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Impedance measurements using microelectrodes on adjacent grains of polycrystals are a valuable tool for a detailed analysis of grain boundary properties of ceramic materials. In order to support proper interpretation of such experiments two fundamental aspects are discussed in the theoretical part of this paper: first, it is shown that “localized” experiments using contact pads are often not reasonable and can lead to severe misinterpretation. Second, results from finite element calculations illustrate as to how far conclusions on the grain boundary resistivity distribution can be drawn from a measured grain boundary resistance statistics. In the experimental part, microelectrode impedance measurements and orientation imaging data on undoped polycrystalline SrTiO 3 are presented and interpreted in terms of the homogeneity of the space charge potentials at grain boundaries.

The effect of the electrode material on the high voltage stoichiometry-polarization (“electrocoloration”) of Fe-doped SrTiO3 single crystals has been studied. For different electrode materials (150-nm Ag/15-nm Cr and 150-nm Au/15-nm Cr) secondary ion mass spectrometry (SIMS) was used to measure the depth profile of the 18O-isotope after high-field stress. The results were compared with spatially resolved impedance measurements on electrocolored Fe-doped SrTiO3 single crystals. For Au/Cr as well as Ag/Cr electrodes a large dc voltage leads to moving color fronts which are usually correlated with a pronounced stoichiometry polarization of the samples due to electrodes blocking the ionic current. However, the local impedance measurements demonstrate that the conductivity profiles near the cathode depend on the electrode material. This finding is in accordance with the SIMS measurements which indicate that the Ag/Cr-electrodes are, in contrast to Au/Cr-electrodes not completely inactive for the oxygen incorporation into the Fe-doped SrTiO3. The results are discussed in terms of defect chemical models.

Microcontacts on adjacent grains of polycrystalline Fe-doped SrTiO3 samples have been used to locally investigate the properties of individual grain boundaries. Impedance spectroscopy was employed to separate bulk and grain boundary impedances. Experiments at about 30 different grain boundaries permit far-reaching conclusions on the distribution of grain boundary resistances, capacitances, and peak frequencies measured between adjacent grains. The rather narrow distribution of the grain boundary peak frequencies indicates a narrow distribution of grain boundary resistivities. All features (e.g., nonlinear current–voltage characteristics, grain boundary thickness, temperature dependence) are in accordance with the assumption of space charge depletion layers (double Schottky barriers) as the origin of the enhanced grain boundary resistivity. The average barrier height measured was about 630 mV. For comparison conventional (macroscopic) impedance measurements on a polycrystal were also performed and a brick layer model was used to extract effective properties. The reasonable agreement between these effective parameters and the average of the locally obtained parameters demonstrates that, in this case, a brick layer analysis of conventional impedance experiments yields satisfying estimates of the grain boundary properties.

Ohmic Ag/YBa 2 Cu 3 O 7-δ -microcontacts on adjacent grains of polycrystalline Fe-doped SrTiO 3 -samples have been used to investigate locally the properties of grain boundaries by means of impedance spectroscopy. Experiments at 30 different single grain boundaries permit reliable conclusions to be made on the distribution of grain boundary properties. For comparison conventional impedance measurements on a polycrystal were performed and a brick layer model was used to extract effective properties. The reasonable agreement between these effective parameters and the average of the locally obtained parameters demonstrates that, in this case, a brick layer analysis of conventional impedance experiments yields satisfactory estimates of the grain boundary properties.

Impedance techniques using circular microelectrodes are introduced in order to locally measure grain boundary properties in polycrystalline materials with highly conductive as well as highly resistive grain boundaries. Finite element calculations allow for a discussion of the general aspects, potentials and limits of such measurements and enable a quantitative analysis of local properties. The applicability of the methods are shown by detecting and analyzing highly conductive grain boundaries in AgCl and by locally measuring impedance spectra of highly resistive grain boundaries in acceptor-doped SrTiO3.

The electrical properties of grain boundaries in polycrystalline materials can vary from boundary to boundary and conventional impedance measurements yield only averaged information. Local measurements at individual grain boundaries using microelectrodes can partly overcome this restriction. Finite element calculations on the impedance of such microelectrode measurements are performed to reveal the interrelations between the measured grain boundary impedance and the grain boundary properties (grain boundary resistivity, permittivity and geometry). We will discuss how far adjacent grain boundaries influence the results. In particular, the calculations revealed that even in the case of identical grain boundaries a considerable part of the current between two microelectrodes may flow across neighboring grain boundaries. It is shown how these additional current paths can be taken into account with respect to a reliable data analysis. Experiments on SrTiO3 polycrystals demonstrate that spatially resolved impedance investigations on single grain boundaries are possible in this way. The results are interpreted in terms of a double Schottky barrier at the grain boundaries.

The hydrozirconations of acyclic alkenes are known to yield the dicyclopentadienyl-1-aklyl-zirconium chlorides regioselectively and in high yields. These alkyl groups may readily be transmetallated to the haloboranes forming regioisomeric pure alkylboranes in good yields. In principle, this migration reaction combines the facile hydrozirconation and unique isomerization reactions of organozirconiums with the established chemistry of boron, expanding the utility of both metals for use in organic synthesis.

The hydrozirconations of acyclic alkenes are known to yield the dicyclopentadienyl-1-aklyl-zirconium chlorides regioselectively and in high yields. These alkyl groups may readily be transmetallated to the haloboranes forming regioisomeric pure alkylboranes in good yields. In principle, this migration reaction combines the facile hydrozirconation and unique isomerization reactions of organozirconiums with the established chemistry of boron, expanding the utility of both metals for use in organic synthesis.

Trialkoxyboranes are conveniently synthesized in good to excellent yields from the corresponding alcohols or phenols and boric acid using calcium hydride as a drying agent. This method is successful for a variety of sterically hindered alcohols. In a new general route, trialkoxyboroxins are formed under mild reaction conditions from these same readily available materials. The trialkoxyboranes and trialkoxyboroxins can be interconverted by addition of stoichiometric amounts of boric acid or the alcohols/phenols, in the presence of calcium hydride. Calcium hydride is most effective for the removal of water. This allows for a simple and convenient route for the preparation of the useful trialkoxyboranes and the previously inaccessible trialkoxyboroxins, for further investigation or applications.

The temperature dependence of the sharp and the broad component of the critical scattering above the cubic-to-tetragonal phase transition of ${\mathrm{SrTiO}}_{3}$ has been studied by means of high-resolution triple-crystal diffractometry using 100--200 keV synchrotron radiation in five samples differing with respect to growth technique and oxygen vacancy concentrations. Emphasis is on changes in the critical behavior, the critical temperature, and the strain fields at the transition from bulk to surface. The sharp component was observed only in surface near regions of highly perfect crystals and is coupled to the occurrence of a long-range strain gradient that was identified by an exponential increase of mosaicity, lattice parameter fluctuations, and Bragg-peak intensity when approaching the surface from the bulk of the sample. Vanishing of the sharp component was observed at the polished/etched surface of a platelet cut off the large perfect crystal after release of strain due to free bending of the platelet. The values of the critical temperature observed in the bulk of the different samples vary between $98.7l~{T}_{c}l~105.8 \mathrm{K}.$ In the surface near regions of a highly perfect float-zone grown crystal a variation of ${T}_{c}$ of about 0.5 K has been found. Concerning the broad component the critical exponent describing the temperature dependence of the inverse correlation length ${\ensuremath{\kappa}}_{b}$ varies between $0.73l~{\ensuremath{\nu}}_{b}l~1.19,$ the exponent for the susceptibility between $1.49l~{\ensuremath{\chi}}_{b}l~2.9,$ however, the ratio of the two exponents is almost sample independent and equal to ${\ensuremath{\chi}}_{b}/{\ensuremath{\nu}}_{b}=2.1$ with a variance of 0.2, in good agreement with the theoretical value of 1.97 obtained by LeGouillou and Zinn-Justin [Phys. Rev. B 21, 3976 (1980)]. The occurrence of the sharp component did not affect significantly the critical exponents for the underlying broad component of the critical scattering. The exponents for the sharp component observed in surface near layers of about 100 $\ensuremath{\mu}\mathrm{m}$ thickness at the highly perfect float zone and flux grown crystals varied between $0.58l~{\ensuremath{\nu}}_{s}l~1.30,$ the values for the ratio varied between $3.3l~{\ensuremath{\chi}}_{s}/{\ensuremath{\nu}}_{s}l~4.6.$ The average value of ${\ensuremath{\chi}}_{s}/{\ensuremath{\nu}}_{s}$ is 3.9 with a variance of 0.5, and is about twice the ratio ${\ensuremath{\chi}}_{b}/{\ensuremath{\nu}}_{b}$ for the broad component as suggested by Harris et al. [Phys. Rev. B 52, 14 420 (1995)].

Trialkoxyboranes are conveniently synthesized in good to excellent yields from the corresponding alcohols or phenols and boric acid using calcium hydride as a drying agent. This method is successful for a variety of sterically hindered alcohols. In a new general route, trialkoxyboroxins are formed under mild reaction conditions from these same readily available materials. The trialkoxyboranes and trialkoxyboroxins can be interconverted by addition of stoichiometric amounts of boric acid or the alcohols/phenols, in the presence of calcium hydride. Calcium hydride is most effective for the removal of water. This allows for a simple and convenient route for the preparation of the useful trialkoxyboranes and the previously inaccessible trialkoxyboroxins, for further investigation or applications.

Highly sensitive radio-immuno assay (RIA) was used for 21 days to study the curves of several steroid hormones, progesterone (P), oestradiol 17 beta (E2), and testosterone (T), as well as of luteinizing hormone (LH) in peripheral blood of six heifers to which oestrus had been induced by prostaglandin F2 alpha. The validity of the authors' RIA for P, E2, and T determination in blood plasma was positively verified by two reliability criteria, correctness and accuracy, as well as by comparative determinations, using reference methods. Only four of the six heifers returned to oestrus, within 21 days from first oestrus induction. A typical cycle-related curve of the P concentration in peripheral blood with peak values between the 13th and 17th days of cycle (15-26 nmol/l) was recorded from four animals. The peak values of pre-ovulatory E2 and LH were between 33.0 and 53.2 pmol/l or between 19.5 and 52.5 micrograms/l. Some of the T rises in peripheral blood were in parallel to E2 concentrations. All hormone curves are presented in detail and are discussed in relation to clinico-physiological findings.

A microcomputer system has been developed and installed at the Mallinckrodt Institute of Radiology to monitor the moment-to-moment activity in a radiology department. Data about department performances are collected, summarized, and displayed in graphs and tables on terminals at stations throughout the department. The system is currently used to monitor a wide variety of functions including patient waiting time, report production time, film-library operations, equipment maintenance, and room use. Since the displays are updated in real-time, it is possible to identify and address problems as they occur, thereby providing a method of evaluation that leads to improved performance as related to patient care.

We developed an interactive computer system to support various educational uses. The system allowed numerous instructors to create lessons or tests in a flexible, personalized fashion. As a pilot project, the system was used to develop and administer a computer-driven final examination in a didactic radiology course for second-year medical students. It was our objective to change the examination into an enjoyable experience that would also bring about long-term learning gains. The computer added immediate feedback to the testing situation.

Plain radiographic examinations were performed on 388 patients (adult and pediatric) using either a calcium tungstate or rare-earth screen. Radiographs were compared for overall image quality, mottle, contrast, density, and detail. The rare-earth screen produced radiographs with significantly more mottle than did the standard screen. However, this was almost never of diagnostic importance. A small but significant decrease in overall image quality also was found with the rare-earth screen. This was the result of minor differences between the grading of optimal and good. A major advantage of rare-earth screens is a 50% or more reduction in radiation exposure. The results here show that rare-earth screens may be used in plain radiographic examinations without a clinically important decrease in image quality.

R. Gilbert Jost, Stephen S. Rodewald, Rexford L. Hill, Thomas HansonSharon Albertina, Armand Diaz, Ronald G. EvensMallinckrodt Institute of Radiology, 510 S. Kingshighway, St. Louis, Missouri 63110IntroductionA key measure of any radiology department's efficiency in performing examinations is itspercentage of repeated films. In reliably obtaining high -quality films, some percentage ofrepeats is certain to occur. Human error and equipment malfunction can never be eliminated.Poor patient positioning, patient movement, film artifacts, exposure problems, and process-ing problems are only some of the reasons for performing a certain view more than once.But keeping this repeat percentage to a minimum should enable the department to use lessstaff time and fewer films, and help ensure that patients wait a shorter time and receiveless radiation exposure. In this way, reducing repeats should both lower the cost and im-prove the quality of patient care.A significant decrease in the number of repeated films can be achieved with a good filmmanagement program with accurate collection and analysis of appropriate data. Manualsystems for tabulating repeated film data are time consuming and require a conscientiouscommitment by technologists and supervisors that can be difficult to sustain. In additionto data collection, a successful film management system must provide for a comprehensiveanalysis of the data once it is collected.A computer system, by assuming these data processing chores, can give technologists andsupervisors on- going, in -depth feedback regarding the performance of radiology examinations.A system of this kind has been developed and installed at the Mallinckrodt Institute ofRadiology (MIR) to monitor all diagnostic examinations performed at the Institute.System description and resultsThe MIR computer system for monitoring examination performance has been designed as anindependent module, capable of running on its own minicomputer. The system can also operatein a mode where interaction takes place with a larger, more comprehensive computer patientinformation system. The MIR system, written in BASIC -PLUS, runs on a Digital EquipmentCorporation PDP -11/34 under the RSTS operating system.In the MIR system, a CRT terminal for film data entry is located in each examinationarea of the department. Once an examination is completed and the final set of films hasbeen obtained, the x -ray technologist who performed the exam enters the film data, (eitherby typing at a standard CRT keyboard or by sweeping a hand -held wand across machine -readablelabels.) Through an interactive dialogue at the computer screen, the technologist entersidentification codes for both patient and technologist, and then indicates the type ofexamination, the number and the size of films used, and the room where the examination wasperformed.If it was necessary to repeat any radiographic exposures, additional data is entered.The computer provides a list of the various positions and sizes of films appropriate forthe examination, as well as possible reasons for the repeat (See Figure 1). The technolo-gist makes the appropriate selections, the selections are verified by the technologist andby the computer, and the information is stored.Whenever film or equipment data is entered anywhere in the department the computersimultaneously sends up -dated information to data display terminals located in key super-visory areas.l Thus each of these on -line display terminals provides an up -to- the -minutepicture of room utilization and film utilization throughout the Institute and supervisorsat all levels can make immediate use of this information (See Figure 2).