Zagrożenia zdrowotne wśród dzieci i młodzieży. T. 2

Praca recenzowana / Peer-reviewed pape

Thermal expansion of coesite determined by synchrotron powder X-ray diffraction

Thermal expansion of synthetic coesite was studied with synchrotron powder X-ray diffraction in the temperature range of 100–1000 K. We determined the unit cell parameters of monoclinic coesite (a, b, c, and β) every 50 K in this temperature range. We observed that a and b parameters increase with increasing temperature, while c decreases. The β angle also decreases with temperature and approaches 120°. As a result, the unit cell volume expands by only 0.7% in this temperature range. Our measurements provide thermal expansion coefficients of coesite as a function of temperature: it increases from 3.4 × 10$^{−6}$ K$^{−1}$ at 100 K to 9.3 × 10$^{−6}$ K$^{−1}$ at 600 K and remains nearly constant above this temperature. The Suzuki model based on the zero-pressure Mie–Grüneisen equation of state was implemented to fit the unit cell volume data. The refined parameters are V$_0$ = 546.30(2) Å$^3$, Q = 7.20(12) × 10$^6$ J/mol and $θ_D$ = 1018(43) K, where $θ_D$ is the Debye temperature and V0 is the unit cell volume at 0 K with an assumption that K′ is equal to 1.8. The obtained Debye temperature is consistent with that determined in a previous study for heat capacity measurements

Low temperature heat capacity measurements of β−Si3N4β-Si_{3}N_{4} and γ−Si3N4γ-Si_{3}N_{4}: Determination of the equilibrium phase boundary between β−Si3N4β-Si_{3}N_{4} and γ−Si3N4γ-Si_{3}N_{4}

Isobaric heat capacities of β-Si$_{3}$N$_{4}$ and γ-Si$_{3}$N$_{4}$ were measured at temperatures between 1.8 and 309.9 K with a thermal relaxation method. The measured heat capacities of γ-Si$_{3}$N$_{4}$ are smaller than those of β-Si$_{3}$N$_{4}$ in this temperature range. Using these data, we determined the standard entropies of β-Si$_{3}$N$_{4}$ and γ-Si$_{3}$N$_{4}$ to be 62.30 J·mol$^{−1}$ K$^{−1}$ and 51.79 J·mol$^{−1}$ K$^{−1}$, respectively. The equilibrium phase boundary between β-Si$_{3}$N$_{4}$ and γ-Si$_{3}$N$_{4}$ was calculated using these values and thermodynamic parameters reported in previous studies. The obtained equilibrium phase transition pressure at 2000 K is 11.4 GPa. It is lower than the experimental pressures at which γ-Si$_{3}$N$_{4}$ was synthesized in previous studies. The calculated Clapeyron slope at this temperature is 0.6 MPa K$^{−1}$, which is consistent with those of theoretical studies

Synthesis, X-ray structure analysis, and Raman spectroscopy of R2TiO5R_{2}TiO_{5}-based (R = Sc, Y) solid solutions

Order–disorder transitions in $xR_{2}O_{3} · (1 − x)TiO_{2}$ (R = Sc, 0.4 ≤ x ≤ 0.5; R = Y, 0.5 ≤ x ≤ 0.6) solid solutions with highly imperfect fluorite-derived structures have been studied using monochromatic synchrotron X-ray diffraction and Raman spectroscopy. The results demonstrate that the synthesis process leads to the formation of a fluorite-like (Fm$_3$m) disordered phase and a nanoscale (~10–100 nm) pyrochlore-like (Fd$_3$m) ordered phase of the same composition, coherent with the disordered phase. We have determined their lattice parameters. The Raman spectra of $Sc_{2}TiO_{5}(Y_{2}TiO_{5})$ contain broad lines in low- and high-frequency regions: at 190, 350, and 775 (134, 188, 365, 404, and 727) cm−1. These lines are characteristic of a pyrochlore-like phase with a varying degree of order and a disordered fluorite-like phase, respectively. The pyrochlore-like phase $Y_{2}Ti_{2}O_7$ has two strong Raman peaks in the low-frequency region: at 312 and 527 cm$^{−1}$. The formation of nanodomains with different degrees of order is caused by the internal stress that arises from the high density of structural defects in the unit cells of the solid solutions

Thermal expansion and P-V-T equation of state of cubic silicon nitride

We performed in-situ X-ray diffraction measurements of polycrystalline cubic silicon nitride samples at high temperatures under atmospheric pressure and at simultaneous high-pressure-temperature conditions. In air, cubic silicon nitride survives metastably up to 1733 K without oxidation. The temperature dependence of the thermal expansion coefficient was determined to be α(T) = a1 + a2T – a3T−2 where a1 = 1.34(6) × 10−5 K−1, a2 = 5.06(44) × 10−9 K−2, and a3 = 0.20(10) K. Using all the experimental data obtained under atmospheric and high pressures, a complete set of parameters of the high-temperature third-order Birch Murnaghan equation of state was obtained: K300,0 = 303(5) GPa, K′300,0 = 5.1(8), and (∂KT,0/∂T)P = –0.017(1) GPa K−1, where K0, K′0, and (∂KT,0/∂T)P are the isothermal bulk modulus, its pressure derivative, and its temperature derivative, respectively. These parameters are necessary to calculate the equilibrium phase boundary between the β and cubic phases in silicon nitride

A Complete Solid Solution with Rutile-Type Structure in SiO2−GeO2\mathrm{SiO_{2}-GeO_{2}} 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 SiO$_{2}$-rich coesite and GeO$_{2}$-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 SiO$_{2}$

Structural Characteristics and Thermophysical Properties of Complex Ceramic Oxides in the System Dy2O3–HfO2Dy_{2}O_{3}–HfO_{2}

The structure and thermophysical properties of materials formed in the system Dy$_2$O$_3$–HfO$_2$ (molar ratio 1 : 3 to 3 : 1) as a result of isothermal firing of x-ray amorphous mixed hydroxides at temperature to 1600°C are investigated. It is shown that for ratios 1 : 3 to 1 : 1 the crystallization process results in the formation of single-phase solid solutions with the structure of defective fluorite and marked nonequivalence of the parameters of the local environment of the Dy and Hf atoms. It is determined that the ceramic based on dysprosium hafnate (Dy$_2$O$_3$: HfO$_2$ = 1 : 1) possesses low, practically temperature independent (to 800°C), thermal conductivity about 1.4 W/(m · K)

Synthesis of Al2O3/SiO2Al_{2}O_{3}/SiO_{2} Nano-Nano Composite Ceramics under High Pressure and Its Inverse Hall-Petch Behavior

We report the synthesis of alumina/stishovite nano-nano composite ceramics through a pressure-induced dissociation in Al2SiO5 at a pressure of 15.6 GPa and temperatures of 1300°C-1900°C. Stishovite is a high-pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain-boundary phase

Synthesis of Al 2

We report the synthesis of alumina/stishovite nano-nano composite ceramics through a pressure-induced dissociation in Al2SiO5 at a pressure of 15.6 GPa and temperatures of 1300°C-1900°C. Stishovite is a high-pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain-boundary phase