Quartz Nanocrystals Research Articles

Hydrothermal synthesis and characterization of quartz nanocrystals — Implications from a simple kinetic growth model

Hydrothermal synthesis and characterization of quartz nanocrystals — Implications from a simple kinetic growth model

Temperature-induced phase transition in quartz nanocrystals dispersed in pseudotachylite

The size and concentration of α-quartz nanocrystals dispersed in samples of pseudotachylite and the internal stresses in these nanocrystals have been determined using infrared spectroscopy in the temperature range 300–800 K. Pseudotachylite is a product of intense crushing of granite that undergoes in the Earth’s crust faults. It has been found that the size of the nanocrystals is ∼20 nm and does not depend on temperature. As the temperature increases, their concentration decreases monotonically and tends to zero at ∼650 K. This process is paralleled by a growth of the concentration of β-quartz nanocrystals. The α-quartz nanocrystal concentration regains its initial level with decreasing temperature. Thus, the α → β phase transition in quartz nanocrystals in pseudotachylite starts at temperatures lower by ∼500 K than that in the bulk of the macrocrystal (846 K), and is stretched by ∼350 K. At room temperature, the unit cell of nanocrystals is compressed by surface tension forces. These forces retard the α → β phase transition. The thermal expansion coefficient of nanocrystals is larger than that of macrocrystals, which entails a decrease of compression and a monotonic decrease of the concentration of α-quartz nanocrystals with increasing temperature.

Quartz nanocrystals in the 2.48 Ga Dales Gorge banded iron formation of Hamersley, Western Australia: Evidence for a change from submarine to subaerial volcanism at the end of the Archean

Banded iron formations (BIF) have recently been used as proxies for tracking the chemical changes associated with the transition from an anoxic to oxic atmosphere around 2.45 billion years ago, known as the Great Oxidation Event (GOE). The timing of the GOE has been ascribed to both the collapse of a methane greenhouse and a decreased overall demand for oxygen due to the production of more oxidizing gases associated with greater subaerial volcanism. The latter is a byproduct of a period of high mantle plume activity and the formation of new continental crust between 2.51 to 2.45 Ga. Here we report unique mineral evidence for momentary subaerial volcanism recorded in hematite-rich layers of the 2.48 Ga BIF from Dales Gorge, Hamersley of Western Australia. The BIF contains euhedral quartz nanocrystals (QNC), which only occur on the surfaces or in cavities of hematite breccias exhibiting soft-sediment features and an exogenous source. These QNCs with an average size of 170–100 nm are distinct to the amorphous chert in the BIF mineral assemblage and have the smallest crystal sizes of well-crystallized quartz ever reported. We suggest that QNCs represent pyroclastic materials that were transported as dust particles to the BIF depositional setting. Although the analysis of one specific BIF unit does not provide proof of changing modes of volcanism during the Archean-Paleoproterozic transition, this high-resolution petrological study does confirm that subaerial volcanism existed at that time.

Structural investigation of sol–gel derived SiO2–GeO2 noncrystalline and nanocomposite materials

Noncrystalline and nanocomposite materials of (1−x)SiO2·xGeO2 system, with Si:Ge ratio from 8:1 to 2:1 (x = 0.111; 0.142; 0.2; 0.333), initially obtained by sol–gel method, were characterized by thermal analyses, X-ray diffraction, nuclear magnetic resonance and Raman spectroscopy. According to DTA and XRD results, the noncrystalline state of the as-prepared samples is stable up to 1,000 °C and only after 30 min heat treatment at 1,200 °C the samples become partial crystalline, due to development of cristobalite and quartz nanocrystals. Solid-state 29Si MAS-NMR was employed in order to characterize the local structure around silicon as a function of composition and thermal history of the samples. The NMR data indicate the presence of Q2, Q3 and Q4 units in all samples. The fraction of the highly interconnected SiO4 tetrahedra increases both with germanium content and with annealing temperature. The Raman spectroscopy results evidence structural changes related to silicon- and germanium-oxygen units but also to their interconnection, that depend on Si:Ge ratio and annealing temperature.

IR spectroscopy of quartz nanocrystals formed during intense crushing of a heterogeneous material (granite)

The spectra of the imaginary part ɛ″(ν) of the permittivity of quartz single crystals and a heterogeneous material, i.e., pseudotachylite, formed during intense crushing of granite in the region of the seismogenic Earth’s crust fault have been calculated from IR reflection spectra. It has been found that all strong bands in the pseudotachylite spectrum ɛ″(ν) correspond to lattice vibrations in quartz nanocrystals. Bands are asymmetrically broadened due to dielectric and phonon confinements. Linear sizes of quartz nanocrystals have been estimated from the broadening as ∼70 nm. The frequency of nanocrystal lattice vibrations is higher than that of the macrocrystal, which is caused by lattice compression. The internal stresses which could cause the observed change in the frequency are ∼200 MPa.

Interaction of corundum and quartz nanocrystals with erythrocyte membranes

Mechanisms of interaction of quartz and corundum nanocrystals with erythrocyte membranes were studied by means of atomic force microscopy and fluorescence analysis. Hydrophobic, chemically inert nanocrystals larger than a critical size (20–25 nm) can bind to erythrocyte membranes without damaging them. If the size of the nanocrystals is less than 15 nm, they can penetrate into the lipid bilayer membranes. This decreases the membrane microviscosity, and pores appear, which leads to cell lysis. A thermodynamic explication of the critical size of the nanocrystals is given.

Doping and calcination effect on nanostructured aluminosilicates processed by sol-gel route

Local order changes determined in an aluminosilicate matrix by calcination and addition of iron, yttrium or dysprosium oxides were investigated by X-ray diffraction and infrared spectroscopy. The nanocrystalline phase is reached only by Fe doping and not by the other two dopings. After calcination at temperatures as high as 1200 °C, nanocrystalline cristobalite alone is formed in the aluminosilicate matrix, while the doped samples favor the additional growth of mullite and quartz nanocrystals. The aluminosilicate spinel phase was not detected in any of these samples. An additional crystalline phase was only observed in the samples doped with yttrium and dysprosium. The structural units evidenced by FTIR analysis are associated with the crystalline phases identified by X-ray diffraction.

Young's moduli and microhardness of glass–ceramics in the system MgO/Al 2O 3/TiO 2/ZrO 2/SiO 2 containing quartz nanocrystals

Glasses with the compositions (60.2 − 2 x)SiO 2·(16.2 + x)Al 2O 3·(16.2 + x)MgO·3.7TiO 2·3.7ZrO 2 ( x = 0, 2.3, 4.65, 6.95, 8.1 and 9.25) were annealed at temperatures in the range from 800 to 1100 °C. This led to the precipitation of high- or low-quartz solid solution nanocrystals, magnesium aluminosilicate, mullite and spinel, depending on temperature and chemical composition. Large SiO 2-concentration ( x = 0) led to the crystallization of magnesium aluminosilicate as the main crystalline phase. Smaller SiO 2-concentrations led to the crystallization of quartz. If these samples were annealed at temperatures ≤1000 °C, high-quartz solid solutions were formed which did not transfer to the low temperature modification during cooling. The microhardness of the glasses were in the range from 7 to 8.5 GPa, while the Young's moduli were 105 to 120 GPa. After annealing, the microhardness was up to 12.3 GPa and the Young's moduli up to 152 GPa.

Amorphous and partly ordered structures in SiO2 rich volcanic glasses. An ED study

Glass structures of obsidian and pumice samples were measured using electron diffraction. Amorphous, partly ordered, and nanocrystalline regions were distinguished and analysed separately. The deconvoluted atomic distances obtained from experimental diffraction patterns through total pair-distribution functions are consistent with distances for ideal SiO 4 tetrahedra. Partly ordered structures in pumices are composed of plate-like fragments of tridymite/cristobalite layers, whereas obsidian contains quartz nanocrystals with abundant moganite-like planar faults. The validity of the structure-model of Goodman for silica glasses is discussed and an alternative interpretation is proposed for silicic volcanic glasses.

Hydrothermal Synthesis of Quartz Nanocrystals

This paper describes for the first time a chemical method for the preparation for nanocrystalline quartz. Submicron quartz powders are initially produced in hydrothermal reactions where soluble silica precursors precipitate as pure crystalline silica. To yield nanocrystalline material these particles can be purified and size selected by dialysis, filtration, and centrifugation. Transmission electron microscopy and X-ray diffraction illustrate that the product is pure phase α-quartz, consisting of isolated (i.e., nonaggregated) nanocrystals. Depending on the size selection method, crystallites with average sizes of 10 to 100 nanometers can be recovered.