Mixed alkali-zinc diphosphates: Synthesis, structure, and properties

A series of mixed alkali-zinc diphosphates in the form of poly- and single crystals have been synthesized. Their physical-chemical, structural, and thermal characteristics have been determined. The compounds Na₂ZnP₂O₇ and K₂ZnP₂O₇ have a layered structural type, whereas LiNaZnP₂O₇, LiKZnP₂O₇, NaKZnP₂O₇, Li₁₂Zn₄(P₂O₇)₅, and K₂Zn₃(P₂O₇)₂ have a framework structural type.     

Phase relationships in the Na<Subscript>2</Subscript>ZnP<Subscript>2</Subscript>O<Subscript>7</Subscript>–LiKZnP<Subscript>2</Subscript>O<Subscript>7</Subscript> system

The phase relationships in the Na₂ZnP₂O₇–LiKZnP₂O₇ system are studied. They are represented by a mixture of the starting components in the subsolidus region. The eutectic was found at a temperature of 640°C and composition of 0.5LiKZnP₂O₇. The phase formation of this system is compared with the previously studied NaKZnP₂O₇–LiKZnP₂O₇ system. It is shown that a structural factor affects the geometry of the state diagrams.     

Crystal Structures of Na2ZnP2O7, K2ZnP2O7, and LiKZnP2O7 Phases in the M2O–ZnO–P2O5 Glass-Forming System (M = Li, Na, and K)

The crystal structures of Na₂ZnP₂O₇ and K₂ZnP₂O₇ are determined from X-ray powder diffraction data. The structure of LiKZnP₂O₇ is determined using a single-crystal sample. In the alkali zinc phosphates studied, the Zn²⁺ cation is coordinated by four oxygen atoms of the phosphate tetrahedra. In the first two compounds, the [ZnP₂O₇]^2– anion has the form of a sheet consisting of Zn and P oxygen-containing tetrahedra that involves only five-membered rings. In the LiKZnP₂O₇ structure, the zinc phosphate anion of the same composition has the form of a three-dimensional skeleton with a three-dimensional system of channels. These channels are linked via six-membered, eight-membered, and ten-membered rings. The atomic coordinates and interatomic distances are reported.     

Synthesis and crystal structure of the low-temperature modification of lithium potassium zinc diphosphate LiKZnP2O7

A new polymorphic modification, α-LiKZnP₂O₇, has been synthesized, which is a low-temperature modification of the LiKZnP₂O₇ compound. The crystal structure is determined by the Rietveld method from the X-ray powder diffraction data. The compound crystallizes in the monoclinic system (space group Pc, a = 12.3621(3) Å, b = 5.0655(1) Å, c = 10.2365(3) Å, β = 90.88(1)°, Z = 2) and has a framework structure similar to that of the high-temperature β phase. The framework is formed by the diphosphate groups and the oxygen tetrahedra of the zinc and lithium atoms, which statistically uniformly occupy equivalent positions in the structure.     

New solid solutions of mixed alkali-zinc diphosphates LiNa1 − x K x ZnP2O7

Phase relationships in the LiNaZnP₂O₇-LiKZnP₂O₇ system have been studied. For the first time a vast region of solid solutions of the rhombic syngony LiNa_{1 ? x} K _x ZnP₂O₇ containing simultaneously three alkali cations was found. The miscibility is broken at the temperature of the polymorphous transformations LiKZnP₂O₇ (270°C) and below, with the formation of the two-phase region of the solid solution with the rhombic and monoclinic structures (0.85 ≤ x ≤ 1 at 25°C).     

Thermal expansion of β-BaB2O4 and BaB4O7 borates

The thermal behavior of β-BaB₂O₄ and BaB₄O₇ borates is investigated using high-temperature X-ray powder diffraction analysis. The components of the thermal expansion tensor and the tensor orientation with respect to the crystallographic axes are calculated. Thermal deformations are analyzed in relation to the crystal structure. The thermal expansion of the β-BaB₂O₄ borate is maximum along the c axis and close to zero in the ab plane. The thermal expansion anisotropy correlates with the layered structure, the orientation of the optical indicatrix, and the thermal ellipsoids of atoms. The thermal expansion of the BaB₄O₇ borate is studied in three high-temperature X-ray diffraction experiments (two experiments with heating and one experiment with cooling). The thermal expansion of this compound is strongly anisotropic: the expansion is maximum in the ac monoclinic plane, whereas the contraction is observed along the b axis.     

Thermophysical performances of (Gd1-xYbx)2AlTaO7 oxides for high-temperature heat-insulation coating applications

Newly developed (Gd_1-xYb_x)₂AlTaO₇ oxides for high-temperature heat-insulation coatings were prepared using a multi-step solid-state fritting method. The important features of synthesized oxides including phase composition, thermal conductivity and expansion performances were studied. It is investigated that the fabricated ceramics are confirmed to possess a sole pyrochlore crystal structure. Owing to the influence of the strain fields and mass fluctuations caused by Yb₂O₃ addition, thermal conductivities of (Gd_1-xYb_x)₂AlTaO₇ oxides are lower than that of Gd₂AlTaO₇ or Yb₂AlTaO₇, and the (Gd_0.9Yb_0.1)₂AlTaO₇ exhibits the lowest thermal expansion coefficient. Due to the synergic effects of the relatively high electro-negativity of Yb³⁺, decedent lattice order, and numerous oxygen vacancies, the thermal expansion coefficients increase gradually with increasing Yb₂O₃ content. The thermophysical performances of (Gd_1-xYb_x)₂AlTaO₇ oxides satisfy the conditions for high-temperature heat-insulation coatings.     

Thermal properties of field-assisted-sintered SiCN–Y2O3 composites

Polymer-derived amorphous SiCN has excellent high-temperature stability and properties. To reduce the shrinkage during pyrolysis and to improve the high-temperature oxidation resistance, Y₂O₃ was added as a filler. In this study, polymer-derived SiCN–Y₂O₃ composites were fabricated by mixing a polymeric precursor of SiCN with Y₂O₃ submicron powders in different ratios. The mixtures were cross-linked and pyrolyzed in argon. SiCN–Y₂O₃ composites were processed using field-assisted sintering technology at 1350°C for 5 min under vacuum. Dense SiCN–Y₂O₃ composite pellets were successfully made with relative density higher than 98% and homogeneous microstructure. Due to low temperature and short time of the heat-treatment, the grain growth of Y₂O₃ was substantially inhibited. The Y₂O₃ grain size was ∼1 μm after sintering. The composites’ heat capacity, thermal diffusivity, and thermal expansion coefficients were characterized as a function of temperature. The thermal conductivity of the composites ceramics decreased as the amount of amorphous SiCN increased and the coefficient of thermal expansion (CTE) of the composites increased with Y₂O₃ content. However, the thermal conductivity and CTE did not follow the rule of mixture. This is likely due to the partial oxidation of SiCN and the resultant impurity phases such as Y₂SiO₅, Y₂Si₂O₇, and Y_4.67(SiO₄)₃O.     

Tailorable thermal expansion in leucite-pollucite materials derived from geopolymers for environmental barrier coatings

The K[AlSi₂O₆]-Cs[AlSi₂O₆] pseudo-binary system was synthesized by geopolymer crystallization. The thermal expansion properties of these materials were studied by in situ high-temperature X-ray diffraction to characterize thermal expansion behavior for potential application as environmental barrier coatings. Tailorable thermal expansion through changing cation stoichiometry allowed reduced thermal expansion mismatch with SiC_f/SiC composites compared to rare-earth-based coatings.     

Thermal properties of RE2AlTaO7 (RE = Gd and Yb) oxides

In the current paper, the synthesis of RE₂AlTaO₇ (RE = Gd and Yb) oxides was performed through a high-temperature sintering method using Gd₂O₃, Yb₂O₃, Al₂O₃ and Ta₂O₅ as raw chemicals. The phase-structure, micro-morphology and thermal-physical properties of RE₂AlTaO₇ (RE = Gd and Yb) oxides were analyzed. The RE₂AlTaO₇ (RE = Gd and Yb) oxides exhibit single pyrochlore-type crystal-structure. Compared to Yb₂AlTaO₇, the Gd₂AlTaO₇ has greater thermal conductivity owning to relatively weaker phonon scattering. Because of higher electro-negativity of Yb, the Yb₂AlTaO₇ has lower coefficient of thermal expansion than Gd₂AlTaO₇. The RE₂AlTaO₇ (RE = Gd and Yb) oxides also exhibit no phase-transition between ambient and 1473?K.     

High-entropy rare earth titanates with low thermal conductivity designed by lattice distortion

High-entropy single-phase rare earth titanates (RE_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₂Ti₂O₇ (RE = Sm, Y, Lu) were designed and synthesized successfully, in which their lattice distortion was quantitatively described by mass disorder and size disorder. It is worth mentioning that (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₂Ti₂O₇ could obtain the low thermal conductivity (1.51 W·m⁻¹·K⁻¹, 1500°C), high thermal expansion coefficient (average, 11.69×10⁻⁶ K⁻¹, RT ∼1500°C) and excellent high-temperature stability. In addition, the relationship between the microstructure and thermal transport behaviors has been studied at the atomic scale. Due to the disorder of A-site ions, severe lattice distortion occurred in specific crystal planes, and the large mass difference between Y³⁺ and other RE³⁺ further causes mass fluctuation and results in lower thermal conductivity. Compared with YSZ, the high-entropy rare earth titanate (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₂Ti₂O₇ has lower thermal conductivity, higher thermal expansion coefficient, and excellent high-temperature stability, which has great potential for application in the thermal protection field.     

High-temperature crystal chemistry of α-Na<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript> and β-NaB<Subscript>3</Subscript>O<Subscript>5</Subscript> layered borates

The thermal expansion of two layered sodium borates is investigated using high-temperature X-ray powder diffraction. It is established that both compounds are characterized by strong anisotropy of thermal expansion due to the hinge mechanism. Special (singular) points are revealed in the temperature dependences of the angular lattice parameters for the α-Na₂B₄O₇ compound. The temperature of manifestation of these singularities corresponds to the glass transition temperature of the melt of the same chemical composition. The assumption is made that the angular lattice parameters (angles between atomic rows) in compounds with a considerable degree of ionic bonding are more sensitive to variations in temperature as compared to the linear lattice parameters (interatomic distances). It is experimentally in situ demonstrated with high-temperature X-ray diffraction that the β-NaB₃O₅ borate is peritectically melted at a temperature of 725 ± 10°C with the formation of the α-Na₂B₈O₁₃ octaborate.     

Preparation of crystalline ytterbium disilicate environmental barrier coatings using suspension plasma spray

In high-speed modern industries, high-temperature stability of materials is essential. A promising high-temperature material currently attracting attention is silicon carbide (SiC)-based ceramic matrix composites (CMC). However, a disadvantage of these materials is their reduced lifetime in an oxidizing atmosphere. To overcome this, environmental barrier coating can be employed. In this study, we aimed to fabricate an environmental barrier coating using suspension plasma spray with Yb₂Si₂O₇, which exhibits excellent oxidation resistance and a similar thermal expansion coefficient to SiC. To prepare the crystalline Yb₂Si₂O₇ coating layer, the gas concentration of the plasma spray was adjusted, and then the suspension manufacturing solvent was adjusted and sprayed. The prepared coating samples were analyzed by X-ray diffraction, scanning electron microscope, transmission electron microscopes, and energy dispersive X-ray spectroscopy to determine phase and microstructure changes. Highly crystalline ytterbium disilicate was observed at low plasma enthalpy with no hydrogen and 20% addition of water.     

Crack-healing behaviour of MoSi2 dispersed Yb2Si2O7 environmental barrier coatings

MoSi₂ doped Yb₂Si₂O₇ composites were designed to extend the lifetime of Yb₂Si₂O₇ environmental barrier coatings (EBCs) via self-healing cracks during high-temperature applications. Yb₂Si₂O₇–Yb₂SiO₅–MoSi₂ composites with different mass fractions were prepared by applying spark plasma sintering. X-ray diffraction results confirmed that the composites consisted of Yb₂Si₂O₇, Yb₂SiO₅, and MoSi₂. The thermal expansion coefficients (CTEs) of the composites increased with an increase in the MoSi₂ content. The average CTE of the 15 wt% MoSi₂ doped Yb₂Si₂O₇ composite was 5.24 × 10^?6 K^?1, indicating that it still meets the CTE requirement of EBC materials. After being pre-cracked by using the Vickers indentation technique, the samples were annealed for 0.5 h at 1100 or 1300 °C to evaluate the crack-healing ability. Microstructural studies showed that cracks in 15 wt% MoSi₂ doped Yb₂Si₂O₇ composites were fully healed during annealing at 1300 °C. Two mechanisms may be responsible for crack healing. First, the cracks were filled with SiO₂ glass formed by MoSi₂ oxidation. Second, the formed SiO₂ continued to react with Yb₂SiO₅ to form Yb₂Si₂O₇, which can cause cracks to heal owing to volumetric expansion. The Yb₂Si₂O₇ formation with smaller volume expansion is more beneficial.     

Thermal Expansion of Rare‐Earth Pyrosilicates

The use of RE₂Si₂O₇ materials as environmental barrier coatings (EBCs) and in the sintering process of advanced ceramics demands a precise knowledge of the coefficient of thermal expansion of the RE₂Si₂O₇. High‐temperature X‐ray diffraction (HTXRD) patterns were collected on different RE₂Si₂O₇ polymorphs, namely A, G, α, β, γ, and δ, to determine the changes in unit cell dimensions. RE₂Si₂O₇ compounds belonging to the same polymorph showed, qualitatively, very similar unit cell parameters behavior with temperature, whereas the different polymorphs of a given RE₂Si₂O₇ compound exhibited markedly different thermal expansion evolution. The isotropy of thermal expansion was demonstrated for the A‐RE₂Si₂O₇ polymorph while the rest of polymorphs exhibited an anisotropic unit cell expansion with the biggest expansion directed along the REO_x polyhedral chains. The apparent bulk thermal expansion coeficcients (ABCTE) were calculated from the unit cell volume expansion for each RE₂Si₂O₇ compound. All compounds belonging to the same polymorph exhibited similar ABCTE values. However, the ABCTE values differ significantly from one polymorph to the other. The highest ABCTE values correspond to A‐RE₂Si₂O₇ compounds, with an average of 12.1 × 10^?6 K^?1, whereas the lowest values are those of β‐ and γ‐RE₂Si₂O₇, which showed average ABCTE values of ~4.0 × 10^?6 K^?1.     

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y,Ho, Er,or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE₂(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)₂O₇ (RE₂HE₂O₇, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE₂HE₂O₇ formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE₂HE₂O₇ ceramics were investigated systematically. The results show that RE₂HE₂O₇ ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10^?6 K^-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m^-1 K^-1, 25 ℃). In addition, RE₂HE₂O₇ ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m^0.5). The outstanding properties of the RE₂HE₂O₇ ceramics indicate that they could be candidates for the next generation of TBC materials.     

A new ternary phase in the system CaO–Al2O3–Cr2O3: Crystal structure and thermal expansion of CaAl2Cr2O7

Polycrystalline material of a novel phase in the system CaO–Al₂O₃–Cr₂O₃ has been obtained by solid-state reactions. Chemical analysis indicated the composition CaAl₂Cr₂O₇. Single-crystal growth of the new compound using borax as a mineralizer was successful. Diffraction experiments at ambient conditions on a crystal with composition CaAl_2.13Cr_1.87O₇ yielded the following basic crystallographic data: space group P 3, a = 7.7690(5) Å, c = 7.6463(5) Å, V = 399.68(6) ų, Z = 3. Structure determination and subsequent least-squares refinements resulted in a residual of R(|F|) = 2.3% for 1440 independent observed reflections and 113 parameters. To the best of our knowledge, the structure of CaAl_2.13Cr_1.87O₇ or CaAl₂Cr₂O₇ represents a new structure type. It belongs to the group of double layer structures where individual double layers contain octahedrally and tetrahedrally coordinated cation positions. Linkage between neighboring sheet packages is provided by additional calcium cations. Furthermore, thermal expansion has been studied in the interval between 29 and 790°C using in situ high-temperature single-crystal diffraction. No indications for a structural phase transition were observed. From the evolution of the lattice parameters the thermal expansion tensor has been obtained. A pronounced anisotropy is evident. The response of structural building units to variable temperature has been discussed.     

Fabrication and thermal insulation properties of ceramic felts constructed by electrospun γ-Y2Si2O7 fibers

Ceramic fiber felts are attractive candidates for high temperature insulation due to their lightweight, high porosity and low thermal conductivity. In this work, ceramic felts constructed by γ-Y₂Si₂O₇ fibers were prepared by a facile method of Y–Si–O/PVB sol-gel electrospinning combined with subsequent high-temperature calcination. Effects of calcination temperature on the phase composition, microstructure and thermal insulation properties of ceramic felts were systematically studied. Results indicated that the organic components in the sol-gel fibers were removed after high temperature calcination, while the fibers kept the original continuous microtopography with high aspect ratios. Ceramic fiber felts of pure γ-Y₂Si₂O₇ phase could be obtained after calcinated at 1300 °C. The as-prepared paper-like γ-Y₂Si₂O₇ fiber felt presented low density of ～120 mg/cm³ and a high porosity up to 97.03%. Combined with the inherent high temperature stability and low thermal conductivity of γ-Y₂Si₂O₇, this light ceramic felts possessed high-temperature resistance and thermal insulating property (low thermal conductivity of 0.052 W m^?1 K^?1). The successful preparation of this ceramic fiber felt may provide a perspective for insulation materials used in harsh environments.     

A Study of Thermodynamic Characteristics of the Zn2P2O7–M4P2O7(M= Li and Na) Systems by Differential Scanning Calorimetry

Glasses, crystals, and melts in the Zn₂P₂O₇–M ₄P₂O₇(M= Li and Na) systems are studied by differential scanning calorimetry (DSC). Temperature dependences of the heat capacity and the enthalpies of melting of Zn₂P₂O₇and Na₂ZnP₂O₇crystals are determined. The thermodynamic characteristics obtained are used for calculating the liquidus curve for the Zn₂P₂O₇–Na₂ZnP₂O₇subsystem on the basis of the Schröder relationship with due regard for the temperature dependences of the enthalpy of melting for components and the nonideal behavior of melts. Reasoning from the Hruby empirical formula, the inference is drawn that the crystallization ability of lithium–zinc phosphate glasses is higher than that of sodium–zinc phosphate glasses. The radius of cooperative motion in the glass transition range for glasses in the Zn₂P₂O₇–Na₄P₂O₇system is estimated in the framework of the phenomenological thermokinetic fluctuation theory. It is assumed that an increase in the radius with an increase in the alkali oxide content is associated with an increase in the ionicity of bonds.     

Phase structure and thermophysical properties of co-doped La2Zr2O7 ceramics for thermal barrier coatings

La₂Zr₂O₇ has high melting point, low thermal conductivity and relatively high thermal expansion which make it suitable for application as high-temperature thermal barrier coatings. Ceramics including La₂Zr₂O₇, (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ were synthesized by solid state reaction. The effects of co-doping on the phase structure and thermophysical properties of La₂Zr₂O₇ were investigated. The phase structures of these ceramics were identified by X-ray diffraction, showing that the La₂Zr₂O₇ ceramic has a pyrochlore structure while the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ exhibit a defect fluorite structure, which is mainly determined by ionic radius ratio r(A_av.³⁺)/r(B_av.⁴⁺). The measurements for thermal expansion coefficient and thermal conductivity of these ceramics from ambient temperature to 1200 °C show that the co-doped ceramics (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ and (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ have a larger thermal expansion coefficient and a lower thermal conductivity than La₂Zr₂O₇, and the (La_0.2Yb_0.8)₂(Zr_0.7Ce_0.3)₂O₇ shows the more excellent thermophysical properties than (La_0.7Yb_0.3)₂(Zr_0.7Ce_0.3)₂O₇ due to the increase of Yb₂O₃ content.