Influence of heating rate on the hydrothermal formation of kilchoanite from lime-quartz mixtures

If mixtures of Ca(OH)₂ and quartz with Ca : Si = 1.3–2.0 are processed hydrothermally at 350 °C and 400–500 bars, the products depend on the heating rate. If this is sufficiently rapid, the main product at Ca : Si = 1.5 is kilchoanite (Ca₆(SiO₄)(Si₃O₁₀)); this is accompanied at Ca : Si = 1.3 by xonotlite and at Ca : Si = 2.0 by calciochondrodite. If the heating rate is slower, the main products are combinations of xonotlite, foshagite and dellaite. If the heating rate is rapid, kilchoanite is formed at 300 °C, and, if it is rapid and sufficiently finely divided quartz is used, at 250 °C; it was not obtained at 200 °C. Broadly similar results were obtained at saturated steam pressures. At Ca : Si = 1.5, 250–350 °C and 400–500 bars, kilchoanite is possibly stable relative to the other solid phases, formation of which at slow heating rates is attributed to nucleation while the temperature is rising. To obtain the fastest heating rates, a new method was used, in which water is not admitted into the reaction vessel until the working temperature has been reached.     

Effects of Na2O on calcium silicate hydrates at elevated temperatures

Pectolite is a stable phase in large areas of the system NCSH at 180 to 325°C; it may coexist, depending on Na₂O concentration and temperature, with tobermorite, xonotlite or foshagite. Sodium carbonate and $\text{N}\text{S}$ reacted with Ca²⁺ in Al₂O₃ substituted tobermorite to form slightly soluble C$\text{C}$ and C$\text{S}$ NSH and NaOH. The possible destructive effects of these reactions are considered. Sodium chloride had no detectable corrosive effect on the tobermorite.     

Temperature‐Stable Relative Permittivity from −70°C to 500°C in (Ba0.8Ca0.2)TiO3–Bi(Mg0.5Ti0.5)O3–NaNbO3 Ceramics

Temperature‐stable relaxor dielectrics have been developed in the solid solution system: 0.45Ba_0.8Ca_0.2TiO₃–(0.55 ? x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃. Ceramics of composition x = 0 have a relative permittivity ?_r = 950 ± 15% over a wide temperature range from +70°C to 600°C. Modification with NaNbO₃ at x = 0.2 decreases the lower limiting temperature to ?70°C, but also decreases relative permittivity such that ?_r ~ 600 ± 15% over the temperature range ?70°C to 500°C. For composition x = 0.3, the low‐temperature dispersion in loss tangent, tan δ, (at 1 kHz) shifts to lower temperature, giving tan δ values ≤0.02 across the temperature range ?60°C to 300°C in combination with ?_r ~ 550 ± 15%. Values of dc resistivity for all samples are of the order of 10¹⁰ Ω m at 250°C and 10⁷ Ω m at 400°C.     

Influence of additives on the autoclave reactions of β-Ca2SiO4 and Ca3SiO5 at 170 °C

The products formed from β-Ca₂SiO₄ and Ca₃SiO₅ in the autoclave at 170°C are influenced by the presence of Al₂O₃, Fe₂O₃, Na₂O and B₂O₃, whether in solid solution in the starting material or added to the mix in suitably reactive compounds. The most important conclusion is that Al₂O₃, Fe₂O₃ and Na₂O all favour the formation of the orthorhombic α-2CaO.SiO₂.H₂O as against hillebrandite or tricalcium silicate hydrate. Addition of Na₂O, or especially of B₂O₃, retards the hydration of both β-Ca₂SiO₄ and Ca₃SiO₅.     

High-pressure steam curing of high-slag cement: I. Composition of the hydrated cement

The phases formed in a high-slag cement on curing in saturated steam at 180 lbf/in², have been investigated. X-ray diffraction revealed that in the presence of suitable addition of fine quartz, 11-Å tobermorite was the predominant phase formed in an autoclaving period of 5 h at maximum pressure; the phases identified in neat slag cement were 11-Å tobermorite and hillebrandite. The same phases were identified in the samples autoclaved for 6 h, in addition to minor amount of xonotlite. Chemical analysis of the same hydrated samples has shown that the molar CaO/SiO₂ ratio was 1.27 for the cement-quartz and 1.49 for the neat cement for the 5-h autoclaving period, and 1.35 and 1.57, respectively, for the 6-h period.     

Effects of the CaO/SiO2 ratio and Cr2O3 on the hydrothermal synthesis of xonotlite

Formation of xonotlite was attempted by hydrothermal reactions of Ca(OH)₂, silica gel and coprecipitated SiO₂Cr₂O₃ gel at C/S molar ratios of 0.65, 0.83, 1.0 and 1.2 at a saturated water vapor pressure with addition of 0–10% of Cr₂O₃. Cr₂O₃ enters 10% or more into CS(I) to form a solid solution, interferes with the formation of tobermorite and expands the range of formation of CSH(I) towards the higher temperature side. The temperature of formation of xonotlite rises above that in the absence of Cr₂O₃: from 210 to 450°C at C/S = 0.83 from 160 to 280°C at C/S = 1.0 and from 250 to 270°C at C/S=1.2.     

Oxidation Behavior of SiC Whiskers at 600–1400°C in Air

The oxidation behavior of SiC whiskers (SiC_W) with a diameter size of 50–200 nm has been investigated at 600°C–1400°C in air. Experimental results reveal that SiC_W exhibit a low oxidation rate below 1100°C while a significant larger oxidation rate after that. This can be attributed to the small diameter size of SiC_W, which determines that it is hard to form a protective SiO₂ layer thick enough to hamper the diffusion of oxygen effectively. Both nonisothermal and isothermal oxidation kinetics were studied and the apparent oxidation energy was calculated to further understand the oxidation behavior of the SiC_W.     

Extended X-ray Absorption Fine Structure Investigation of Calcium Silicate Hydrates

²⁹Si MAS-NMR, and Ca-EXAFS experiments have been conducted on calcium silicate hydrates (C-S-Hs) with structure derived from wollastonite. Crystalline compounds (wollastonite, xonotlite, hillebrandite, foshagite, 1.1 nm and 1.4 nm tobermorites, and jennite) and C-S-H were synthesized and characterized. ²⁹Si NMR provides information on silicate chains and EXAFS on calcium environment. The refined EXAFS values are in agreement with XRD data, except for tobermorite. The calcium order in C-S-H (C/S molar ratio from 0.7 to 1.4) is similar to that of tobermorite but different from that of jennite. Structural models of C-S-H are discussed.     

Dielectric Properties of Ba0.8Ca0.2TiO3–Bi(Mg0.5,Ti0.5)O3–NaNbO3 Ceramics

Ceramics in the system 0.45Ba_0.8Ca_0.2TiO₃–(0.55?x)Bi(Mg_0.5Ti_0.5)O₃–xNaNbO₃, x = 0–0.02 were fabricated by a conventional solid‐state reaction route. X‐ray powder diffraction indicated cubic or pseudocubic symmetry for all samples. The parent 0.45Ba_0.8Ca_0.2TiO₃–0.55Bi(Mg_0.5Ti_0.5)O₃ composition is a relaxor dielectric with a near‐stable temperature coefficient of relative permittivity, ε_r = 950 ± 10% across the temperature range 80°C–600°C. Incorporation of NaNbO₃ at x = 0.2 extends the lower working temperature to ≤25°C, with ε_r = 575% ± 15% from temperatures ≤25°C to >400°C, and tan δ < 0.025 from 25°C to 400°C. Values of dc resistivity ranged from ~10⁹ Ω·m at 250°C to ~10⁶ Ω·m at 500°C. The properties suggest that this material may be of interest for high‐temperature capacitor applications.     

Some factors affecting formation of truscottite and xonotlite at 300–350°C

Formation of truscottite and xonotlite was studied at Ca/(Si + A?) = 0.6, 300–350°C and saturated steam pressures. Truscottite is favored by use of silicic acid rather than quartz, or of β-C₂S rather than Ca(OH)₂, and by the presence of small amounts of alkali. Partial replacement of Si by A? has no apparent influence on whether truscottite or xonotlite is formed. In runs at 350°C and saturated steam pressure using commercial Class G or Class J oil-well cements, truscottite was slowly replaced by xonotlite, but wollastonite may be the ultimately stable product under these conditions.     

Kinetics of the CaO  quartz  H2O reaction at 120° to 180°C in suspensions

Kinetics of hydrothermal reactions have been studied for mixtures of CaO and quartz (<10 μm 10–20 μm) with Ca/Si = 0.8 and 1.0 in stirred suspensions at 120 – 180°C. Reaction proceeds through the sequence: Ca(OH)₂ + SiO₂ → Ca-rich C-S-H + SiO₂ (at 120°C) → poorly crystalline tobermorite (at 140°C)→ highly crystalline tobermorite (at 180°C) → xonotlite at 180°C and Ca/Si = 1.0 and 180°C and Ca/Si = 0.8 if 10–20 μm quartz is used. Reaction is controlled by dissolution of the quartz. For both Ca/Si ratios the radius of the 10–20 μm quartz decreases at a constant rate, viz 0.85 μm/h at 180°C, 0.13 μm/h at 140°C, 0.04 μm/h at 120°C.     

Direct Formation and Luminescence of Nanocrystals in the System Eu2Sn2O7–Gd2Sn2O7 Complete Solid Solutions

Nanocrystals with orange‐reddish luminescence based on the pyrochlore‐type complete solid solutions with cube‐like morphology in the Eu₂Sn₂O₇–Gd₂Sn₂O₇ system were directly formed from the precursor solutions of SnCl₄, GdCl₃, and EuCl₃ under weakly basic hydrothermal conditions at temperatures higher than 180°C for 5 h. The crystallite of Gd₂Sn₂O₇ pyrochlore gradually grew from 10 to 37 nm as the hydrothermal treatment temperature rose from 180°C to 240°C. The lattice parameter of cubic phase linearly increased with increased europium concentration according to the Vegard's law. The characteristic orange‐reddish photoluminescence spectra of Gd₂Sn₂O₇:Eu³⁺ cubelike nanocrystals with crystallite size from 34 to 37 nm that were formed at 240°C for 5 h were attributed to the most sharp orange (586 nm) luminescence with high intensity and quite broad red (610–630 nm) emission with weak intensity, according to the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions of Eu³⁺, respectively. At a composition of (Eu_0.09Gd_0.91)₂Sn₂O₇, the intensity of orange emission reached the maximum. The Red‐to‐Orange (⁵D₀→⁷F₂/⁵D₀→⁷F₁) (R/O) emission intensity ratio was in the low range from 0.10 to 0.14, which was a characteristic of Gd₂Sn₂O₇:Eu³⁺.     

The specific volume of poly(4-methylpentene-1) as a function of temperature (30°–320°C) and pressure (0–2000 kg/cm2)

The dependence of the specific volume of a commercial sample of poly(4-methylpentene-1) (Mitsui TPX, RT-20, abbr. PMP) on temperature (30°–320°C) and pressure (0–2000 kg/cm²) has been determined. Results are reported in tabular form and as approximate fits, making use of the Tait equation. The data show that the crystalline melting transition of this type of PMP is completed at 235°C under zero pressure and gives indication of a glass transition temperature T_g at about 20°C at p = 0. Its approximate pressure dependence is given by dT_g/dp ≈ 0.015°C kg^?1 cm². The zero pressure results on the melting and glass transitions are in agreement with DTA results. The p-v-T data, quenching experiments, and a determination of the crystalline unit cell (tetragonal, a = b = 18.70 Å, c = 13.54 Å) confirm earlier work indicating that the room-temperature crystalline specific volume of PMP is greater than the amorphous specific volume. This unusual density behavior persists to a temperature of 50–60°C at p = 0 and to temperatures as high as 230–240°C under a pressure of 2000 kg/cm².     

Synthesis of normal and anomalous tobermorites

Tobermorites were made from several starting materials at 105 – 180°C and Ca/(Si + A?) 0.8 – 1.0. Reaction gives in succession C-S-H, normal, mixed and anomalous tobermorites, and finally xonotlite. High C/S ratio (1.0), short time, low temperature, stirring, presence of A?, and if quartz is used, small particle size, all tend to stop it at normal tobermorite. In some cases, this effect is due to promotion of crystal growth of normal tobermorite. Low C/S ratio (0.8), long time, high temperature, no stirring, presence of A? plus alkali, and, if quartz is used, large particle size, all tend to give anomalous tobermorite. However, at 180° C this changes easily into xonotlite if C/S > 0.9.     

Cyclic Oxidation of Ternary Layered Ti2AlC at 600–1000°C in Air

The cyclic oxidation of bulk Ti₂AlC at intermediate temperatures of 600–1000°C in air was studied by thermogravimetric analysis. It was demonstrated that Ti₂AlC exhibited good cyclic‐oxidation resistance at temperatures above 700°C. The cyclic‐oxidation kinetics approximately follows a parabolic rate law at 700–1000°C range. The surface scales are dense, resistant to spalling and adhesive to Ti₂AlC substrate. An abnormal oxidation whose cyclic‐oxidation kinetics obeys a linear law is observed at 600°C. As revealed by scanning electron microscope (SEM), oxidation‐induced cracks present at 600°C results in poor protectivity and accounts for the abnormal oxidation. The cracks are caused by the stress associated with the volume expansion due the formation of anatase TiO₂ in the scale.     

A Complete Solid Solution with Rutile‐Type Structure in SiO2–GeO2 System at 12 GPa and 1600°C

High pressure and temperature synthesis of compositions made of (Si_1?x,Ge_x)O₂ where x is equal to 0, 0.1, 0.2, 0.5, 0.7, and 1 was performed at 7–12 GPa and 1200–1600°C using a Kawai‐type high‐pressure apparatus. At 12 GPa and 1600°C, all the run products were composed of a single phase with a rutile structure. The lattice constants increase linearly with the germanium content (x), which indicates that the rutile‐type phases in the SiO₂–GeO₂ system form a complete series of solid solutions at these pressure and temperature conditions. Our experimental results show that thermodynamic equilibrium state was achieved in this system at 12 GPa and 1600°C, but not at 1200°C. At lower pressures (7 and 9 GPa) and 1600°C, we observed the decomposition of (Si_0.5,Ge_0.5)O₂ into SiO₂‐rich coesite and GeO₂‐rich rutile phases. The silicon content in the rutile structure increases sharply with pressure in the vicinity of the coesite–stishovite phase transition pressure in SiO₂.     

Dynamic mechanical properties of poly(vinyl chloride)–xonotlite composite system

The filler effect of xonotlite (6CaO.6SiO₂.H₂O; needle-shaped) on dynamic mechanical properties, such as storage modulus (E′), loss modulus (E″), and tan δ was studied for the PVC—xonotlite composite system. And the properties of the system containing mechanically or chemically disaggregated particle of xonotlite were compared with those of the system-filled aggregates. The dynamic mechanical properties obviously depends on the dispersion condition of xonotlite particle. The aggregates of xonotlite produces a remarkably high modulus, an increase in T_g, and a decrease in mechanical damping near T_g in the system. On the other hand, the disaggregates, especially the chemical disaggregate one, bring softer or more rubbery properties in these systems. The interaction between matrix and filler was the strongest in the aggregates system and decreases in the order, mechanical disaggregates system, chemical disaggregates system.     

High‐temperature calorimetric study of oxide component dissolution in a CaO–MgO–Al2O3–SiO2 slag at 1450°C

Blast‐furnace slags are formed, as iron ore is reduced to metal, as a molten a mixture of refractory and not easily reducible oxides, largely silica, alumina, lime, and magnesia. Their relatively low silica content makes them basic and poor glass formers. Their thermodynamic properties, though important for modeling their formation and reactivity, as well as furnace heat balance, are poorly known. Solution calorimetry of small amounts of solid oxides in a molten oxide solvent at high temperature (up to about 1500°C) permits direct assessment of energetics of dissolution. The enthalpies of solution of slag forming oxides: CaO, SiO₂, Al₂O₃, MgO, and Fe₂O₃ in a simplified model slag of composition: CaO (45.9 mol%), SiO₂ (35.1 mol%), Al₂O₃ (8.3 mol%), MgO (10.7 mol%) were measured by high‐temperature drop solution calorimetry at 1450°C. For this slag composition, enthalpies of solution become more exothermic in the order: Fe₂O₃ (279.3 ± 20.8 kJ/mol), MgO (56.7 ± 9.1 kJ/mol), Al₂O, (41.6 ± 11.3 kJ/mol), CaO (?4.3 ± 2.3 kJ/mol), and SiO₂, (?20.4 ± 4.4 kJ/mol), reflecting the relatively basic character of this low‐silica melt. Within these fairly large experimental errors, characteristic of calorimetry at this high temperature, there is little or no discernible concentration dependence for these heats of solution. The trends seen for these five solutes parallel those seen for heats of solution of the same oxides in other melts at various temperatures, with changes in magnitude reflecting the differences in acid‐base character of the melts. The new data for quartz show systematic behavior which extends the range of basicity studied for the enthalpy of dissolution of silica. The results provide reliable data for future modeling of the thermal balance of steel‐making furnaces and geologic and ceramic systems.     

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

The confined crystallization of poly(?‐caprolactone) (PCL) block in poly(?‐caprolactone)–poly(l ‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry     

Solubility of potassium ferrate in 12 M alkaline solutions between 20°C and 60°C

The solubility of potassium ferrate (K₂FeO₄) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As₂O₃. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO₄⁻. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K⁺, and the ferrate anion, KFeO₄⁻, was found to be 0·044 ± 0·006 mol² dm⁻⁶ for 20°C, 0·093 ± 0·004 mol² dm⁻⁶ for 40°C and 0·15 ± 0·09 mol² dm⁻⁶ for 60°C. From these results the heat of dissolution of K₂FeO₄ was calculated as −14·3 kJ mol⁻¹. At 60°C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20°C or 40°C.