Showing papers in "Journal of research of the National Bureau of Standards in 1958"

Infrared studies on polymorphs of silicon dioxide and germanium dioxide

Abstract: The infrared spectra of coesite, low-temperature tridymite, low-temperature cristobalitelow-~emperature quartz, vitreous silica, hexagonal Oe02, tetragonal Ge02, and vitreous germallla are reported from ~,OOO to 300 C~-l. Wherever possible an assignment of frequencies has been made on t he basls of the selectlOn rules for the crystal symmetry. Three characteris.tic gro up frequencies. near 1,lOq, 800, and 480 cm-1 are common to all the polymorphs of Sl02. Thes.e frequ~nCle.s respectlvely correspond to a str~tching mode involving displacements assoclated prnnanly wlth the oxygen atoms, a stretclung mode involving displacements associated primarily . wi~h the. sil~co~ atoms, and a Si-O bending mode. The presence of these group frequencles III coeslte mdlcates that t he coordination of silicon in coesite is tetrahedral and that its high density is associated with the packinrr of tetrahedral units at an angle approximatiI?g 120 degrees .. The tetragonal and hexagon~l GC02 polymorphs show a marked dlfference III spectra due III part to the change from sixfold to fourfold coordination. The assignment of obscrved frequencies in hexagonal Ge02 is consistent with that made for low-temperature quartz if allowance is made for the heavier mass of t he Ge atom.

Thermal degradation of cellulosic materials

Abstract: Fortisan, cellulose triacetate, and NOroxidized cellulose were pyrolyzed in a vacuum in the temperature range 1800 to 465 0 C. Cotton cellulose and cellulose t ri acetate were also pyrolyzed in nitrogen at atmospheric pressure. The tar yields were in the decreasing order from: Cotton, Fortisan, cellulose triacetate, and oxidized cellulose. The other volatiles consisted mainly of acetic acid, carbon dioxide, and carbon monoxide, from the triacctate; and water, carbon dioxide, and carbon monoxide from the other celluloses. In all cases there was a carbonaceous residue (volatilization end point), the amount depending on t he nature of the cellulose and the temperature of pyrolysis. When pyrolyzed in nitrogen at atmospheric pressure, cotton cellulose and cellulose triacetate yielded less tar than when pyrolyzed in a vacuum. T he tar from cellulose triacetate consisted of a compound whose infrared spectrum resembled that of the original triacetate. Cotton cellulose, Fortisan, and cellulose triacetate do not differ much in t heir initial rates and activation energies of thermal degradation. Oxidized cellulose has very high initial rates of thermal degra dation .

Infrared spectra of thermally degraded poly(vinyl-chloride)

Abstract: The cha nges in chemical structure occu ring in pol y (vinyl chloride) as a res ul t of heati ng ina vacu um in t he range 100° to 400° C were studied using infrared spectropho to metry. The pri ncipa l changes occ urri ng in t he residue during p yrolysis in a vacuum were the format ion of unsaturated s tructures a nd a change from an al iphatic spectrum to one showing aromatic absorption. The data are used to suppor t a previously proposed mechani sm of decomposition for poly (vinyl chloride) .

Specific volume and degree of crystallinity of semicrystalline poly(chlorotrifluoroethylene), and estimated specific volumes of the pure amorphous and crystalline phases

Abstract: The specific volume of poly (chlorotrifiuoroethylene) has been measured from -400 to + 260 0 C. Both q uenched and well-crystallized. ~pecimens prepared by reproducible procedures were studied. Well-defined glass tranSitIOns were found close to 52 0 C in both specimens. Hence t he glass temperature of t his polymer is essentially independent of t h e ~egree of ?rystallinit:y. The. quasi~eq~ilibrium melting point of the particular type of crystal~Ized speCimen used III the l~lVestlgatlOn was 216 0 C. The equilibrium melting temperature IS undoubtedly somewhat higher, and probably lies between 2200 and 225 0 C. The d egree ?f crystallinity of the crystallized and quenched specimens was calculated at T I from speCific-volume J?easurements alone, using a straightforward thermodynamic procedure. The method aVOids a long \"unguided\" extrapolation of the liquid data to low te~per:;ttures, and does not involve the inexact approximation that the volu me-temperature denvattv.e.of the glas y and cryst~lline states are the same. It has the further advantage of not regumn g a pure crystal denSity from another source, such as a unit-cell determination from X-ray ? ata. It was fo und that th.e quenched sample was 39 p ercent crystalline and the well-cry:st.alhzed one 82 percent. A sl~ple extension of the theory permit the degree of crystalh1l1ty to be comp uted as a functlOn of temperature. The results are compared with those obtained in earlier investigations. The sp ecific vo.lumes and volum~-temperature derivatives of the pure supercooled liquid, g~3:s y, and crystallme phases are estimated over a wid e range of temperature. Certain quant ities re lated to the free volume of the glassy state are discussed . The methods outli ned may be of utility in analyzing specific-volume-temp erature data on other semi crystalline polymers where the rapid onset of crystallization interferes with a direct s~udy of the sLIpercoole~ liquid and glassy states, and where independent data on the properties of the pure crystallme phase are not availa ble.

Transmission and reflection of electromagnetic waves in the presence of stratified media

Abstract: A genera l analysis is presented for t he electromagnetic response of a plane stratified medium consist ing of any number of parallel homogeneous layers. The solution is first developed for pla ne-wave incidence and then generalized to both cylindrical and sphericalwave incidence. Numerical results for interesting special cases a re presented and discussed . The application of t he r esults to surface-wave propagation over a s tratified ground is considered in some detail.

Self-ignition temperatures of materials from kinetic-reaction data

Abstract: R esults of experimental determinations of the kinetic constants of the self-heating reaction are presented for wood fiberboard, cotton linters, sugar pine, cork, crepe rubber, GRS rubber, natural, synthetic, and blended foam rubber (with and without additive), various oils (raw linsecd, cottonseed, rapeseed, sperm, olive, castor, and neatsfoot) applied to cotton gauze in a ratio of 1. part of oil to 6 parts of cotton by weight, ammonium perchlorate, and nitrocellnlose plastic. Under the assumption that self-heating follows a first-order r eaction, these constants were used to calculate the critical radii of spherical piles for each of four surface temperatures likely to be experi enccd in long-period storage. Calcuhted self-ignition temperatures of piles of ~~-inch-diameter to 22-inch-diameter spheres of wood fiberboard and JIg-inch-diameter to 2-inch-diameter sph eres of cotton linters were in r casonable agreement with previous mcasurements by N. D. Mitchell (National Fire Protection Association Quarterly

Dielectric constant of deuterium oxide

Abstract: An equal ratio-arm, capacitance-conductance bridge operated at frequencies below 100 kc was used to measure the dielectric constant of deuterium oxide with an accuracy of 0.1% or better in the range 4 to 100 deg C. A value of 77.94 was found for the dielectric constant at 25 deg C. The data fit the relation epsilon = 87.48/sub 2/ -- (0.40509)t + (9.638 x 10/sup -4/)t/sup 2/ -- (1.333 x 10/sup -6/)t/sup 3/, with a maximum deviation of less than 0.01 unit in dielectric constant. These values were obtained by using a smnple having a deuterium oxide content of 99.38 mole %. Deviation of these values from those for pure deuterium oxide, as a result of isotopic contamination, is estimated to be less than 0.005 unit. Some aspects of the temperature dependence of the dielectric constant and of that of a macroscopic polarization are discussed. (auth)

Sample calculations of gamma-ray penetration into shelters: Contributions of sky shine and roof contamination

Abstract: An approximate method is presented for calculating the penetration of gamma radiation in shelters. Sample calculations, for an assumed source energy of 1 million electron volts, are given for the following problems: (1) Dose rate inside houses and umderground shelters whose roofs are covered with radioactive fallout, and (2) dose rate in open holes due to reflected radiation (sky shine) from fallout contamination on the surrounding ground. A detailed examination is made of the dependence of the dose rate in a shelter on the shape of the shelter, and on the position of the detector within the shelter. The estimated accuracy of the calcillations is plus or minus 30 percent. (auth)

Mechanism of stress-corrosion cracking in the az31b magnesium alloy

Abstract: The mechanism of s tre?s-co rrosion crt;tcking of annealed AZ31B magnesium a lloy in an aqueous ~aClI(2CrO. solutIOn was lI1vestlgated . Cracking was predomina nt ly an electrochemical pro.cess a nd was shown to be dependent on t he rate of stra in in t he specimen f01l0wtng load lll g ] 11 tensIOn. It ] S postu lated t ha t cracks develop if t he protective fi lm on t he metal surface IS r uptu red (over narrow segments of t he specimen) a t a greater rate than it is repa ired 1Il t he corroding medium . T his wou ld expose film-free metal t hat is anodic t o the fi lmed m eta l.

Cavity ionization as a function of wall material

Abstract: A study has been made of the ionization vdtnn a flat cavity chamber under irradiation by x- and gamma rays in the energy region 38 to 1,250 kilovolts effective (kev). Chamber walls were made of carbon, aluminum, copper, tin, and lead, and the wall separation was varied from 0.5 to 10 millimeters. Results are compared with cavity theory. (auth)

Additional abscissas and weights for Gaussian quadratures of high order: Values for n=64, 80, and 96

Abstract: In a previou paper ,2 tables of zeros of the Legendre polynomials P n(x) were published for various values of n up to n=48. It was also stated there that values were available for n = 64 , 80, and 96 . Despite a brisk demand for these values, they were not published with the other because doubt was cast on their accuracy. It was feared that the round-off error accumulated in using the recursion formula for Legendre polynomials

Some thermodynamic properties of the systems polybutadiene-benzene and polyisobutene-benzene

Abstract: Experimental data are rcported on densities at 26. go C of solutions of polyisoblllcnc in benzcne covcring thc composition range from 0 to 10 percent by wcight of polyisobutene, and on rclative vapor pressures of solutions of polyblltadicnc a nd of polyisobutene in benzene covcring the range from 0 to 50 perccnt of bcnzenc. Valucs of apparcnt spccific volume of polyisobutene in benzcnc \" 'crc calculated from thc densities of solvcnt and so lutions. The results of the vapor pressure measurements were compared with thc Flory-Huggins cquation. Thc dcviations from this equation for the po lys iobll tene-benzelle system arc 1l111Cll greater than for the polybutadiene-benzene system. The following cquations represent the activitics of the so lvent for the polybutadicne-benzene and polysiobutenc-bcnzene systcms, respectively.

Measurement of current with the National Bureau of Standards current balance

Abstract: Prior to the adjustment of the electrical units in 1948, t h e value of a curren t bad been determined in absolute units by means of a current balance and simultaneously measured in NBS amperes by comparison with standard resistors and standard cells. Th is work was reported in RP1449. Similar measurements made recent ly with an electrody namometer indicate a possible change in t he values of t he standards . The present paper reports a repetit ion of t he work described i'n" RP1449. The purpose of this remeasurement was to determine whether or not the ~t'andards had cbanged . Only minor changes were made in t he equipment in order that factors which might ha ve introduced small systematic errors in the results would r emain unchanged. According t o the work described in t his paper, 1 N BS ampere = 1.000008 ab30lu te amperes. R ecent work with the P ellat electrodynamometer gave the result 1 BS a mpcre 1.000013 absolute amperes. The weighted mean of t hese two values is

Thermal degradation of polyacrylonitrile, polybutadiene, and copolymers of butadiene with acrylonitrile and styrene

Abstract: Polybutadiene and a copolymer consisting of 76.5 per cent butadiene and 23.5 per cent of styrene were investigated as to the rate of their thermal degradation. The rates for polybutadiene indicated an activation energy of 62 kilocalories per mole. The copolymer had very high initial rates of degradation followed by a rapid drop, so that it was not possible to obtain a reliable activation energy. Polyacrylonitrile and a copolymer consisting of 31 per cent of acrylonitrile and 69 per cent of butadiene were investigated with regard to the nature and distribution of volatile products, as well as to the rates of their thermal degradation. The rates were very high initially, but dropped rapidly so that it was not possible to determine accurately the activation energy. The acrylo-nitrile-butadiene copolymer showed a decomposition pattern similar to that of polybutadiene. Here, too, the activation energy could not be deduced accurately from the rate curves. Comparative thermal stability in terms of tempe...

On the motion of 2 cylinders in an ideal fluid

Abstract: The comp lex potent ial of t wo cylinders moving in an infin it e liquid is determ in ed by the m et hod of image do uble ts, a nd t he solution is expressed as an infin ite series in rectangular coordinates. Approxima te solut ions in fi n ite form are g iven for various cases. A method for generali zing t h e solution for t he ease of more than two cylinders is indicated . Applications to t he fl ow indn ced by a cylinder movin ~ in t he p resence of p]n,ne boundaries are given and t he st ream lines are illustrated in cer tain cases.