Celia I. Merzbacher

Electronic connection to the interior of a mesoporous insulator with nanowires of crystalline RuO2

Highly porous materials such as mesoporous oxides are of technological interest for catalytic, sensing and remediation applications: the mesopores (of size 2–50 nm) permit ingress by molecules and guests that are physically excluded from microporous materials. Connecting the interior of porous materials with a nanoscale or ‘molecular’ wire would allow the direct electronic control (and monitoring) of chemical reactions and the creation of nanostructures for high-density electronic materials. The challenge is to create an electronic pathway (that is, a wire) within a mesoporous platform without greatly occluding its free volume and reactive surface area. Here we report the synthesis of an electronically conductive mesoporous composite—by the cryogenic decomposition of RuO 4—on the nanoscale network of a partially densified silica aerogel. The composite consists of a three-dimensional web of interconnected (∼4-nm in diameter) crystallites of RuO2, supported conformally on the nanoscopic silica network. The resulting monolithic (RuO2∥SiO 2) composite retains the free volume of the aerogel and exhibits pure electronic conductivity. In addition to acting as a wired mesoporous platform, the RuO2-wired silica aerogel behaves as a porous catalytic electrode for the oxidation of chloride to molecular chlorine.

29Si NMR and infrared reflectance spectroscopy of low-silica calcium aluminosilicate glasses

Abstract Selected calcium aluminosilicate glass compositions have been investigated using nuclear magnetic resonance (NMR) and infrared reflectance spectroscopy, with emphasis on compositions in the silica-poor portion of the system. In particular, a structural explanation for a maximum observed in the glass transition temperature at roughly 15 mol% SiO 2 was sought. 29 Si chemical shifts from magic-angle spinning NMR data are nearly constant in the low-silica glasses and are consistent with depolymerized Q 2 species, or fully polymerized Q 4 tetrahedra with 4 Al next-nearest neighbors. Static 29 Si NMR spectra bear no evidence of asymmetric Q 2 or Q 3 sites; however, the asymmetric component could be hidden by extreme broadening due to structural disorder. The functions ϵ″ and −Imag(1/ϵ) have been calculated from infrared reflectance spectra by a Kramers-Kronig transformation. A change in lineshape which parallel the maximum in T g is attributed to variation in the distribution of tetrahedral aluminate species.

Carbon aerogels as broadband non-reflective materials

Abstract We report the reflectance properties over a wide spectral range of five carbon aerogels prepared by different synthetic routes. Carbon aerogels that were prepared by the conventional base-catalyzed hydration and polymerization of a resorcinol–formaldehyde (RF) solution followed by pyrolysis in an inert atmosphere have the lowest reflectance among all the aerogels investigated. The directional hemispherical reflectance (DHR) of the low density 0.07 g/cm 3 conventional carbon aerogel studied is on the order of 0.3% across the entire 40,000–700 cm −1 ( 0.25–14.3 μm ) region. The other carbon aerogels studied all have DHR values below ∼6% in the infrared and below 2.5% in the ultraviolet. Increasing the incidence angle from 8° to 30° leads to an increase of ∼0.2% in the DHR of each material. The low reflectivity of carbon aerogels is attributed to high absorption by carbon along with the rough internal and external surfaces of the aerogel structure.

Tailoring advanced nanoscale materials through synthesis of composite aerogel architectures

Using silica sol as a glue, we have developed a generalmethod to prepare composites of particulate guests and na-noscale silica that are highly porous, have high surface areas,and can be nanoscopically tailored to suit specific applica-tions. The properties of the particulate guest are retained inthe composite, thereby creating a composite that unites thecontinuous mesoporous network and high surface area of sil-ica gel networks with pre-selected chemical, physical, and op-tical properties of the guest solid. The use of sol–gel chemis-try to create these composites affords enormous flexibility totailor them with solution- and gas-phase reagents at variousstages of the wet gel preparation or after supercritically dry-ing the wet gel to form the aerogel, as shown in Figure 1.Sol–gels and sol–gel composites can be prepared with low-er densities and higher porosities and surface areas than con-ventional ceramics, which confers on them novel physicaland chemical properties.

Dissolution Kinetics of a Simple Analogue Nuclear Waste Glass as a Function of Ph, Time and Temperature

We have measured the dissolution rate of a simple five-component borosilicate glass (Na 2 O, CaO, Al 2 O 3 , B 2 O 3 , SiO 2 ) using a flow-through system. The experiments were designed to measure the dissolution rate constant over the interval pH 1 through pH 13 at 3 temperatures (25°, 50° and 70°C). Dilute buffers were used to maintain a constant pH. Analyses of solutions and solid surfaces provided information that is used to develop a kinetic model for glass dissolution. Under all conditions we eventually observed linear dissolution kinetics. In strongly acidic solutions (pH 1 to pH 3) all components but Si were released in their stoichiometric proportions and a thick, Si-rich gel was formed. In mildly acidic to neutral solutions the gel was thinner and was both Si- and Al-rich, while the other components were released to solution in stoichiometric proportions. In mildly to strongly alkaline solutions all components were released to solution in stoichiometric proportions. By varying the flow rate at each pH we demonstrated a lack of transport control of the dissolution rate. The dissolution rates were found to be lowest at near-neutral pH and to increase at both low and high pH. A rate equation based on transition-state theory (TST) was used to calculate dissolution rate constants and reaction order with respect to pH over two pH intervals at each temperature. At 250C between pH 1 and pH 7 based on the Si release rate the log rate constant for glass dissolution (g glass/m20d) was −0.77 and the order with respect to pH was −0.48. Between pH 7 and pH 13 the log rate constant for glass dissolution was −8.1 and the order with respect to pH was +0.51. The measured simple glass dissolution rate constants compare very well with constants estimated by fitting the same TST equation to experimental results obtained for SRL-165 glass and to dissolution rate estimates made for synthetic basaltic glasses.

X-ray absorption studies of Ge and Ga environments in BaOGa2O3GeO2 glasses

The local structure surrounding Ge and Ga cations in BaOGa2O3GeO2 glasses has been studied using X-ray absorption spectroscopy. Based on comparisons with results for crystalline standards, the Ge and Ga in all the glasses are predominantly in tetrahedrally coordinated sites which are disordered relative to the crystals. While bond length and coordination stay fairly constant for all the glasses, disorder steadily increases with BaO content, especially in the second shell surrounding Ge and Ga. Previously observed trends in certain macroscopic physical properties, therefore, cannot be explained by coordination changes but may be due to longer-range structural variation.

Aerogels as a tool to study the electrical properties of ruthenium dioxide

RuO2 is a well-known oxidation catalyst for electrolytic generation of Cl2. Although bulk RuO2 is a metallic conductor, studies of mixed-oxide aerogels containing RuO2 confirm that the surface of RuO2 is a hydrous oxide and a mixed conductor. Our previous work showed that at room temperature, protonic and electronic conductivity occurs in mixed phase aerogels of RuO2+TiO2 (designated as (Ru–Ti)Ox). Impedance measurements up to 340°C demonstrate that electron hopping and/or semiconduction also occurs in (Ru–Ti)Ox aerogels. Electron hopping may occur between Ru3+ and Ru4+ ions in the ruthenia, while n-type semiconduction may be introduced into the titania by Ru3+ defects. The multiplicity of charge-transfer mechanisms measured on the surfaces of RuO2-based aerogels is indicative of the complexity of the catalytic and electrocatalytic chemistry performed on RuO2.

Raman spectroscopic studies of BaOGa2O3GeO2 glasses

Raman spectra were collected for barium gallogermanate glasses along two compositional joins to determine relationships among glass structure, chemistry, and physical property variations. As GeO2 content decreases for both glass series, the mid-frequency envelope shifts to higher frequencies while the high-frequency features increase in intensity and shift to lower frequencies. These trends in the Raman spectra parallel those reported for binary Li, Na and K germanate glasses, and are interpreted in terms of a reduction in average ring size, as well as an average lengthening of TO bonds. The more extreme trends for the most barium-rich glasses indicate partial depolymerization of the tetrahedral network and a concomitant increase in the number of non-bridging oxygens. There is no evidence for octahedral Ge or Ga in any of the barium gallogermanate glasses investigated in this study. The findings determined from the Raman data are consistent with earlier conclusions drawn from Ge and Ga X-ray absorption spectra.

Modifying nanoscale silica with itself: a method to control surface properties of silica aerogels independently of bulk structure

Abstract A nanoscale morphological composite of silica can be prepared in which the bulk properties (density, transparency) of the rough colloidal silica network are retained while the surface properties are dictated by a ramified form of nanoscale acid-catalyzed silica. This silica-modified silica composite is created by chemically modifying a base-catalyzed silica gel with a silica sol prepared by acid-catalyzed hydrolysis and condensation. Such structural nanocomposites of base-catalyzed, acid-modified (BCAM) silica gels can be supercritically dried to create BCAM aerogels in which the nanoscale mesoporous material is transparent to visible light while the surface of the silica network adopts the chemical nature (silanol-rich, hydrophilic) typical of an acid-catalyzed silica aerogel. Allowing an acid-catalyzed silica sol to gel about a base-catalyzed silica gel creates BCAM gels-in-gels that retain high porosity (>85%), yet which permit non-covalent entrapment of molecules at the base-catalyzed interface under conditions in which they would otherwise freely diffuse out of the gel or aerogel structure. The BCAM approach may be extended to create morphological nanocomposites other than SiO2/SiO2 and is demonstrated for a titania-modified silica gel and aerogel.

Electron spin resonance studies of defect centers induced in a high-level nuclear waste glass simulant by gamma-irradiation and ion-implantation

Abstract Samples of an iron-free soda-lime boro-aluminosilicate high-level nuclear waste (HLW) glass simulant were implantated with 160-keV protons or He + ions or irradiated by 60 Co γ-rays. Electron spin resonance (ESR) spectra were recorded before and after implantation or irradiation and after postirradiation isochronal anneals to 550°C. The most numerous paramagnetic states in the unannealed samples following γ-irradiation were boron–oxygen hole centers and Ti 3+ trapped-electron centers. The most numerous paramagnetic states in the as-implanted samples were peroxy radicals (PORs). Annealing of the γ-irradiated samples at temperatures >300°C caused recombination of the trapped electrons and holes, revealing an underlying POR spectrum which annealed in stages at ∼400°C and 550°C, similar to the annealing of PORs in the He + -implanted sample. For equivalent depositions of ionizing energy, the initial POR concentration in the He + -implanted sample was ∼100 times that in the γ-irradiated sample and at least 10 times larger than the (unmeasurable) number of PORs in the proton-implanted sample. Since TRIM calculations indicate that 160-keV protons deliver twice the ionizing dose of He + ions of the same energy but displace only 1/10 as many atoms, it is evident that the PORs induced by He + implantation result from displacements of oxygens in elastic collision cascades. The absence of trapped electrons and holes in the implanted samples is ascribed to annealing during irradiation associated with the fraction of the implantation energy deposited as heat. One of the fundamental defects to be expected in HLW glasses containing α-particle emitters is therefore the POR. Several distinct families of radiation-induced PORs are unambiguously identified in our samples by computer line-shape simulations constrained by the Kanzig–Cohen g -value formulae. Possible reasons for the non-observation of PORs in similarly-irradiated glasses containing several wt% Fe 2 O 3 are proposed.