflection of a conically convergent shock in the specimen yields a disk‐shaped front at a pressure of several megabars, depending on the type of explosive employed and the specimen material; this front is accessible to Hugoniot equation‐of‐state measurements. Results for copper at a pressure of 1.9 Mbar are in good agreement with published values.

ISSN:0021-8979    

DOI:10.1063/1.1714313

出版商:AIP    

年代:1965

数据来源: AIP