Ionic Conductivity of the Fluorite-Type Hafnia–R2O3 Solid Solutions

The ionic conductivity of the hafnia-scandia, hafnia-yttria, and hafnia-rare earth solid solutions with high dopant concentrations of 8, 10, and 14 mol% was measured in air at 600° to 1050°C. Impedance spectroscopy was used to obtain lattice conductivity. A majority of the investigated samples exhibited linear Arrhenius plots of the lattice conductivity as a function of temperature. For all investigated dopant concentrations the ionic conductivity was shown to decrease as the dopant radius increased. The activation enthalpy for conduction was found to increase with dopant ionic radius. The fact that the highest ionic conductivity among 14-mol%-doped systems was obtained with HfO₂─Sc₂O₃ suggested that the radius ratio approach should be used to predict the electrical conductivity behavior of HfO₂─R₂O₃ systems. A qualitative model based on the Kilner's lattice parameter map does not seem to apply to these systems. For the three systems HfO₂─Yb₂O₃, HfO₂─Y₂O₃, and Hf₂O₃─Sm₂O₃ a conductivity maximum was observed near the dopant concentration of 10 mol%. Deep vacancy trapping is responsible for the decrease in the ionic conductivity at high dopant concentrations. Formation of microdomains of an ordered compound cannot explain the obtained results. A comparison between the ionic conductivities of doped HfO₂ and ZrO₂ systems indicated that the ionic conductivities of HfO₂ systems are 1.5 to 2.2 times lower than the ionic conductivities of ZrO₂ systems.     

Activity–Composition Relations in FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 Solid Solutions at 1500° and 1600°C

Activity–composition relations of FeCr₂O₄–FeAl₂O₄ and MnCr₂O₄–MnAl₂O₄ solid solutions were derived from activity–composition relations of Cr₂O₃–Al₂O₃ solid solutions and directions of conjugation lines between coexisting spinel and sesquioxide phases in the systems FeO–Cr₂O₃–Al₂O₃ and MnO–Cr₂O₃–Al₂O₃. Moderate positive deviations from ideality were observed.     

Preparation and Cu+-Ion-Conducting Properties of the Glasses in the System CuBr–Cu2MoO4–Cu3PO4

Cu⁺-ion-conducting glasses were prepared in the pseudoternary system CuBr–Cu₂MoO₄–Cu₃PO₄, and the ion-conducting properties of the glasses obtained were compared to those of the glasses in the system CuI—Cu₂MoO₄—Cu₃PO₄, which contain CuI instead of CuBr. The CuBr—Cu₂MoO₄—Cu₃PO₄ glasses showed high ion conductivities in the range of 10⁰ to 10⁻² S · m⁻¹ at room temperature. The change in ion-conducting properties caused by the substitution of PO^3-₄ anions for MoO²⁻₄ anions was larger than the change caused by the substitution of Br⁻ anions for I⁻ anions in the glasses containing a constant amount of the cuprous halides.     

Effective Ionic Radius of Y3+ Determined from Lattice Parameters of Fluorite-Type HfO2 and ZrO2 Solid Solutions

Fluorite type HfO₂ and ZrO₂ solid solutions were prepared by doping with 8 to 14 mol% of Ho₂O₃ and Y₂O₃, and their lattice parameters were determined. In both HfO₂ and ZrO₂ systems, the lattice parameters of the solid solutions containing Ho₂0₃ were consistently greater than those containing the same amounts of Y₂O₃. This indicated that the ionic radius of Ho³⁺ was larger than that of Y³⁺ in the fluorite structure solid solutions. The effective ionic radius of Y³⁺ in eightfold coordination was estimated to be 0.1011 nm by using the measured lattice parameters and the empirical equations to predict the lattice parameters of the fluorite-type solid solutions.     

Structure Change of Ca2SiO4 Solid Solutions with Ba Concentration

A series of Ba-bearing Ca₂SiO₄ solid solutions (C₂S( ss )), (Ba _x Ca_{1− x} )₂SiO₄ with 0.075 x 0.30, were prepared and examined by X-ray and electron beam diffraction. They are all made up of orthorhombic domains 120° different in orientation around the common c axis of the former α phase. The C₂S solid solution with x = 0.075 shows a superstructure incommensurate along the a axis with λ (modulation wavelength) = 3.5 and commensurate along the c axis with Δ= 3. With x = 0.15, modulation is observed only along the a axis and Δ= 3.4. No evidence of superstructure is found with x = 0.24; the space group and cell dimensions are comparable with those of pure α '_H-C₂S. The C₂S( ss ) with x = 0.30 gives a superlattice with the cell-edge length of 3 b . All the C₂S( ss ), when reheated at 1000°C for 24 h, produced lamellae of the trigonal phase T nearly in parallel with (001) of the host α '_L phase. The crystallographic orientation between the two phases is

This indicates that the above Ba-bearing C₂S( ss ) phases occur as precursors to the thermodynamically more stable two-phase mixtures.     

Control of Grain-Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping

The control of the microstructure of Ce-doped Al₂O₃/ZrO₂ componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al₂O₃ grains with intragranular ZrO₂ particles and small Al₂O₃ grains with intergranular ZrO₂ particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO₂, Ce³⁺ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce⁴⁺ causes no breakaway and produces a normal intergranular ZrO₂ distribution. The dramatic effect of Ce³⁺ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO₂ second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO₂ is not exceeded.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

Direct Synthesis of New Zircon-Type ZrGeO4 and Zr(Ge,Si)O4 Solid Solutions

A new phase of ZrGeO₄ was hydrothermally synthesized from a mixture of ZrOCl₂ solution and GeO₂ powder at 240°C. X-ray diffractometry patterns of synthesized ZrGeO₄ particles, which had better crystallinity than ZrSiO₄ particles prepared by the same process from ZrOCl₂ and tetraethoxysilane, could be indexed for a zircon-type tetragonal structure, having a ₀= 0.6694(0) and c ₀= 0.6265(7) nm. The c / a ratio of synthesized ZrGeO₄ (0.9360) was larger than that of ZrSiO₄ (0.9054). Solid solutions with zircon-type structure over the whole composition range in the ZrGeO₄-ZiSiO₄ system were also directly synthesized through the same solution route. The secondary particle size of zircon-type Zr(Ge,Si)O₄ solid solutions decreased, and its morphology gradually changed from octahedron-like to blood-red cell-like with decreased GeO₂ content.     

Structural Evolution and Vanadium Distribution in the Preparation of V4+-ZrSiO4 Solid Solutions from Gels

Vanadium-containing ZrSiO₄-gel precursors with nominal compositions V _x -ZrSiO₄ with x = 0.0, 0.002, 0.004, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.1, and 0.2 were prepared using a previously reported procedure and thermally treated over a range of temperature up to the formation of the V⁴⁺-ZrSiO₄ solid solution. The structural evolution and the V⁴⁺ location and its homogenous distribution were followed using powder X-ray diffractometry and electron spin resonance spectroscopy (ESR). Our experiments showed that a tetragonal form of V⁴⁺-ZrO₂ was the first crystalline phase obtained on heating the gels. On further heating, a phase transformation to the monoclinic form of V⁴⁺-ZrO₂ took place. Finally, the monoclinic form reacted with amorphous SiO₂ to form the V⁴⁺-ZrSiO₄ solid solution. A semiquantitative determination of the vanadium solubility in the ZrSiO₄ phase using ESR confirmed previously reported results obtained by lattice parameter variation over the solid-solution series.     

Pyroxene-Type (K,Na,Li)VO3 Solid Solutions

Phase relations in the system were studied between 600° and 350°C. All three end members have the pyroxene-type structure. Both LiVO₃ and NaVO₃ are monoclinic, whereas KVO₃ takes the orthorhombic symmetry. At 360°C, the join LiVO₃–NaVO₃ is characterized by two series of pyroxene-type solid solutions: NaVO₃–(Na_0.44Li_0.56)VO₃ and LiVO₃-(Na_0.16Li_0.84)-VO₃. (K_0.5Na_0.5)VO₃ and (K_0.5Li_0.5)VO₃ are two other stable phases at 360°C. The pyroxene-type (K_0.5Na_0.5)VO₃ has a range of solid solution from (K_0.6Na_0.4)VO₃ to (K_0.4Na_0.6)VO₃ along the join and extends into the ternary field with a maximum of 13 mol% LiVO₃. (K_0.5Li_0.5)VO₃ has no detectable range of solid solution, and its X-ray powder diffraction data cannot be indexed based upon either the monoclinic or the orthorhombic unit cell.     

Structure–Property Relations in xBaTiO3–(1−x)La(Mg1/2Ti1/2)O3 Solid Solutions

The microwave dielectric properties and microstructures of compounds in the solid solution series x BaTiO₃–(1− x )La(Mg_1/2Ti_1/2)O₃ (BTLMT) have been investigated. The structural phase transitions that occur as a function of x have been studied and are related to changes in the dielectric properties. For compounds where x ≤ 0.1, X-ray diffraction (XRD) showed evidence of 1:1 ordering between Mg and Ti cations. For x ≤ 0.3, XRD and electron diffraction revealed that compounds were tilted in both antiphase and in-phase. However, for 0.3 < x < 0.7, only antiphase tilting was present. The temperature coefficient of resonant frequency (τ_f) vs the relative permittivity (ɛ_r) was linear until x = 0.5 at which point in the solid solution the transition to a nontilted structure resulted in nonlinear behavior. τ_f values close to zero (−2 ppm/°C) were achieved at x = 0.5 (ɛ_r∼ 60), which had a quality factor ( Q · f _o) of 9600 GHz.     

Solubility of Si3N4 in Liquid SiO2

The nitrogen solubility in the SiO₂-rich liquid in the metastable binary SiO₂-Si₃N₄ system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N₂. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si₃N₄ and to adjust the computed phase diagram. The solubility of Si₃N₄ in fused SiO₂ is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si₃N₄ without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si₃N₄-SiO₂ interface that leads to amorphous regions of higher N content.     

High-Brightness LaPO4:Ce3+, Tb3+ Nanophosphors: Reductive Hydrothermal Synthesis and Photoluminescent Properties

This paper definitely reveals that LaPO₄:Ce³⁺, Tb³⁺ (LAP) nanophosphors prepared by normal hydrothermal method suffer significant loss of luminescence due to the oxidation of Ce³⁺–Ce⁴⁺ at hydrothermal stage. To effectively protect Ce³⁺ from oxidation, a reductive hydrothermal process using hydrazine hydrate as a protecting agent is proposed to synthesize LAP nanophosphors with different Ce³⁺ and Tb³⁺ concentrations, which exhibited much stronger green photoluminescence (PL) and longer lifetime than the products prepared by normal hydrothermal method. Furthermore, the high-brightness LAP nanophosphors exhibited high-quenching concentration of Tb³⁺; the La_0.4Ce_0.4Tb_0.2PO₄ nanophosphor showed almost the same PL intensity as that of the commercially used La_0.7Ce_0.2Tb_0.1PO₄ bulk powder.     

Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3

In the system TiO₂—Al₂O₃, TiO₂ (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al₂O₃) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al₂O₃. At higher temperatures, anatase solid solutions transform into TiO₂ (rutile) with the formation of α-Al₂O₃. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.     

Dielectric Properties and Relaxation in (1−x)BiScO3–xBa(Mg1/3Nb2/3)O3 Solid Solutions

The dielectric properties and frequency dispersion associated with a dielectric relaxation were evaluated within the perovskite (1− x )BiScO₃– x Ba(Mg_1/3Nb_2/3)O₃ solid solution systems (0.7 ≤ x ≤ 1). With increasing BiScO₃, the room-temperature dielectric permittivity at low frequency (100 Hz) increased up to 115 at x = 0.7, and a dielectric relaxation phenomenon was evident. Relaxation parameters were analyzed using several Arrhenius-type equations, and the microwave dielectric property measurement using rectangular wave-guide method enabled confirmation of the extrapolated value of the Arrhenius plot. The result of the microwave dielectric property measurement was also checked with J -function fitting based on the frequency-dependent Gaussian distribution of the associated dielectric loss data at low frequency.     

Ionic Conductivity of Li4SiO4, Li4GeO4, and Their Solid Solutions

Conductivity was measured for Li₄SiO₄ and its solid solutions with Li₄GeO₄ over a wide frequency range to separate clearly the effects of electrode polarization, conductance relaxations, etc., and to obtain true "dc" conductivities. The conductivities of all the electrolytes are markedly temperature-dependent, ranging from 10⁻⁸ to 10⁻¹⁰Ω⁻¹ cm⁻¹ at 100°C to 10⁻² to 10¹⁰Ω⁻¹ cm⁻¹ at 700°C. For solid solutions with the Li₄GeO₄ structure, conductivities fit the Arrhenius equation over a wide temperature range, but at higher temperatures a change in activation energy occurs, corresponding to a first-order phase transition. In contrast, solid solutions with the Li₄SiO₄ structure show changes in activation energy which do not correspond to phase transitions, but which appear to indicate changes in the conduction mechanism.     

Solubility Limits of α'-SiAION Solid Solutions in the System Si,Al,Y/N,O

The solubility limit of α'-SiAION solid solutions on the Si₃N₄─YN:3AIN composition join in the system Si₃N₄─YN─AIN has been determined at 1800°C. The end members of these solid solutions are Y_0.43Si_10.7Al_1.3N₁₆ and Y_0.8Si_9.6Al_2.4N₁₆. Unit-cell dimensions of the α'-SiAION solid solutions in the system Si,Al,Y/N,O can be expressed as follows: a _o(Å) = 7.752 + 0.045 m + 0.009 n , c _o(Å) = 5.620 + 0.048 m + 0.009 n , where the α'-SiAION solid solution has the formula Y _x Si_{12-( m+n )}Al _m+n N_{16- n} O _n . The single-phase boundary of the solid solution α'-SiAION on the composition triangle Si₃N₄─YN:3AIN─AIN:Al₂O₃ is delineated. The present paper also reports the phase relationships involving α'-SiAION.     

Residual Stress Distributions in the Solid Solution Eutectic, Co1−xNixO/ZrO2(CaO)

This study investigates thermal mismatch stresses in the lamellar microstructure of the solid solution directionally solidified eutectic (DSE) oxide Co_{1− x} Ni _x O/ZrO₂(CaO). X-ray and neutron diffraction measurements were performed on isolated eutectic domains to measure the residual strain and stress tensors. Maximum principal residual stresses on the order of 1 GPa were recorded, with the Co_{1− x} Ni _x O and the ZrO₂(CaO) phases maintaining states of tensile and compressive stress, respectively. The stress tensors for these materials are compared with measurements for similar DSE oxide systems and suggest that solid solution DSEs might be used to tailor the residual stress states in DSE composites.