Structural Degradation of Tobermorite during Vibratory Milling

Mechanochemistry of hydrothermally prepared Al-free and Al-substituted 1.1 nm tobermorite was studied using ²⁹Si NMR, XRD, and TGA-DTA. Tobermorite has a double-chain structure and Al/Si substitution occurs preferentially at bridging tetrahedra. By using the vibration mill, both tobermorites were observed to decompose as a function of milling time to form amorphous C-S-H-like phases. The decomposition rate was higher for the Al-substituted tobermorite. The decomposition process occurs mainly at the bridge portion of the silicate double chain. In the Al-substituted tobermorite, the breakage seems to occur preferentially at the points where Al has replaced Si. On heating the amorphous C-S-H-like phase, wollastonite is formed. However, no decrease was observed at the wollastonite formation temperature.     

Ion acceleration in bias-enhanced nucleation of diamond at relatively high pressures

In order to explain the bias-enhanced nucleation (BEN) of diamond at pressures of approximately 6 kPa, the flux and energy of hydrogen ions to a negatively biased substrate in microwave plasma chemical vapor deposition (MWP-CVD) were investigated. Ion flux increased rapidly upon increasing the negative bias voltage to greater than 150 V when the pressure was higher than 3 kPa. This phenomenon was attributed to the additional ionization in the vicinity of the substrate due to biasing. Optical emission spectroscopy at 6 kPa clarified that the emission intensity of the Swan band of C₂ radicals was reduced, and intramolecular temperatures were enhanced in this bias region. A numerical simulation on the basis of the measured ion flux and density revealed that the most frequent energy was less than 2 eV under the conditions of 200 V and 6 kPa, and that the average number of collisions in the plasma sheath reached 60. We propose, on the basis of these results, that the substrate biasing in BEN at pressures above 3 kPa causes the additional ionization and activation of neutral species in the sheath, while the direct momentum transfer of ions to the substrate is strongly suppressed in BEN in this pressure range.     

Characterization of carbon nanotubes by nanoprobe manipulation in transmission electron microscope

Two types of telescopic sliding deformation processes in multiwalled carbon nanotubes were observed dynamically and their sliding forces were measured experimentally by employing nanoprobe manipulation in a transmission electron microscope (TEM). The sliding force was measured using a commercially available Si cantilever installed in the manipulator. A constant sliding force of approximately 70nN was measured during the telescopic sliding of a inner shell of diameter 5.8 nm. However, the sliding force in multilayered shells was variable. The nanoscale measurements performed using the TEM contributed to the characterization of the structural details during the deformation.     

Formation of 11 Å tobermorite from mixtures of lime and colloidal silica with quartz

Mixtures of lime, colloidal silica and quartz (<10 μm, 10–20 μm) were treated hydrothermally in stirred suspensions at 180°C to prepare 11 Å tobermorite with Ca/Si = 0.8. The runs made using the colloidal silica and lime quickly formed CSH; but did not convert into crystalline tobermorite even after 20 h. The runs with the mixtures of colloidal silica and quartz gave highly crystalline 11 Å tobermorite after 5 h through the reaction of Ca-rich C-S-H and quartz. The reaction of quartz was controlled by its rate of dissolution. The thermal behaviour of the tobermorites was normal, trending to mixed with increase in processing time.     

Influence of aluminium on the conversion of calcium silicate hydrate gels into 11 Å tobermorite at 90°C and 120°C

Mixtures of CaO and colloidal silica with and without γ-A?₂O₃, CaO and alkali-bearing A?₂O₃-SiO₂ gels, or CaO and clinoptilolite were treated hydrothermally at 90°C or 120°C for 4 hr – 4 weeks. Reaction seemed always to proceed through formation of C-S-H gels to 11 Å tobermorite. In the absence of A?, tobermorite crystallized more rapidly at c/S = 1.0 than at C/S = 0.8 but in the presence of A?, it crystallized more rapidly at Ca/(Si + A?) = 0.8 than at Ca/(Si + A? = 1.0. Where the starting materials contained both A? and alkali the tobermorite showed anomalous thermal behaviour similar to that of the natural mineral from Loch Eynort, but where they contained A? but no alkali, the thermal behaviour of the tobermorite was complex.     

α-Dicalcium Silicate Hydrate: Preparation, Decomposed Phase, and Its Hydration

α-C₂SH can be synthesized by hydrothermal treatment of lime and silicic acid for 2 h at 200°C. When heated to 390–490°C, α-C₂SH dissociates through a two-step process to form an intermediate phase plus some γ-C₂S. This appears to be a new dicalcium silicate different from known dicalcium silicates—α, α'_L, α'_H, β, and γ phase—and is stable until around 900°C. At 920–960°C, all the phases are transformed to the α'_L phase. The intermediate phase has high crystallinity and is stable at room temperature. ²⁹ Si MAS NMR measurements indicate the possibility that it contains both protonated and unprotonated monosilicate anions. The intermediate phase that has passed through the α'phase at higher temperature yields β-C₂S on cooling. The intermediate phase is highly active, and completed its hydration in 1 day ( w/s = 1.0, 25°C). Among the crystalline calcium silicate hydrates with Ca/Si = 2.0, it is hillebrandite that yields β-C₂S at the lowest temperature.     

Phase Evolution during Autoclaving Process of Aerated Concrete

The reactions were investigated that occur when lime, cement, and quartz sand are mixed together and molded, then treated at 180°C under saturated steam pressures to produce autoclaved aerated concrete. The hydrothermal treatment of mixtures gives Ca-rich C-S-H with varying Ca/Si ratios as an initial product, which reacts further with silica dissolved from quartz to form 1.1-nm tobermorite with increase of curing time. During autoclaving, the composition of C-S-H and tobermorite as a binder continues to change until after 8 h, when the Ca/(Al + Si) ratio becomes constant at 0.8. As the reaction proceeds, the number of micropores increases, and the strength also increases due to the binder effect of the tobermorite. However, the total pore volume does not change, remaining constant values.     

Estimation of long-chain branching in ethylene–propylene terpolymers from infinite-dilution viscoelastic properties

A method is outlined for estimation of small degrees of long-chain branching in polymers with moderately narrow molecular weight distribution (M?_w/M?_n <1.4). The storage and loss shear moduli, G′ and G″, are measured in dilute solution by the Birnboim-Schrag multiple-lumped resonator and extrapolated to infinite dilution, choosing a suitable solvent viscosity and frequency range such that the data lie in the terminal zone where G′ and G″ are proportional to the second and first powers of frequency, respectively. The intrinsic reduced steady-state shear compliance is determined from these data and corrected for moderate molecular weight heterogeneity (assuming a Gaussian distribution) from knowledge of M?_w/M?_n and the Mark-Houwink exponent a. The resulting value of S₂/S (where S₁ = Στ_p/τ₁, S₂ = Σ(τ_p/τ₁)², the τ_p's being the relaxation times and τ₁ the longest one) is compared with values calculated by the Zimm-Kilb theory as evaluated by Osaki for comb polymers of regular geometry and different numbers of branch points. The method has been illustrated by measurements on four ethylene–propylene copolymers. One containing no termonomer and one containing a saturated termonomer appeared to be linear; two containing unsaturated termonomers showed small degrees of branching. The method appears to be promising for detecting from one to four branch points per molecule.