920 Matching Results

Search Results

Advanced search parameters have been applied.

Light weight cellular structures based on aluminium

Description: An interesting form of lightweight material which has emerged in the past 2 decades is metallic foam. This paper deals with the basic concepts of making metallic foams and a detailed study of foams produced from Al-SiC. In addition, some aspects of cellular solids based on honeycomb structures are outlined including the concept of producing both two-phase foams and foams with composite walls.
Date: February 1, 1997
Creator: Prakash, O.; Embury, J.D.; Sinclair, C.; Sang, H. & Silvetti, P.
Partner: UNT Libraries Government Documents Department

Characterization of SiC fibers by soft x-ray photoelectron and photoabsorption spectroscopies and scanning Auger microscopy

Description: Synchrotron radiation soft x-ray photoelectron and photoabsorption spectroscopy was used to characterize commercially obtained SiC fibers produced by CVD on a W core and followed by a C passivating layer. Depth profiling of the fiber through the C/SiC interface was done by making Si 2p and C 1s core level PES and PAS, as well as scanning Auger microscopy, measurements following Ar{sup +} sputtering. No significant changes in either photoemission or absorption or Auger line shapes were observed versus depth, indicating no significant interfacial reaction. The line shapes of the carbonaceous coatings are predominantely graphite-like and those of the CVD SiC coatings are microcrystalline, with disorder present to some extent in both cases.
Date: August 1996
Creator: Ma, Qing; McDowell, M. W. & Rosenberg, R. A.
Partner: UNT Libraries Government Documents Department

ICP Etching of SiC

Description: A number of different plasma chemistries, including NF{sub 3}/O{sub 2}, SF{sub 6}/O{sub 2}, SF{sub 6}/Ar, ICl, IBr, Cl{sub 2}/Ar, BCl{sub 3}/Ar and CH{sub 4}/H{sub 2}/Ar, have been investigated for dry etching of 6H and 3C-SiC in a Inductively Coupled Plasma tool. Rates above 2,000 {angstrom} cm{sup {minus}1} are found with fluorine-based chemistries at high ion currents. Surprisingly, Cl{sub 2}-based etching does not provide high rates, even though the potential etch products (SiCi{sub 4} and CCl{sub 4}) are volatile. Photoresist masks have poor selectivity over SiC in F{sub 2}-based plasmas under normal conditions, and ITO or Ni are preferred.
Date: February 4, 1999
Creator: Grow, J.M.; Lambers, E.S.; Ostling, M.; Pearton, S.J.; Ren, F.; Shul, R.J. et al.
Partner: UNT Libraries Government Documents Department

Research on microwave joining of SiC. Final report

Description: Work on microwave joining of sintered SiC has showed that small samples could be jointed using Si interlayer (applied as pressed powder); SEM showed a smooth, homogeneous interlayer 50 {mu}m wide. Objective of this contract is to optimize these joints. Results showed that the interlayer could be reduced to 10-20 {mu}m using an oil-based slurry made from Si powder, and to less than 5 {mu}m by plasma spraying Si on one of the SiC surfaces. Direct joints were made in reaction bonded SiC, using the residual Si. Excellent joints with good mechanical properties were obtained in both small specimens and in small scale tube assemblies like in heat exchanger and radiant burner tubes. In situ reaction synthesis from powders to produce a SiC-TiC-SiC joint was demonstrated, as well feasibility of producing SiC from microwave-assisted decomposition of polymer precursors. Finally, new applicator designs, including a compound adjustable iris and a mitered bend single mode cavity, were demonstrated to provide improved heating of larger and longer specimens. This work provides the foundation for scaleup of microwave joining to SiC components for industrial applications.
Date: June 30, 1993
Partner: UNT Libraries Government Documents Department

New perspectives on the fracture of Nicalon fibers

Description: Experimental studies have been conducted to examine the strength and fracture behavior of monofiliment Nicalon{sup 3} SiC fibers ranging in diameter from 8 to 22 {mu}m. The effects of varying fiber diameter, flaw location and flaw population on the tensile behavior of individual fibers were investigated using fractography. Results indicate that variations in fiber diameter influence the apparent fiber fracture toughness K{sub 1c}, with higher K{sub 1c} values observed for decreasing fiber diameters. Observations also suggest that the location of the critical flaw may play a role in the fracture of Nicalon fibers. In addition to surface flaws, three distinct internal flaw populations are seen to cause fracture in Nicalon fibers.
Date: November 1, 1997
Creator: Taylor, S.T.; Lowe, T.C. & Butt, D.P.
Partner: UNT Libraries Government Documents Department

A study of the effect of grain size on the ballistic performance of silicon carbide

Description: The depth of penetration method was used to ballistically evaluate the performance of silicon carbide as a function of grain size. The hot-pressed silicon carbide was backed by 4340 steel Rc = 35 and impacted by tungsten heavy metal projectiles of L/D = 4 at velocities of 1.6 and 1/75 km/s. The hot-pressed silicon carbide was also compared with reaction-sintered silicon carbide of identical thickness in the current study. Results are compared with data previously reported by others.
Date: March 1, 1995
Creator: Cline, C.F.
Partner: UNT Libraries Government Documents Department

Task 6.3/6.7.4 - Engineering Performance of Advanced Structural Materials

Description: Future energy systems will be required to fire low-grade fuels and meet higher energy conversion efficiencies than today's systems. The steam cycle used at present is-limited to a maximum temperature of 550C, because above that the stainless steel tubes deform and corrode excessively. However, to boost efficiency significantly, much higher working fluid temperatures are required. Although high-temperature alloys will suffice for the construction of these components in the near term, the greatest efficiency increases can only be reached with the use of advanced structural ceramics
Date: November 16, 1998
Creator: Hurley, John P. & Kay, John P.
Partner: UNT Libraries Government Documents Department

Molecular-jet chemical vapor deposition of SiC

Description: SiC films have been deposited by molecular-jet chemical vapor deposition (MJCVD) on Si(001) substrates. Methylsilane (MS) diluted in He was used as a precursor for deposition under conditions which produced a MS molecular beam with 0.365 eV translational energy. Films grown at temperatures between 1000 and 1150 C and above {approx}1200 C were single crystal as judged by electron channeling, while those grown at intermediate temperatures were polycrystalline. Films grown at lower temperatures generally had a smoother surface morphology for moderate thicknesses, although all films showed at least some degree of faceting. The best thick films, up to 4 {mu}m, were obtained for substrate temperatures of {approx}1210 C under flow conditions which produced a deposition rate of {approx}1200 {angstrom} per minute.
Date: September 1, 1995
Creator: Lubben, D.; Jellison, G.E. & Modine, F.A.
Partner: UNT Libraries Government Documents Department

Task 6.3/6.7.4 - Silicon Carbide Joining

Description: Future energy systems will be required to fire low-grade fuels and meet higher energy conversion efficiencies than today's systems. The steam cycle used at present is limited to a maximum temperature of 550 "C, because above that the stainless steel tubes deform and corrode excessively. To boost efficiency significantly, much higher working fluid temperatures are required. Although high-temperature alloys will suffice for the construction of these components in the near-term, the greatest efficiency increases can only be reached with the use of advanced structural ceramics such as silicon carbide (SiC). However, SiC does not melt, but instead sublimes at temperatures over 2000 "C. Therefore, it is not possible to join pieces of it through welding, and most brazing compounds have much lower melting points so the joints lose strength at temperatures much lower than the maximum use temperature of the SiC. Since larger objects, such as heat exchangers, cannot be easily created from smaller ceramic pieces, the size of the SiC structures that can presently be manufactured are limited by the size of the sintering furnaces (approximately 10 feet for sintered alpha silicon carbide). In addition, repair of the objects will require the use of field joining techniques. Some success has been made by causing silicon and carbon to react at 1400 0-1 500 "C to form SiC in a joint (Rabin, 1995) but these joints contain continuous channels of unreacted silicon which cause the joints to corrode and creep excessively at temperatures below 1260 "C (Breder, 1996). At present, no joining techniques are available that allow sintered alpha SiC to be used to its full potential.
Date: February 1, 1998
Creator: Hurley, John P. & Kay, John P.
Partner: UNT Libraries Government Documents Department

Determining the toughness of ceramics from Vickers indentationsusing the crack-opening displacements: An experimental study

Description: Recently, a method for evaluating the fracture toughness of ceramics has been proposed based on the computed crack-opening displacements of cracks emanating from Vickers hardness indentations. In order to verify this method, experiments were carried out to determine the toughness of a commercial silicon carbide ceramic, Hexaloy SA, by measuring the crack-opening profiles of such Vickers indentation cracks. While the obtained toughness value of Ko = 2.3 MPavm was within 10% of that measured using conventional fracture toughness testing, the computed crack-opening profiles corresponding to this toughness displayed poor agreement with those measured experimentally, raising concerns about the suitability of this method for determining the toughness of ceramics. The effects of subsurface cracking and cracking during loading are considered as possible causes of such discrepancies, with the former based on evidence observed for secondary radial cracking which affected the crack opening profile and deduced toughness values.
Date: October 30, 2002
Creator: Kruzic, J.J. & Ritchie, R.O.
Partner: UNT Libraries Government Documents Department

On the Development of MMCS Containing Copper with Silicon Carbide Reinforcement using Nanomaterials and Dynamic Compaction

Description: Metal matrix composites (MMCs) are promising engineering materials for a wide spectrum of applications. There are many possible matrix-reinforcement combinations including MMCs containing copper or copper alloy matrices [1-3]. The present study is concerned with copper reinforced with SiC particles. The materials studied here were processed from nano-scale matrix powders and consolidated using dynamic compaction.
Date: April 10, 2002
Creator: Popov, V A; Lesuer, D R; Kotov, I A; Ivanov, V V; Smirnov, O M; Marmulev, A V et al.
Partner: UNT Libraries Government Documents Department

Microstructure development in hot-pressed silicon carbide: Effects of aluminum, boron, and carbon additives

Description: SiC was hot-pressed with aluminum, boron, and carbon additives. The Al content was modified either to obtain SiC samples containing a continuous Al gradient, or to vary the average Al content. In both cases, dramatic changes in microstructure, phase composition, and grain boundary structure were observed as a result of the Al variation. Similar processing and characterization were done with modified boron and carbon average contents. The systematic experiments allowed identification of the roles of Al, B, and C in developing grain morphology and phase composition. The experiments also clarified the mechanical property responses to microstructural modification. Tailoring of the SiC microstructure to suit different applications would be possible.
Date: December 18, 2002
Creator: Zhang, Xiao Feng; Yang, Qing & De Jonghe, Lutgard C.
Partner: UNT Libraries Government Documents Department

Research on microwave joining of SiC

Description: Results: identification of optimum joining temperature range for reaction bonded Si carbide at 1420-1500 C; demonstration that specimens joined within this range have fracture roughness greater than as-received material; and demonstration of ability to use SiC formed in situ from the decomposition of polycarbosilane as a joining aid for sintered Si carbide. In the latter case, the interlayer material was also shown to fill any pores in the joining specimens near the interlayer. Together with the demonstration of leaktight joints between tube sections of reaction bonded and sintered SiC under the previous contract, these results provide the foundation for scaleup to joining of the larger and longer tubes needed for radiant burner and heat exchanger tube assemblies. The formation of SiC in situ is important because maintaining roundness of these large tubes is a technical challenge for the tube manufacturer, so that formation of a leaktight joint may require some degree of gap filling.
Date: July 31, 1995
Creator: Silberglitt, R.
Partner: UNT Libraries Government Documents Department

On the statistical strength of nicalon fibers and its characterization

Description: Nicalon fibers are one of the most attractive ceramic fibers for reinforcing high temperature structural composites. Experiment show that the diameter variation (from 8 to 22 {mu}m) in a tow of commercial Nicalon fibers has an effect on statistical strength distribution of Nicalon fibers. Therefore, an appropriate characterization of the statistical distribution of fiber strength, capable of accounting for the effect of diameter variation, is required. The two-parameter single-modal Weibull distribution is found inadequate for characterizing the strength of Nicalon fibers. This work demonstrates that a 3-parameter modified Weibull distribution can successfully characterize the strength of Nicalon fibers; this distribution yields a higher {beta} value, which indicates less scatter in fiber strength than with the 2-parameter single modal Weibull distribution. It more accurately treats the strength variation caused by diameter variation. It is also much easier to use than the 4-parameter bimodal. Moreover, it is seen to more accurately treat the strength variation caused by diameter variation than the single modal 2-parameter analysis.
Date: February 1, 1997
Creator: Zhu, Y.T.; Taylor, S.T.; Stout, M.G.; Butt, D.P.; Blumenthal, W.R. & Lowe, T.C.
Partner: UNT Libraries Government Documents Department

The effect of neutron irradiation on silicon carbide fibers

Description: Nine types of SiC fiber have been exposed to neutron radiation in the Advanced Test Reactor at 250 C for various lengths of time ranging from 83 to 128 days. The effects of these exposures have been initially determined using scanning electron microscopy. The fibers tested were Nicalon{trademark} CG, Tyranno, Hi-Nicalon{trademark}, Dow Corning SiC, Carborundum SiC, Textron SCS-6, polymethysilane (PMS) derived SiC from the University of Michigan, and two types of MER SiC fiber. This covers a range of fibers from widely used commercial fibers to developmental fibers. Consistent with previous radiation experiments, Nicalon fiber was severely degraded by the neutron irradiation. Similarly, Tyranno suffered severe degradation. The more advanced fibers which approach the composition and properties of SiC performed well under irradiation. Of these, the Carborundum SiC fiber appeared to perform the best. The Hi-Nicalon and Dow Corning Fibers exhibited good general stability, but also appear to have some surface roughening. The MER fibers and the Textron SCS-6 fibers both had carbon cores which adversely influenced the overall stability of the fibers.
Date: January 1, 1997
Creator: Newsome, G.A.
Partner: UNT Libraries Government Documents Department

Long-term performance of ceramic matrix composites at elevated temperatures: Modelling of creep and creep rupture

Description: The models developed, contain explicit dependences on constituent material properties and their changes with time, so that composite performance can be predicted. Three critical processes in ceramic composites at elevated temperatures have been modeled: (1) creep deformation of composite vs stress and time-dependent creep of fibers and matrix, and failure of these components; (2) creep deformation of ``interface`` around broken fibers; and (3) lifetime of the composite under conditions of fiber strength loss over time at temperature. In (1), general evolution formulas are derived for relaxation time of matrix stresses and steady-state creep rate of composite; the model is tested against recent data on Ti-MMCs. Calculations on a composite of Hi-Nicalon fibers in a melt-infiltrated SiC matrix are presented. In (2), numerical simulations of composite failure were made to map out time-to-failure vs applied load for several sets of material parameters. In (3), simple approximate relations are obtained between fiber life and composite life that should be useful for fiber developers and testers. Strength degradation data on Hi-Nicalon fibers is used to assess composite lifetime vs fiber lifetime for Hi-Nicalon fiber composites.
Date: July 31, 1996
Creator: Curtin, W.A.; Fabeny, B.; Ibnabdeljalil, M.; Iyengar, N. & Reifsnider, K.L.
Partner: UNT Libraries Government Documents Department

The synthesis, characterization and formation chemistry of Si-C-N-O-M ceramic and composite powders. Final technical report

Description: Brief summaries are given for the work in each of the following areas: (1) general pattern of behavior in the conversion processes of SiNC precursors; (2) thermal and oxidative stability of SiNC ceramics; and (3) modification of PMVSEDA(poly(methylvinyl)silylethylenediamine) with borane-dimethyl sulfide.
Date: August 1, 1998
Creator: Mariam, Y.H.
Partner: UNT Libraries Government Documents Department

Growth of silicon carbide on silicon via reaction of sublimed fullerenes and silicon

Description: Epitaxial silicon carbide films are grown on Si(100) substrates at a surface temperature of 1,200 K via fullerene precursors. Films have been grown up to a thickness of 2,500 {angstrom}. The growth rate of the SiC film is not limited by the surface reaction rate of fullerene with silicon at these temperatures, rather by the arrival rate of the reactants Si (by diffusion from substrate or from gas phase) or fullerene. This results in rapid film growth. Films have been characterized by low energy electron diffraction, ultraviolet photoelectron spectroscopy and Auger electron spectroscopy. Stoichiometric, epitaxial SiC films are grown. Supply of silicon to the growing SiC surface via sublimation greatly reduces the tendency for silicon diffusion to form voids at the Si/SiC interface.
Date: February 1, 1996
Creator: Hamza, A.V. & Balooch, M.
Partner: UNT Libraries Government Documents Department

Fracture toughness and impact properties of laminated metal composites

Description: Laminated metal composites consist of alternating metal (or metal matrix composite) layers bonded together. These materials can provide fracture toughness and impact properties superior to those of the component materials. These properties are a function of component material properties, laminate architecture (volume fraction, thickness) and interface properties. Properties are compared for seven lightweight materials.
Date: March 4, 1996
Creator: Lesuer, D.R.; Riddle, R.A.; Gogolewski, R.P.; Syn, C.K. & Cunningham, B.J.
Partner: UNT Libraries Government Documents Department

Influence of fiber lay-up sequence on mechanical properties of SiC(f)/SiC composites

Description: Mechanical properties of Nicalon-fiber-reinforced silicon carbide matrix composites with two different fiber lay-up sequences (0{degree}/40{degree}/60{degree} and 0{degree}/45{degree}) were evaluated at various temperatures ranging from ambient to 1300{degree}C. Composites with 0{degree}/40{degree}/60{degree} fiber lay-up sequence showed a higher average first matrix cracking stress than that of 0{degree}/45{degree} composites. The measured room-temperature ultimate strength of the 0{degree}/40{degree}/60{degree} composites was 300 MPa compared to 180 MPa for the 0{degree}/45{degree} composites. These measured ultimate strengths were correlated to the predictions made with an analytical model and to in-situ fiber strength characteristics. The large difference in room-temperature ultimate strengths between the two sets of composites is attributed to the relative contributions of the off-axis fibers to the load-bearing capacity of each composite. Up to 1200{degree}C, ultimate strength and work-of-fracture in each set of composites increased, but then declined above 1300{degree}C. The decreases were correlated to in-situ Nicalon fiber strength and fiber/matrix interface degradation.
Date: March 1996
Creator: Singh, D.; Singh, J. P. & Sutaria, M.
Partner: UNT Libraries Government Documents Department