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All sourcesNASA NTRSDOE OSTI
NASA NTRSDate not supplied

SiO2 Activity Measurements in the Gd2O3-SiO2 System Via Knudsen Effusion Mass Spectrometry (KEMS)

Environmental barrier coatings (EBCs) are required for the use of silicon carbide (SiC)-based ceramic matrix composites (CMCs) in gas turbine engines, as they protect the SiO 2 thermally grown oxide (TGO) formed at the CMC/coating interface from volatilization in a combustion environment. Current-generation EBCs consist of a silicon bond coat and a rare earth (RE) silicate topcoat. The RE silicate topcoat is exposed to high-velocity steam during engine operation, and SiO 2 within the coating can preferentially volatilize to form Si(OH) 4 gas species, albeit at a lesser degree than SiO 2 alone. Therefore, it is of interest to investigate the stability of these materials in steam as a function of their RE. In this study, SiO 2 activity, which can be related to the partial pressure of Si(OH) 4 , was measured across the Gd 2 O 3 -SiO 2 phase diagram, which includes Gd 2 SiO 5 , Gd 9.33 (SiO 4 ) 6 O 2 , and Gd 2 Si 2 O 7 as line compounds, via Knudsen Effusion Mass Spectrometry (KEMS). Values obtained for the Gd 2 O 3 -SiO 2 system were compared to those of other RE2O3-SiO2 systems and contextualized in terms of EBC viability.

EBCs↗
NASA NTRS1984

XPS study of the Al/SiO2 interface viewed from the SiO2 side

The first nondestructive measurement of the chemical and physical characteristics of the interface between bulk SiO2 and thick aluminum films is presented. Both X-ray photoelectron spectroscopy (XPS) and electrical measurements of unannealed resistively evaporated Al films on thermal SiO2 indicate an atomically abrupt interface. Postmetallization annealing (PMA) at 450 C induces reduction of the SiO2 by the aluminum, resulting in the layer ordering SiO2/Al2O3/Si/Al. The XPS measurement is performed from the SiO2 side after removal of the Si substrate after etching with XeF2 gas and thinning of the SiO2 layer with HF:ETOH. This represents a powerful new approach to the study of metal-insulator and other interfaces.

Hecht, M. H.↗
NASA NTRS1993

Mass spectrometric studies of SiO2 deposition in an indirect plasma enhanced LPCVD system

Reaction pathways for the low temperature deposition of SiO2 from silane and indirect plasma-excited oxygen-nitrogen mixtures are proposed based on experimental evidence gained from mass spectrometry in an indirect plasma enhanced chemical vapor deposition chamber. It was observed that about 80-85 percent of the silane was oxidized to byproduct hydrogen and only about 15-20 percent to water. Such conversion levels have led us to interpret that silanol (SiH3OH) could be the precursor for SiO2 film deposition, rather than siloxane /(SiH3)2O/ which has generally been cited in the literature. From mass spectrometry, we have also shown the effects of the plasma, and of mixing small amounts of N2 with the oxygen flow, in increasing the deposition rate of SiO2. Free radical reaction of nitric oxide, synthesized from the reaction of oxygen and nitrogen in the plasma chamber, and an *ncrease in atomic oxygen concentration, are believed to be the reasons for these SiO2 deposition rate increases. Through mass spectrometry we have, in addition, been able to identify products, presumably originating from terminating reactions, among a sequence of chemical reactions proposed for the deposition of SiO2.

Iyer, R.↗
NASA NTRS1998

SiC Recession Due to SiO2 Scale Volatility Under Combustion Conditions: Thermodynamics and Gaseous Diffusion Model - Part 2

In combustion environments, volatilization of SiO2 to Si-O-H(g) species is a critical issue. Available thermochemical data for Si-O-H(g) species were used to calculate boundary layer controlled fluxes from SiO2. Calculated fluxes were compared to volatilization rates Of SiO2 scales grown on SiC which were measured in Part 1 of this paper. Calculated volatilization rates were also compared to those measured in synthetic combustion gas furnace tests. Probable vapor species were identified in both fuel-lean and fuel-rich combustion environments based on the observed pressure, temperature and velocity dependencies as well as the magnitude of the volatility rate. Water vapor is responsible for the degradation of SiO2 in the fuel-lean environment. Silica volatility in fuel-lean combustion environments is attributed primarily to the formation of Si(OH)4(g) with a small contribution of SiO(OH)2(g).

Opila, Elizabeth J.↗
NASA NTRS2016

Space Weathering of Silicates Simulated by Successive Laser Irradiation: in Situ Reflectance Measurements of Fo90, Fo99+, and SiO2

Pulsed-laser irradiation causes the visible-near-infrared spectral slope of olivine (Fo90 and Fo99+) and SiO2 to increase (redden), while the olivine samples darken and the SiO2 samples brighten slightly. XPS analysis shows that irradiation of Fo90 produces metallic Fe. Analytical SEM and TEM measurements confirm that reddening in the Fo90 olivine samples correlates with the production of "nanophase" metallic Fe (npFe0) grains, 20-50 nm in size. The reddening observed in the SiO2 sample is consistent with the formation of SiO or other SiOx species that absorb in the visible. The weak spectral brightening induced by laser irradiation of SiO2 is consistent with a change in surface topography of the sample. The darkening observed in the olivine samples is likely caused by the formation of larger npFe0 particles, such as the 100-400 nm diameter npFe0 identified during our TEM analysis of Fo90 samples. The Fo90 reflectance spectra are qualitatively similar to those in previous experiments suggesting that in all cases formation of npFe0 is causing the spectral alteration. Finally, we find that the accumulation of successive laser pulses cause continued sample darkening in the Vis-NIR, which suggests that repeated surface impacts are an efficient way to darken airless body surfaces.

Airless planetary bodies↗
NASA NTRS1975

Electron radiation effects on time-dependent dielectric breakdown in SiO2 films

An experiment testing the effect of ionizing radiation on breakdown characteristics of SiO2 films is presented. Silicon wafers were oxidized and metallized, and a capacitor array was etched into a control sample while the rest were first irradiated with 1 MeV electrons and then etched. Time-dependent dielectric tests were made on all the capacitors, and the average characteristics of 96 capacitors are illustrated graphically. The curves are consistent with the model of holes trapped in the SiO2 film during irradiation leading to a retarding field for positive ion emission and drift toward the interface. It is shown how an externally applied field is reduced by the trapped charge, and that changes in the dielectric breakdown properties of the SiO2 film after irradiation depend on the positive trapped charge near the metal interface.

Li, S. P.↗
NASA NTRS2008

High Temperature Aerogels in the Al2O3-SiO2 System

Al2O3-SiO2 aerogels are of interest as constituents of thermal insulation systems for use at high temperatures. Al2O3 and mullite aerogels are expected to crystallize at higher temperatures than their SiO2 counterparts, hence avoiding the shrinkages that accompany the formation of lower temperature SiO2 phases and preserving pore structures into higher temperature regimes. The objective of this work is to determine the influence of processing parameters on shrinkage, gel structure (including surface area, pore size and distribution) and pyrolysis behavior.

Hurwitz, Frances I.↗
NASA NTRS2003

Process for Smoothing an Si Substrate after Etching of SiO2

A reactive-ion etching (RIE) process for smoothing a silicon substrate has been devised. The process is especially useful for smoothing those silicon areas that have been exposed by etching a pattern of holes in a layer of silicon dioxide that covers the substrate. Applications in which one could utilize smooth silicon surfaces like those produced by this process include fabrication of optical waveguides, epitaxial deposition of silicon on selected areas of silicon substrates, and preparation of silicon substrates for deposition of adherent metal layers. During etching away of a layer of SiO2 that covers an Si substrate, a polymer becomes deposited on the substrate, and the substrate surface becomes rough (roughness height approximately equal to 50 nm) as a result of over-etching or of deposition of the polymer. While it is possible to smooth a silicon substrate by wet chemical etching, the undesired consequences of wet chemical etching can include compromising the integrity of the SiO2 sidewalls and undercutting of the adjacent areas of the silicon dioxide that are meant to be left intact. The present RIE process results in anisotropic etching that removes the polymer and reduces height of roughness of the silicon substrate to less than 10 nm while leaving the SiO2 sidewalls intact and vertical. Control over substrate versus sidewall etching (in particular, preferential etching of the substrate) is achieved through selection of process parameters, including gas flow, power, and pressure. Such control is not uniformly and repeatably achievable in wet chemical etching. The recipe for the present RIE process is the following: Etch 1 - A mixture of CF4 and O2 gases flowing at rates of 25 to 75 and 75 to 125 standard cubic centimeters per minute (stdcm3/min), respectively; power between 44 and 55 W; and pressure between 45 and 55 mtorr (between 6.0 and 7.3 Pa). The etch rate lies between approximately equal to 3 and approximately equal to 6 nm/minute. Etch 2 - O2 gas flowing at 75 to 125 stdcm3/min, power between 44 and 55 W, and pressure between 50 and 100 mtorr (between 6.7 and 13.3 Pa).

Turner, Tasha↗
NASA NTRS2013

Passivation of Flexible YBCO Superconducting Current Lead With Amorphous SiO2 Layer

Adiabatic demagnetization refrigerators (ADR) are operated in space to cool detectors of cosmic radiation to a few 10s of mK. A key element of the ADR is a superconducting magnet operating at about 0.3 K that is continually energized and de-energized in synchronism with a thermal switch, such that a piece of paramagnetic salt is alternately warm in a high magnetic field and cold in zero magnetic field. This causes the salt pill or refrigerant to cool, and it is able to suck heat from an object, e.g., the sensor, to be cooled. Current has to be fed into and out of the magnets from a dissipative power supply at the ambient temperature of the spacecraft. The current leads that link the magnets to the power supply inevitably conduct a significant amount of heat into the colder regions of the supporting cryostat, resulting in the need for larger, heavier, and more powerful supporting refrigerators. The aim of this project was to design and construct high-temperature superconductor (HTS) leads from YBCO (yttrium barium copper oxide) composite conductors to reduce the heat load significantly in the temperature regime below the critical temperature of YBCO. The magnet lead does not have to support current in the event that the YBCO ceases to be superconducting. Cus - tomarily, a normal metal conductor in parallel with the YBCO is a necessary part of the lead structure to allow for this upset condition; however, for this application, the normal metal can be dispensed with. Amorphous silicon dioxide is deposited directly onto the surface of YBCO, which resides on a flexible substrate. The silicon dioxide protects the YBCO from chemically reacting with atmospheric water and carbon dioxide, thus preserving the superconducting properties of the YBCO. The customary protective coating for flexible YBCO conductors is silver or a silver/gold alloy, which conducts heat many orders of magnitude better than SiO2 and so limits the use of such a composite conductor for passing current across a thermal gradient with as little flow of heat as possible to make an efficient current lead. By protecting YBCO on a flexible substrate of low thermal conductivity with SiO2, a thermally efficient and flexible current lead can be fabricated. The technology is also applicable to current leads for 4 K superconducting electronics current biasing. A commercially available thin-film YBCO composite tape conductor is first stripped of its protective silver coating. It is then mounted on a jig that holds the sample flat and acts as a heat sink. Silicon dioxide is then deposited onto the YBCO to a thickness of about 1 micron using PECVD (plasma-enhanced chemical vapor deposition), without heating the YBCO to the point where degradation occurs. Since SiO2 can have good high-frequency electrical properties, it can be used to coat YBCO cable structures used to feed RF signals across temperature gradients. The prime embodiment concerns the conduction of DC current across the cryogenic temperature gradient. The coating is hard and electrically insulating, but flexible.

Johannes, Daniel↗
NASA NTRS2016

High-Temperature, Perhaps Silicic, Volcanism on Mars Evidenced by Tridymite Detection in High-SiO2 Sedimentary Rock at Gale Crater, Mars

The Mars Science Laboratory (MSL) rover, Curiosity, has been exploring sedimentary rocks within Gale crater since landing in August, 2012. On the lower slopes of Aeolis Mons (a.k.a. Mount Sharp), drill powder was collected from a high-silica (74 wt% SiO2) outcrop named Buckskin (BK). It was a surprise to find that the Buckskin sample contained significant amounts of the relatively rare silica polymorph tridymite. We describe the setting of the Buckskin sample, the detection of tridymite by the MSL Chemistry and Mineralogy (CheMin) X-ray diffraction instrument, and detection implications. Geologic setting: The Buckskin outcrop is part of the Murray formation exposed in the Marias Pass area. The formation was previously studied by CheMin in the Pahrump Hills member [1] where three samples of drill fines were analyzed (Confidence Hills (CH), Mojave2 (MJ) and Telegraph Peak (TP) [2]). Assuming approximately horizontal bedding, the Buckskin outcrop is approx.15 m stratigraphically above the bottom of the Pahrump Hills member. Mudstone, generally characterized by fine lamination, is the dominant depositional facies [1]. Buckskin Mineralogical and Chemical Composition: The CheMin instrument and XRD pattern analysis procedures have been previously discussed [3-6]. The diffraction pattern used for quantitative XRD analysis (Fig. 1) is the sum of the first 4 of 45 diffraction images. The remaining images are all characterized by both on-ring and off-ring diffraction spots that we attributed to poor grain motion and particle clumping. Coincident with particle clumping was a significant decrease in the intensity of the tridymite diffraction peaks (Fig. 2a). The derived mineralogical composition of the crystalline component (derived from the first 4 diffraction images) is given in Table 1. The tridymite is well-crystalline and its pattern is refined as monoclinic tridymite (Fig 1). Mineral chemical compositions were derived from XRD unit cell parameters or obtained from stoichiometry. The XRD-calculated amorphous component was 50 +/- 15 wt%. We constrained the value to 60 wt% because it is the minimum value necessary to give a positive Al2O3 concentration for the amorphous component using APXS data for the post-sieve dump pile (Table 2). The amorphous component has high SiO2 (approx.77 wt%) and high anion (SO3+P2O5+Cl ~10 wt%) concentrations. Calculation shows that a cation-anion balance is achieved if the cations in the amorphous component except SiO2 and TiO2, which do not readily form salts, are assumed to be present as amorphous mixed-cation sulfates, phosphates, and chlorides (or perchlorates/ chlorates).

Morris, R. V.↗
NASA NTRS2021

Melting and Crystallization Behavior of CaO-MgO-Al2O3-SiO2 Silicates Relevant to Turbine Engine Applications

The melting and crystallization behavior of four quaternary CaO-MgO-Al2O3-SiO2(CMAS) silicates were investigated. The CaO:SiO2 ratios of these systems were based on various terrestrial sources of ingested particles relevant to gas turbine engine operating environments. Melting behavior was characterized using differential scanning calorimetry, and high temperature intrinsic crystallization products were determined by furnace heat treatments of the glasses at 1200°C, 1300°C, and 1400°C. The silicates exhibited a wide range of melting temperatures from ~1240°C up to ~1500°C, with most of the compositions exhibiting incongruent melting behavior. High temperature crystallization products included CaSiO3,CaAl2Si2O8, Ca2MgSi2O7, and Ca(Mg,Al)Si2O6, although SiO2 was the only crystalline phase observed at 1400°C.

Jamesa L. Stokes↗
NASA NTRS1974

Silicon ions below 100 km: A case for SiO2(+)

Silicon ions are normally detected at altitudes above 100 km and within sporadic E layers. Traces have rarely been observed within the more permanent metallic layer near 93 km. This is surprising since silicon is an important constituent of chondritic meteorites, which ablate material in this region to provide a primary source of the metallic species observed there. Evidence is presented that Si(+)ions form SiO2(+) at the lower altitudes, and exist in this ionic state prior to recombination. A rocket launched from El Arenosillo, Spain on 3 July 1972, at 0743 LMT, during the predicted period of the Beta Taurids meteor shower, passed through a continuous belt of metallic ions that began near 85 km, ended near 115 km, and exhibited an order of magnitude increase in the form of a layer near 114 km. Si(+)was measured in and below the ledge down to 103 km. It showed a rapid decrease below this height. Radiative association is offered as a primary mechanism for SiO2(+) production.

Goldberg, R. A.↗
NASA NTRS1978

Shear strength of metal - SiO2 contacts

The strength of the bond between metals and SiO2 was studied by measuring the static coefficient of friction of metals contacting alpha-quartz in ultrahigh vacuum. It was found that copper with either chemisorbed oxygen, nitrogen, or sulphur exhibited higher contact strength on stoichiometric SiO2 than did clean copper. Since the surface density of states induced by these species on copper is similar, it appears that the strength of the interfacial bond can be related to the density of states on the metal surface.

Pepper, S. V.↗
NASA NTRS1978

Shear strength of metal - SiO2 contacts

The strength of the bond between metals and SiO2 is studied by measuring the static coefficient of friction of metals contacting alpha-quartz in ultrahigh vacuum. It was found that copper with either chemisorbed oxygen, nitrogen or sulphur exhibited higher contact strength on stoichiometric SiO2 than did clean copper. Since the surface density of states induced by these species on copper is similar, it appears that the strength of the interfacial bond can be related to the density of states on the metal surface.

Pepper, S. V.↗
NASA NTRS1983

Shock temperatures of SiO2 and their geophysical implications

High pressure shock state temperatures of SiO2 were measured in specimens of single crystal alpha-quartz and fused quartz. Projectile impact and pyrometry techniques were employed to impart pressures in the 60-140 GPa range. Shock-induced phase transformations were observed near 70 and 50 GPa along the alpha- and fused-quartz Hugoniots. It is suggested that the transformation consists of melting of the shock-synthesized stishovite, with melting delayed by the superheating of the crystalline phase. Details of the stishovite-liquid phase boundaries have been determined, including stishovite melting at 4500 K under 70 GPa, with the melting accompanied by a relative volume change of about 2.7% and a latent heat of fusion of about 2.4 MJ/kg. At 100 GPa, an approximate limit of 3500 K for the melting of SiO2 is indicated for solid mantle mineral assemblages, such as the earth's mantle

Lyzenga, G. A.↗
NASA NTRS1984

Behavior of Al2O3 and SiO2 with heating in a Cl2 + CO stream

Differential thermal analysis (DTA) and Thermogravimetric analysis (TGA) were used to study the chlorination of alpha-Al2O3, gamma-Al2O3 and amorphous SiO2 in a Cl + CO stream, for the preparation of AlCl3 and SiCl4. The chlorination starting temperatures were 235 deg for Al2O3 and 680 deg for SiO2. The chlorination of alpha- and gamma-Al2O3 takes place via the formation of AlOCl as an intermediate product, and its subsequent dissociation at 480 to 560 deg, according to 3AlOCl yields AlCl3 + Al2O3. The chlorination activation energies are given for the three oxides.

Shchetinin, L. K.↗
NASA NTRS1983

Characteristics of Si3N4-SiO2-Ce2O3 compositions sintered in high-pressure nitrogen

Full-density Si3N4-SiO2-Ce2O3 compositions were prepared by sintering with 2.5 MPa nitrogen pressure at temperatures of 1900 and 2090 C. Room-temperature flexural strengths near 700 MPa for sintered material compared favorably with the strength of hot-pressed material. At 1370 C, where flexural strengths as high as 363 MPa were obtained, it was observed that the coarsest structure was the strongest and the finest structure was the weakest. One of the compositions tested, Si3N4-8.7 wt pct SiO2-8.3 wt pct-Ce2O3, was found to have excellent 200-h oxidation resistance at 700, 1000, and 1370 C, without incidence of 700 to 1000 C phase instability and cracking.

Sanders, W. A.↗
NASA NTRS1985

Study of the kinetics and mechanism of the thermal nitridation of SiO2

X-ray photoelectron spectroscopy (XPS) has been used to study the nitridation time and temperature dependence of the nitrogen distribution in thermally nitrided SiO2 films. The XPS data show that the maximum nitrogen concentration near the (SiO(x)N(y)/Si interface is initially at the interface, but moves 20-25 A away from the interface with increasing nitridation time. Computer modeling of the kinetic processes involved is carried out and reveals a mechanism in which diffusing species, initially consisting primarily of nitrogen, react with the substrate, followed by formation of the oxygen-rich oxynitride due to reaction of the diffusing oxygen displaced by the slower nitridation of the SiO2. The data are consistent with this mechanism provided the influence of the interfacial strain on the nitridation and oxidation kinetics is explicitly accounted for.

Vasquez, R. P.↗