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Notes: A study is made of the properties of cermets as electrode materials for thermionic energy converters. For thermodynamic reasons it is expected that all cermets composed of pure Mo and refractory oxides have the same bare work function. From data on the work function of Mo in an oxygen atmosphere this bare work function is estimated to be Φ=4.9 eV (at T=1400 °C). Experimentally, the bare work function of Al2O3-Mo cermets was found to be Φ=4.5 eV, independent of the relative amounts of Al2O3 and Mo. The cesiated work function of the Al2O3-Mo cermets was found to be 0.15 eV lower than the cesiated work function of pure Mo. The bare work function of Mo3Al was found to be Φ=4.0 eV. The cesiated work function of Mo3Al at collector temperature conditions was 0.3 eV lower than the cesiated work function of pure Mo. The electrical power density of a diode with an Al2O3-Mo cermet emitter was 0.4 W/cm2 at 1300 °C. The barrier index at this temperature was 2.36 V. The high barrier index is attributed to a high plasma voltage drop Vd=0.91 V.

Notes: A research diode with polycrystalline molybdenum electrodes is described. Voltage-current characteristics are presented as a function of the cesium reservoir temperature. A power density of 4 W/cm2 is obtained at an emitter temperature of 1400 °C. The influence of the temperatures of the emitter, collector, and cesium reservoir and of the interelectrode distance is experimentally investigated. Physical explanations for the various effects are given. The work function of the polycrystalline molybdenum emitter in a cesium atmosphere is evaluated as 2.5–2.7 eV depending on the reduced emitter temperature (i.e., the emitter temperature divided by the cesium reservoir temperature). A barrier index Vb =2.0 eV and a cesium plasma drop Vd =0.4 eV are found.

Notes: The purpose of this study was to determine if a classification of subgingival plaque into distinct categories of microbial forms would be beneficial for detecting differences in the severity of periodontal disease. Three thousand and thirty-two subgingival plaque samples were collected and evaluated from three different populations having 284, 160, and 180 individuals. Plaque and gingival indices, probing depth, and attachment level were measured at each sampling site. Overall, the results indicated that mean plaque index, probing depth, and loss of attachment were successively higher at sites classified into four distinct microbial categories in the order (1) “Low Motile Organism - High Cocci,” (2) “Low Motile Organism - Low Cocci.” (3) “High Motile Organism - High Motile Rods,” and (4) “High Motile Organism - High Spirochetes.” The consistent pattern of differences in severity of periodontal disease between these microbial categories suggest that microscopic interpretation of plaque may be improved by this classification.

Notes: The association between Actinobacillus actinomycetemcomitans and periodontal disease in juveniles has been well documented. The purpose of this investigation was to determine the prevalence and proportions of A. actinomycetemcomitans in supragingival and subgingival plaque samples from the maxillary first molars of a large number of young adults. The study population included 284 adults, aged 20–40, ranging in periodontal disease status from healthy to moderate periodontitis but with the majority exhibiting early periodontal disease. The clinical characteristics of probing depth, attachment level, plaque index, and gingival index were measured. Supragingival and subgingival plaque samples were evaluated microscopically for microbial forms. They were also cultured on supplemented blood agar and various selective agar media including selective media for A. actinomycetemcomitans. The prevalence of A. actinomycetemcomitans in subgingival and supragingival plaque for individuals in the population was 13.0% (37/284) and 4.9% (14/284), respectively. Proportions of actinobacilli, based on total anaerobic counts, were found at or below 1% in 87% of 47 subgingival sites from 37 subjects. Supragingival and subgingival sites with actinobacilli were compared to sites without actinobacilli. Subgingival sites with A. actinomycetemcomitans had a significantly higher mean plaque index, with 79% of these sites having a plaque index greater than 1.0 compared to 30% of sites without actinobacilli. The mean gingival index, probing depth, and attachment level of sites with actinobacilli were also higher, but not significantly, than those without. Of the microbial forms enumerated, only spirochetes had a significantly higher mean proportion at subgingival sites when compared to sites without actinobacilli. Mean proportions of the cultivable microorganisms, Veillonella spp. and Streptococcus spp., were significantly lower at sites with A. actinomycetemcomitans. Differences in the mean proportions of certain microorganisms were compared between the 47 subgingival sites with actinobacilli divided into three groups by probing depth. Mean proportions of A. actinomycetemcomitans were significantly higher at intermediate probing depths between 3.0 and 5.0 mm compared to deeper sites with probing depths above 5.0 mm. On the other hand, dark-pigmented Bacteroides spp. mean proportions were significantly higher at deeper probing depths than at either intermediate or shallow, less than or equal to 3.0 mm, probing depths. There were no significant differences in the mean proportions of spirochetes between shallow, intermediate, or deeper probing depths of the 47 subgingival sites with actinobacilli.

Notes: The goal of this study was to investigate the relationship between probing depth, total microscopic counts, and specific microbial forms in subgingival plaque from young adults. Subgingival plaque samples were collected and probing depth recorded from 415 mesial surfaces of maxillary first molars in 266 individuals aged 20–40. Using dark field microscopy in a Petroff-Hausser chamber, total organisms in each sample were estimated, as well as proportions of spirochetes, motile and non-motile rods, cocci, and total motile organisms. Limits for decile ranks based on total organisms were determined and the mean percentage of each microbial form within each decile rank was calculated. Correlation coefficients (r) were computed between the dependent variables of microbial forms and the independent variables of total organisms, probing depth, and the combination of total organisms and probing depth. The percentage variability of microbial forms that could be explained by the regression of each dependent variable on each independent variable or combination of independent variables was estimated by r2. The results indicate: (1) differences in bacterial composition of subgingival plaque are associated with differences in total subgingival microorganisms and in probing depths, (2) approximately 15–20% of the variability in the proportions of microbial forms in subgingival plaque can be explained by differences in total subgingival microorganisms, (3) two to four times more variability in the proportions of microbial forms in subgingival plaque can be explained by differences in total subgingival microorganisms than can be explained by differences in probing depths, and (4) considerable variability in proportions of microbial forms in subgingival plaque remains that cannot be explained by either differences in total subgingival microorganisms or by differences in probing depths.

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