L.R. Holland

Determining the shortest production time for glassy carbon ware

Abstract Because of the high production rate of gaseous reaction products in critical temperature ranges where out-diffusion is relatively slow, glassy carbon ware is difficult to make in thick section by pyrolysis of phenolic resin, without causing kilning faults. Using wedge shapes of cured phenolic resin we found the greatest thickness possible for a fixed heating rate during postcuring (400–500 K) and precarbonization (500–875 K), the stages in which failures occur. In postcuring, the critical heating rate varies inversely as the fifth power of critical thickness; in precarbonization, it varies inversely as the third power. Heating rate can be raised much faster at other stages of pyrolysis, leading to fully carbonized ware at 1500 K. Mass spectrometry shows the main gas product is steam; carboniferous gases are also evolved during precarbonization. We discuss diffusion models applicable to any firing process in which volatiles need to diffuse from solids.

Efficiency and radiation hardness of phosphors in a proton beam

Abstract A new system has been used to measure the relative peak efficiency and radiation damage endurance of several phosphor-binder combinations on aluminum substrates. The phosphors tested are Gd 2 O 2 S doped with Pr, Tb, and Eu; Y 2 O 2 S doped with Tb and Eu; YAG doped with Ce; and ZnS doped with Ag. The binders used are a polysiloxane resin and sodium silicate. Binder with suspended phosphor was sprayed on the heated substrates. Tests were done on a 3 MeV proton beam at the University of Lowell Van de Graaff accelerator. The aluminum substrates are formed as rotatable turrets to facilitate sample changes. Light measurement was by means of a silicon photodiode with no optical filter except a glass window. Ce doped YAG shows the best radiation hardness, while Y 2 O 2 S doped with Eu shows the highest fluorescence efficiency.

Crucible surface, thermal refraction, boundaries, and interface shape in melt growth

Abstract A heat conduction analog of Shells law of refraction is described, which can be used to calculate the kinks in isotherms where they pass the boundaries between media of different thermal conductivity. An application in Bridgman crystal growth is discussed, and it is shown that the edge of a melt growth crystal face should meet the crucible wall at a smaller angle than is usually supposed. The angle approaches 180° for metals, and 0° for a large class of semiconductors, which may have a detrimental effect on the crystallinity of the latter. Some examples are cited.

Continuous production of fullerenes by pyrolysis of acetylene at a glassy carbon surface

Abstract A reactor is described for generating fullerenes from hydrocarbon gas or vapor. Distinguishing features are a thermal system in which the fullerene product is not exposed to destructive energetic photons (as in an arc) and ease of control. Acetylene was chosen as the first feedstock to reduce the hydrogen content. Argon is the carrier gas. The reactor is a radio-frequency induction heated cylinder of glassy polymeric carbon having multiple holes through which the gas mixture passes. Operation as high as 2500xa0K is possible, but fullerene production is seen only below 1500xa0K. Preliminary tests give

A study of the thermally induced carbonization of phenolformaldehyde by combined ion beam and surface specific analyses

Abstract Rutherford backscattering spectrometry (RBS), Raman microprobe spectroscopy and electron spectroscopy for chemical analysis (ESCA) have been used to study thermally induced structural changes in phenolformaldehyde resin. Resin samples were heat treated in an inert gas environment at temperatures ranging from 200 to 2500°C. Results obtained from RBS spectra showed that the content of oxygen and other gaseous products of carbonization is reduced at higher heat treatment temperatures (HTT), which is in agreement with ESCA results. Raman microprobe spectroscopy clearly indicates that a key step in the carbonization process occurs between 500 and 575°C. The concentration ratios of various constituents to carbon were used as a complementary tool to the results obtained by ESCA to clarify the processes of thermally induced purification.

Spectroscopic analysis of proton induced fluorescence from cerium doped yttrium aluminum garnet

Abstract We have determined the effect of proton induced damage on relative scintillation efficiency for yttrium aluminum garnet doped with cerium (YAG:Ce). The YAG:Ce phosphor samples were exposed to a 3 MeV proton beam, and substrate temperature was limited to control heating damage. Real time in situ measurements of the fluorescence spectra permitted observation of the spectral characteristics of scintillator deterioration due to particle induced damage. Fluorescence from YAG:Ce is relatively dim when compared to other rare earth oxysulfide compounds and the light intensity drops rapidly with dose. Spectra from proton irradiated YAG:Ce exhibit a broad fluorescence peak that is much wider than is typical in other yttrium and gadolinium phosphor compounds. The physical processes in YAG:Ce are very different from other bright-line phosphors as shown by the large difference in the observed fluorescence peak width. Light intensity decreases with dose, following the Birks and Black empirical model.

Development of the fluorescent materials test chamber

Abstract Alabama A&M university and Nichols Research Corporation have recently completed construction and preliminary testing of the fluorescent materials test chamber (FMTC) with its associated vacuum subsystem at the Alabama A&M University Pelletron accelerator laboratory. The FMTC will provide physicists and material scientists with the capability of exposing a variety of samples in a controlled temperature environment to light ion bombardment. Efforts have also been completed toward the design for a low cost beam pulser subsystem for the FMTC. This equipment will allow the measurement of the fluorescent prompt decay time for a wide variety of phosphors. Current design and equipment capabilities allow for prompt fluorescence decay time measurements down to 1 μs. A vacuum tube based grid leak bias Hartley LC oscillator will be used to generate beam pulses with approximately a 2.2% duty cycle at electric field deflection frequencies of 1 and 100 kHz. This technique provides low-cost post-acceleration beam pulsing when compared to more traditional pulsed ion source arrangements.

Temperature dependent fluorescence from Gd2O2S:Tb induced by 45 MeV proton irradiation

Gadolinium oxysulfide doped with terbium was exposed to 45 MeV proton irradiation in the specially designed High Energy Materials Irradiation Chamber at the Indiana University Cyclotron Facility. This exposure was accomplished to determine the functional relationship between sample temperature and fluorescence intensity. Luminescent intensity dropped to half of the measured 113°C value at a temperature of 133°C. This effect was analyzed to determine the activation energy for radiationless transition. This result will provide a good understanding of the radiation hardness and spectral response for this fluor for possible use in the future development of a high energy accelerator beam positioning system.

Measurement of accumulated contaminants in glassy carbon by RBS, ERD and NRA☆

Abstract It is important to remove water and other gaseous products of carbonization to produce good quality glassy (polymeric) carbon from resin precursors by pyrolysis. In this work, we have used Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis (NRA) to detect impurity elements in glassy carbon specimens fabricated in our carbon processing laboratory. Elastic recoil detection (ERD) was used to measure hydrogen content. Resonance reactions such as (p,p) and (α,α) were used to enhance the detectability of various light elements. The results obtained are in agreement with those obtained by electron spectroscopy for chemical analysis (ESCA), thermogravimetric analysis (TGA), and residual gas analysis (RGA).

Measurement of fluorescence phenomena from yttrium and gadolinium fluors using a 45 MeV proton beam

Abstract In a program to determine proton induced damage and its effect on relative scintillation efficiency for various yttrium and gadolinium based fluors, materials were selected for high efficiency, fast prompt fluorescence response, and minimal delayed fluorescence. These fluors were exposed to a 45 MeV proton beam at the Indiana University Cyclotron Facility (IUCF) using the High Energy Materials Irradiation Chamber (HEMIC). Data obtained at IUCF indicates that the measured 45 MeV proton dose to reduce the fluorescence brightness to half its original value (half brightness dose - N 1 2 ) was found to be 2.2 to 7.3 times larger than that measured at 3 MeV. The corresponding average 45 MeV half brightness dose values, measured at elevated temperature, were 4.0 to 83.7 times larger than at 3 MeV. Although the interaction cross section for 45 MeV protons is smaller than at 3 MeV, each of the resulting high energy reactions causes more damage to the fluor. This research was completed as part of the Fluorescent Materials and Bonding: Extended Energy program under contract DASG60-90-C-0144 for the U.S. Army Space and Strategic Defense Command (USASSDC).