G.M. Jenkins

Biomedical applications of carbon fibre reinforced carbon in implanted prostheses

Abstract Carbon fibre reinforced carbon has been manufactured in both Brazil and Swansea with strengths and stiffnesses comparable with metals in normal surgical use. The biocompatibility of the carbon with living tissue, especially when implanted percutaneously, has been demonstrated and specific applications in bone adjustment and heart-valve fabrication are described.

Ion beam promoted lithium absorption in glassy polymeric carbon

Abstract Glassy Polymeric Carbon (GPC) samples prepared from a precursor possess accessible pore volume that depends on the heat treatment temperature [G.M. Jenkins and K. Kawamura, Polymeric Carbons - Carbon Fiber, Glass and Char (Cambridge University Press, Cambridge, 1976) p. 140]. We have shown that lithium percolates without diffusion into the accessible pores of GPC samples immersed in a molten lithium salt bath at 700°C [D. Ila, G.M. Jenkins, L.R. Holland, A.L. Evelyn and H. Jena, Vacuum 45 (1994) 451]. Ion bombardment with 10 MeV Au atoms increases the total pore volume available for lithium occupation even for samples normally impermeable to lithium. The lithium concentration depth profile is measured using Li 7 (p,2α) nuclear reaction analysis. We will report on lithium percolation into GPC prepared at temperatures between 500°C and 1000°C and activated by a 10 MeV gold ion bombardment.

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.

RBS and Raman spectroscopy study of heat-treatment effect on phenolformaldehyde resin☆

Abstract Phenolformaldehyde resin, which is used to make items of glassy carbon hollow ware, contains residual alkaline catalyst and other impurities. Pyrolysis reduces the concentrations of hydrogen and oxygen during condensation and dehydrogenation reactions, which occur between 150 and 1000°C as the resin is converted to a polymeric glassy carbon. Molded samples heat-treated from 150 to 600°C and sprayed samples heat-treated to 1000 and 2500°C were studied using Rutherford backscattering spectrometry (RBS) and Raman spectroscopy. Raman spectroscopy was used to study the structural changes induced by heat treatment. Raman spectroscopy showed that most carbonization occurred at 500 to 600°C while the RBS data from resin samples at each temperature showed that oxygen and sodium concentrations were reduced at higher temperatures.

Production of Heart Valves from Glassy Polymeric Carbon

When certain resins are pyrolized they transform into a glassy polymeric carbon (GPC) with no change in shape. Using this process we have made all-carbon heart valves by rapid molding of the component pieces out of precursor resin, assemblage in the resin stage and pyrolysis to at least 1,000°C to form an accurately articulated device. A heart valve with two occluders set in a carbon ring manufactured out of highly polished carbon has been tested with an excellent record of wear, fatigue, resistance and biocompatibility, especially in contact with blood.

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.

A Carbon Drug Delivery System for Lithium

Lithium was introduced into pyrolyzed phenolic resins by dissolving lithium nitrate (5, 10, and 15% by mass) in a resol precursor. Impregnated specimens were pyrolyzed at 500°C, 575°C, and 650°C in inert atmosphere. After pyrolysis, samples were placed in 5 ml of phosphate buffered saline solution, refrigerated at 5°C, for various Li release times. Inductively coupled plasmaatomic spectroscopy was used to analyze these solutions, which were tested every 24 hours for 5 days; another set was tested after 60 days. Samples containing 5% lithium salt, fired to 500°C, released Li + at a lower rate than those fired at higher temperature. At the early stages of exposure to saline, samples fired at 575°C and 650°C released Li + at a higher rate, which fell to that of 500°C samples after many days. After leaching, nuclear reaction analysis using alpha radiation, with an exposure time of I hour, allowed us to analyze [Li + ] and gradient up to 121μm below the surface. This indicates that a smaller [Li+] remains in 650°C samples than in those fired at 575°C and 500°C. For 500°C samples, [Li + ] near the surface was lower than that for samples fired at 575°C and 650°C. This indicates that 500°C samples release Li + from near the surface, whereas samples fired at higher temperature release Li + from deep below the surface, probably because of higher permeability. Li + release rates of samples fired at 500°C and below 650°C follow a simple diffusion law, with diffusivities between 10 17 and 10 -18 m 2 /s. Li + rate may controlled over long time by a multilayered sprayed precursor with variable concentration.

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.