WELDED ZIGZAG WINDINGS AND REFRACTORY SHAPES FOR HIGH-TEMPERATURE FURNACES1

A brief account is given of published methods for making high-temperature furnaces using W and Mo windings, placed both internally and externally with respect to the furnace tube. Several new methods of furnace construction are here mentioned:

• 1 A furnace tube may be built up of a series of rings made of refractory substances for external windings.
• 2 A furnace tube may be built up of a series of tall and flat rings, alternately placed, made of refractory substances for an internal winding, the winding being threaded through small holes drilled in the flat rings.
• 3 A zigzag winding with welded joints has been developed for W and Mo wire. The winding may be conveniently sheathed with refractory tubes, as the winding runs the length of the furnace tube. The entire zigzag winding may be put in series, or divided into a split parallel winding, each type having its own advantages and faults.
• 4 Details of windings and refractories for furnaces reaching 1600°C and 2000°C are given. Windingq are protected from oxidation by hydrogen.

     

EXTRUSION OF REFRACTORY OXIDE INSULATORS FOR VACWM TUBES1

A method is described whereby nonplastic materials such as refractory oxides are given a temporary plasticity, sufficient for extrusion. The organic plastic gel used consists of flour or starch paste with ammonia as an electrolyte. A short detailed account is given of the manufacture of twin-hole insulators made of MgO 98% and talc 2% for radio 227 vacuum tubes, with supplemental experimental results on other materials. The refractory oxides Al₂O₃, BeO, ZrO₂, MgO, and ThO₂ are compared from the standpoint of their physical properties and their ability to act as insulators in contact with hot tungsten filaments. Examples are given of the use of such extruded oxide insulators for hot filament supports.     

IMPACT AND STATIC TRANSVERSE STRENGTH OF WET PROCESS ELECTRICAL PORCELAIN1

A brief review is given of the apparatus used for determining the resistance to impact of ceramic materials, namely, apparatus of the “pile driver” type and of the pendulum type. A Charpy (pendulum) transverse impact machine of 2 ft.-lbs. maximum capacity suitable for porcelain has been built and is described. Specimens from 1/2 to 11/8 inches in diameter can be accommodated on movable supports. Data were obtained for 1/2, 3/4, 1, and 11/8 inch diameter cylinders of wet process porcelain on a 4-inch span, the average values for 1/2-inch diameter being 1.24 ft.-lbs. per sq. in., and the average value for 3/4, 1, and 11/8 inches being 1.53 ft.-lbs. per sq. in. with an average deviation from the mean of about 2.5%. Data on specimens of wet process porcelain 1/2, 3/4, 1, and 11/8 inch in diameter fractured on 4- and 5-inch spans give equivalent values for the modulus of rupture. The average of all values is 11,000 pounds per square inch.     

DIELECTRIC PROPERTIES OF GLASSES AT ULTRA-HIGH FREQUENCIES AND THEIR RELATION TO COMPOSITION *

The dielectric constants and dielectric losses of 104 glasses of a wide range of compositions have been measured at 10-cm. and 3-cm. wave lengths by the resonating cavity method (3000 and 10,000 megacycles, respectively). By correlating the power-factor data with the compositions of these glasses, the authors propose a qualitative explanation of the mechanisms producing energy absorption and dielectric losses in the microwave range. These mechanisms are determined by the nature of the bonds joining atoms and ions in the randomly oriented atomic networks of glasses. The rigid and continuous networks of SiO₂ and B₂O₃ glasses are relatively transparent to centimeter wave lengths. Energy absorption and dielectric losses are low. Addition of network-modifying oxides yields glasses of greater energy absorption owing to the oscillation of the interstitial ions thus introduced. Increasing the content of any one of these ions in a glass results in higher losses while the coexistence of a variety of these ions generally results in lower losses. Alkali ions in glasses give rise to high losses, which increase as the number of ions present increases. Glasses containing a combination of alkalis show lower losses than the equivalent compositions with only one alkali. Divalent ions do not contribute as much to losses as alkalis, but high power factors are shown by glasses with high BaO or PbO contents. Using combinations of these oxides instead of only one, slight reductions in power factor are effected. Dissimilar interstitial ions interact in ultra-high frequency fields with the result that energy absorption is reduced. The losses of high-lead glasses are thus reduced by alkalis and, on the other hand, the presence of RO lowers the losses of glasses containing R₂O. Alumina increases the dielectric losses of glasses in much the same manner as other network modifiers.     

COMPOSITIONS AND PROPERTIES OF SOME HIGH-TITANIA CERAMICS*

The compositions are given for high-titania ceramic bodies which mature at cone 10. One series contains calcium titanium silicate as an essential flux, and the other series contains alkali-free titania glasses. Electrical measurements cover the range of 60, 1000, and 1,000,000 cycles for both power factor and dielectric constant. Detailed consideration is given to the type of electrodes used. The properties covering dielectric strength, temperature coefficient of capacity, effect of humidity, and thermal expansion are given for one composition. An X-ray diffraction pattern of this body shows the formation of rutile crystals.     

STUDY OF SEGREGATION IN PORCELAIN FILTER CAKE BY PETROGRAPHIC METHODS1

The segregation which takes place in a filter cake of a porcelain mixture has been studied in thin sections made from slices of the filter cake fired to cone 10. A lenticular segregation is illustrated by photomicrographs.     

SELENIUM DIOXIDE AS A CONSTITUENT OF GLASSES1

SeO₂ is added as selenious acid (H₂SeO₃) and as sodium selenite (Na₂SeO₃ to the oxides Na₂O, CaO, BaO, PbO, B₂0₃, and SiO₂ to form glasses and crystalline melts containing apprcciable amounts of SeO₂. The chemical analysis of glasses shows the vaporization of SeOn to be high during the melting stage. Most of these glasses are unstable in air or water. A relatively stable glass contained Na₂O 33.4%, CaO 12.4%, B₂O₃ 37.6%, SiO₂ 5.8%, SeO₂ 10.8%. Melting several compositions in a closed container yielded opaque or crystalline melts, whereas the same compositions melted in air with a partial volatilization of SeO₂ gave clear glasses.     

THE PRODUCTS OF THE CALCINATION OF FLINT AND CHALCEDONY1

Outline of the Investigation. —This investigation includes the measurement of the following physical properties; specific gravity at 0°, 25° and 98°, the index of refraction, the coefficients of thermal expansion up to 300°, the inversion temperature and volume change on inversion, and the X-ray spectra, the materials studied being quartz, tridymite, cristobalite, silica glass and raw and calcined flint and chalcedony. New Methods —For specific gravity, a special vacuum pycnometer was employed and the immersed powder was later subjected to a pressure of 1000 atmospheres. Resultss .—The numerical data obtained are summarized in Table I and the X-ray spectra are shown in Fig. 10. The specific gravity of calcined chalcedony was raised 2% by fine grinding. Conclusions .—All of the known facts concerning chalcedony are in harmony with the theory that the raw material is colloidal quartz and the calcined material colloidal cristobalite.     

METHODS OF TESTING and THE PHYSICAL PROPERTIES OF WET-PROCESS ELECTRICAL PORCELAIN1

Data are given for the compressive, transverse, and tensile strengths of wet-process electrical porcelain. Compressive Strength The height of the sample is an important variable. Initial failure and ultimate failure are discussed and it is suggested that as the latter is the more definite and constant of the two, the, ultimate failure be always determined and recorded as the “compressive crushing strength.” The cylindrical specimen 1 sq. in. in area (11/8 in. diameter) and 11/8 in. high is recommended. Transverse Strength The load applied to cause rupture is directly proportional to the cube of the diameter of the cylinder over a wide range of diameters. A cylinder with an area of 1sq. in is suggested. Cylindrical specimens give constant and high values. Tensile Strength As the area of minimum cross-section increases the tensile strength decreases rapidly. The diameter should be given with determined values. An apparatus for making tensile strength determinations on a specimen with conically shaped ends and minimum area of 1sq. in is described. Relationship of specimen size and effective dimensions to the average general dimensions of the wale and their methods of manufacture is discussed.     

A SIMPLE “SEALED-IN MERCURY ELECTRODE” APPARATUS FOR TESTING THE BREAKDOWN VOLTAGE OF HEAVY INSULATING MATERIALS1

A method has been developed whereby thick plates of insulating materials (of small area) may be electrically punctured under oil. The specimen is cemented between two hollow electrode holders by means of red sealing wax, as illustrated. One electrode holder, the elbow, may be made of ordinary cast-iron piping or glass. Mercury poured into both tubes gives the necessary contact. It is possible by these means to puncture a porcelain cylinder of diameter 1 ⅛ inches and 1 ⅛ inches thick at 142 kv., and with a porcelain cylinder 2 inches in diameter and 2 inches thick to reach 176 kv. without arcing. The “sealed-in mercury electrode” method has been compared with two other methods employing spheres and disks as electrodes, by puncturing cast polished plate glass specimens. The new method gives consistent results, the values being on the average 15 % higher than values obtained by the other two methods.