Pressureless Sintering of α-Si3N4 Porous Ceramics Using a H3PO4 Pore-Forming Agent

A new method for preparing high bending strength porous silicon nitride (Si₃N₄) ceramics with controlled porosity has been developed by using pressureless sintering techniques and phosphoric acid (H₃PO₄) as the pore-forming agent. The fabrication process is described in detail and the sintering mechanism of porous ceramics is analyzed by the X-ray diffraction method and thermal analysis. The microstructure and mechanical properties of the porous Si₃N₄ ceramics are investigated, as a function of the content of H₃PO₄. The resultant high porous Si₃N₄ ceramics sintered at 1000°–1200°C show a fine porous structure and a relative high bending strength. The porous structure is caused mainly by the volatilization of the H₃PO₄ and by the continous reaction of SiP₂O₇ binder, which could bond on to the Si₃N₄ grains. Porous Si₃N₄ ceramics with a porosity of 42%–63%, the bending strength of 50–120 MPa are obtained.     

Reactivity of Alumina toward Phosphoric Acid

Reaction between alumina and 33.3 wt% orthophosphoric acid was investigated by monitoring the heat liberated under isothermal conditions at temperatures from 25° to 90°C. In a separate set of experiments, the H₃PO₄ concentration was varied from 0 to 50 wt%, at 25°C. Reactivities of five aluminas (three anhydrous and two hydrated) differing in particle size, surface area, and crystallinity were studied. Relationships between the properties of the aluminas and their reactivities toward phosphoric acid were established. The aluminas with the highest surface areas and the lowest degrees of crystallinity react more rapidly and produce overall more heat. Increasing the temperature and phosphoric acid concentration were also shown to increase heat evolution. However, increasing the H₃PO₄ concentration beyond 33.3 wt% (molar Al/P ratio = 1.0) for the anhydrous aluminas, and beyond 40 wt% for boehmite, does not result in a significant increase in the amount of heat evolved. Gibbsite continues to release greater amounts of heat when reacting with increasing concentrations of H₃PO₄ (up to 50 wt%). The anhydrous aluminas generally react faster than do the hydrates. Within the range of H₃PO₄ concentrations from 0 to 33.3 wt% the hydrates and the most reactive anhydrous alumina exhibit approximately the same degree of reactivity on a per mole of Al₂O₃ basis.     

Alumina/Epoxy Interpenetrating Phase Composite Coatings: I, Processing and Microstructural Development

Interpenetrating phase composite (IPC) coatings consisting of continuously connected Al₂O₃ and epoxy phases were fabricated. The ceramic phase was prepared by depositing an aqueous dispersion of Al₂O₃ (0.3 μm) containing orthophosphoric acid, H₃PO₄, (1–9.6 wt%, solid basis) and heating to create phosphate bonds between particles. The resulting ceramic coating was porous, which allowed the infiltration and curing of a second-phase epoxy resin. The effect of dispersion composition and thermal processing conditions on the phosphate bonding and ceramic microstructure was investigated. Reaction between Al₂O₃ and H₃PO₄ generated an aluminum phosphate layer on particle surfaces and between particles; this bonding phase was initially amorphous, but partially crystallized upon heating to 500°C. Flexural modulus measurements verified the formation of bonds between particles. The coating porosity (and hence epoxy content in the final IPC coating) decreased from ∼50% to 30% with increased H₃PO₄ loading. The addition of aluminum chloride, AlCl₃, enhanced bonding at low temperatures but did not change the porosity. Diffuse reflectance FTIR showed that a combination of UV and thermal curing steps was necessary for complete curing of the infiltrated epoxy phase. Al₂O₃/epoxy IPC coatings prepared by this method can range in thickness from 1 to 100 μm and have potential applications in wear resistance.     

Fiber Strength Retention of Lanthanum- and Cerium Monazite-Coated Nextel™ 720

Nextel™ 720 fibers were coated with LaPO₄ and CePO₄ monazite. The coatings were applied using washed and unwashed rhabdophane sols derived from La(NO₃)₃/(NH₄)₂HPO₄ and a washed sol derived from Ce(NO₃)₃/H₃PO₄. The coatings were cured in-line at 900°–1300°C. Multiple coatings were also applied. Fiber strength was retained after coating with washed sols, but not with unwashed sols. These results are consistent with earlier work on LaPO₄ monazite fiber coatings derived from La(NO₃)₃/H₃PO₄.     

Gelcasting of Magnesium Aluminate Spinel Powder

A stoichiometric MgAl₂O₄ spinel (MAS) powder was processed in aqueous media and consolidated by gelcasting from suspensions containing 41–45 vol% solids loading. The MAS powder was first obtained by heat treating a compacted mixture of α-Al₂O₃ and calcined caustic MgO at 1400°C for 1 h, followed by crushing and milling. Then, its surface was passivated against hydrolysis using an ethanol solution of H₃PO₄ and Al(H₂PO₄)₃. The as-treated surface MAS powder could then be dispersed in water using tetra methyl ammonium hydroxide and an ammonium salt of poly-acrylic acid (Duramax D-3005) as dispersing agents. The as-obtained stable suspensions were gelcast, dried, and sintered at 1650°C for 1–3 h. For comparison purposes, the treated powder was also compacted by die pressing of freeze-dried granules and sintered along with gelcast samples. Near-net-shape MAS components with 99.55% of the theoretical density could be fabricated by aqueous gelcasting upon sintering at 1650°C for 3 h. The MAS ceramics fabricated by gelcasting and die pressing exhibited comparable properties.     

Two Phase Monazite/Xenotime 30LaPO4–70YPO4 Coating of Ceramic Fiber Tows

Equiaxed yttrium–lanthanum phosphate nanoparticles (Y_0.7,La_0.3)PO₄·0.7H₂O were made and used to continuously coat Nextel^™ 720 fiber tows. The particles were precipitated from a mixture of yttrium and lanthanum citrate chelate and phosphoric acid (H₃PO₄), and characterized with differential thermal analysis and thermogravimetric analysis, X-ray diffraction, transmission electron microscopy, and scanning electron microscopy. The coated fibers were heat treated at 1000°–1300°C for 1, 10, and 100 h. Coating grain growth kinetics and coated fiber strengths were determined and compared with equiaxed La-monazite coatings. The relationships between coating porosity, coating hermeticity, and coated fiber strength are discussed.     

Dissolution and Reaction of Yttria-Stabilized Zirconia Single Crystals in Hydrothermal Solutions

Dissolution and reaction of yttria-stabilized zirconia (YSZ) single crystals were investigated in various solutions at 600° to 780°C under 100 MPa. YSZ crystals were not corroded in pure H₂O and neutral solutions such a LiF, LiCl, NaNO₃, KCl, KBr; K₂SO₄, and Na₂SO₄ even under severe conditions at 600°C, 100 MPa. They were, however, dissolved and reprecipitated in basic solutions such as NaF, K₂CO₃, KOH, NaOH, and LiOH with partial decompositon (destabilization in the last three solutions. YSZ crystals were completely decomposed into m -ZrO₂ in acidic solutions of Li₂SO₄, H₂SO₄, and HCl, whereas they reacted with the solution and formed other compounds in KF, NH₄F, and H₃PO₄ solutions.     

Synthesis of Nanosized Spherical Rhabdophane Particles

Spherical 10 nm rhabdophane (LaPO₄·H₂O) particles were made by controlled precipitation in water using lanthanum citrate chelate and phosphoric acid (La-Cit⁺/H₃PO₄) at a temperature of 30°C. Rod-shaped 10 nm × 100 nm rhabdophane particles were made by direct precipitation from lanthanum nitrate and phosphoric acid (La(NO₃)₃/H₃PO₄). The lanthanum nitrate to phosphoric acid molar ratios (La:P) were fixed at 1:1 and 1:5 for both methods to study their effect on particle size and shape. The particles were characterized with DTA/TGA, XRD, TEM, SEM, Fourier Transform infrared spectroscopy, and ζ potential measurements. The spherical particle surfaces had absorbed citric acid. The point of zero charge of the rod-shaped and spherical particles was pH 5.4 and 4.3, respectively. Formation mechanisms for the spherical particles are discussed.     

Low-Temperature Formation of Aluminum Orthophosphate

Factors influencing the low-temperature formation of AIPO₄ and its precursor phases, AIPO₄· x H₂O (1 x 2), were investigated. AIPO₄ formed by reaction between 33.3 wt% H₃PO₄ solution and alumina. Five aluminas (three anhydrous and two hydrated) were utilized. Each differed in particle size, surface area, and crystallinity. The reaction temperatures investigated were 113°, 123°, and 133°C. The high-surface-area aluminas were sufficiently reactive in the phosphoric acid solution at these temperatures to produce crystalline reaction products. However, only hydrated forms of AIPO₄, AIPO₄· x H₂O (1 x 2), crystallized directly out of solution. x generally decreased as the curing temperature was increased. Upon dehydration of these hydrated reaction products, anhydrous AIPO₄ was formed, primarily in the berlinite and/or cristobalite modifications. Both the temperature of reaction and the alumina used influence the hydrates that form. In turn, the hydrates which form, the macroscopic assemblages into which they may crystallize, and the morphologies of the crystallites all affect the polymorphic form and the crystallinity of the anhydrous AIPO₄ phase ultimately produced on dehydration. Phase-pure and highly crystalline AIPO₄-cristobalite (the high-temperature modification) was formed by the dehydration of AIPO₄·H₂O at a temperature as low as 113°C.     

Solid Solubility and Thermal Expansion in a LaPO4–YPO4 System

The composition and lattice parameters of co-precipitated (La_0.3Y_0.7) orthophosphate were studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX). The results indicate that the as-precipitated powder consists of single-phase (La_0.3Y_0.7PO₄·H₂O) rhabdophane nanoparticles. Heat treatment at 950°C caused the decomposition of rhabdophane into a (La_{1− x} Y _x )PO₄ monazite solid solution and YPO₄ xenotime. The solid solubility of Y in LaPO₄ monazite from 1000° to 1600°C was studied using XRD, TEM, and EDX. The implications of the findings for controlling the coefficient of thermal expansion of the prospective two-phase monazite–xenotime fiber coatings for ceramic composites applications are discussed.     

Phase Stability and Ionic Conductivity of Solid Solutions with NASICON Structure

Formation of heterovalent Zr-substituted solid solutions (up to 7 mol%) for Yb³⁺ in Na₆Yb₃(PO₄)₅ and LiNa₅Yb₃(PO₄)₅ complex phosphates was studied by ceramic technique at 950°C. Obtained samples were investigated with X-ray powder diffraction, infrared, and impedance spectroscopy. Zr-substituted (7 mol%) Na₆Yb₃(PO₄)₅ has ionic conductivity of 1.6·10⁻² S/cm at 300°C and 4.8·10⁻⁵ S/cm at room temperature. An updated version of phase diagram for ScPO₄–Na₃PO₄–Li₃PO₄ quasi-ternary system was provided.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

Sintering of Boron Carbide Heat-Treated with Hydrogen

Hydrogen gas (H₂) was used to extract B₂O₃ coatings from boron carbide (B₄C) particles, permitting a lower temperature onset of sintering and restricting coarsening via solution and precipitation of B₄C in B₂O₃ liquid. Remnant H₂ had to be removed from the furnace before specimens were heated through temperature ranges in which evaporation-condensation coarsening competed with sintering (2010°–2140°C), because the presence of H₂ increased the B₄C vapor pressure. Heat treatment of B₄C compacts in a 50:50 H₂-He mixture at 1350°C, followed by a purge of the H₂ gas and then rapid heating to 2230°C, resulted in a percentage of theoretical density of 94.7%. This is higher than the value of 92.8%, which was the highest achieved without the use of H₂.     

Preparation and Properties of Hydrothermally Stable γ-Alumina Membranes

Supported mesoporous γ-Al₂O₃ membranes deteriorate and blister in steam-containing environments at high temperatures. This deterioration led us to the development of a new type of supported γ-Al₂O₃ membrane with significantly improved stability under hostile conditions. Two measures were taken to achieve this result. First, the γ-Al₂O₃ itself was stabilized by an addition of 6 mol% La₂O₃ to suppress pore growth of the mesoporous structure. Second, the adherence of the γ-Al₂O₃ membrane to the α-Al₂O₃ support was significantly improved by application of phosphate bonding between the membrane layer and the support, using an Al(H₂PO₄)₃ precursor solution. Membranes applied without phosphate bonding were separated from the α-Al₂O₃ support during high-temperature steam treatment, resulting in complete loss of separative properties. The newly developed membranes could be operated for 100 h at 600°C in H₂O/CH₄= 3/1 (by volume) at 2.5 MPa total pressure with no delamination or cracking in the membrane–support interface and with no significant pore growth in the γ-Al₂O₃ membrane.     

Chemical Reactions of Ultraphosphate Glasses with Water at Various Temperatures

The chemical reactions between P₂O₅-ZnO-H₂O ultraphosphate glasses and water were characterized between room temperature and 500°C, using thermogravimetry, differential scanning calorimetry, X-ray diffraction, and ³¹P nuclear magnetic resonance. Water adsorption and hydrolysis reactions of the glass leads to the formation of H₃PO₄ and crystalline ZnH₂P₂O₇ below 200°C. The rate of water adsorption increases, owing to the hygroscopicity of the hydrolysis products of the glass. Devitrification occurs at 250°C via surface reactions. The microstructure of the devitrified glass consists of crystalline Zn₂P₄O₁₂ and a liquid phase containing hydrolysis products of P₂O₅ like metaphosphoric acid (HPO₃) _n. Devitrification is finally followed by water desorption at higher temperatures.     

Thermal, Mechanical, and Chemical Properties of Sintered Xenotime-Type RPO4 (R = Y, Er, Yb, or Lu)

Xenotime-type RPO₄ (R = Y, Er, Yb, or Lu) powder was dry-pressed into disks and bars. The disks and bars could be sintered to a relative density of greaterthan equal to98% in air without cracking at 1300° (R = Yb or Lu) or 1500°C (R = Y or Er), depending on the grain size. The linear thermal expansion coefficient (at 1000°C), thermal conductivity (at 20°C), and bending strength (at 20°C) of the xenotime-type RPO₄ ceramics were 6.2 10^-6/°C, 12.02 W(mK)^-1, and 95 ± 29 MPa for R = Y; 6.0 10^-6/°C, 12.01 W(mK)^-1, and 100 ± 21 MPa for R = Er; 6.0 10^-6/°C, 11.71 W(mK)^-1, and 135 ± 34 MPa for R = Yb; and 6.2 10^-6/°C, 11.97 W(mK)^-1, and 155 ± 25 MPa for R = Lu. The xenotime-type RPO₄ ceramics did not react with SiO₂, TiO₂, Al₂O₃, ZrO₂, or ZrSiO₄, even at 1600°C for 3 h in air, and were stable in aqueous solutions of HCl, H₂SO₄, HNO₃, NaOH, and NH₄OH at 20°C.     

Crystallization Kinetics of a Complex Lithium Silicate Glass-Ceramic

Volume crystallization of this glass is nucleated by Li₃PO₄. On heating from room temperature, Li₂SiO₃ appears around 650°C and then converts to Li₂Si₂O₅ around 850°C by reaction with SiO₂ from the melt. Preheating the glass at 1000°C forms larger Li₃PO₄ nuclei that promote additional crystallization of cristobalite in the 650° to 850°C range. Crystallization activation energies calculated from scan-rate dependence of DTA peaks are 270 kJ/mol for Li₂SiO₃ and 360 to 570 kJ/mol for Li₂Si₂O₅.     

Uniformly Porous Al2O3/LaPO4 and Al2O3/CePO4 Composites with Narrow Pore-Size Distribution

Porous Al₂O₃/20 vol% LaPO₄ and Al₂O₃/20 vol% CePO₄ composites with very narrow pore-size distribution at around 200 nm have been successfully synthesized by reactive sintering at 1100°C for 2 h from RE₂(CO₃)₃· x H₂O (RE = La or Ce), Al(H₂PO₄)₃ and Al₂O₃ with LiF additive. Similar to the previously reported UPC-3Ds (uniformly porous composites with a three-dimensional network structure, e.g. CaZrO₃/MgO system), decomposed gases in the starting materials formed a homogeneous open porous structure with a porosity of ∼40%. X-ray diffraction, ³¹P magic-angle spinning nuclear magnetic resonance, scanning electron microscopy, and mercury porosimetry revealed the structure of the porous composites.     

Aluminum Phosphates Derived from Alumina and Alumina-Sol–Gel Systems

Aluminum phosphate products formed by the reactions of alumina and alumina-gel systems with acidic phosphates were analyzed. Drying of alumina-gel to form microcrystalline boehmite and conversion to γ-alumina by thermal treatment was indicated by the appearance of octahedral, pentacoordinate, or tetrahedral sites, which were established using ²⁷Al magic-angle-spinning solid-state nuclear magnetic resonance spectroscopy. Crystalline aluminum phosphate products and amorphous material were identified using this technique. α-alumina and heat-treated alumina-gel that were reacted with phosphate in an Al:P ratio of 1:1 yielded dramatically different aluminum orthophosphate:aluminum metaphosphate product ratios of 8.2:1 and 1:1.1, respectively. When alumina-gel was heat-treated with phosphate, an abundance of aluminum orthophosphate, aluminum metaphosphate, and hydrated aluminum phosphate products were affected by varying conditions of temperature and time of heat treatment and by the amount of phosphate present. An α-alumina/alumina-gel composite sol–gel phase that was reacted with phosphoric acid (H₃PO₄) in a Al:P ratio of 1:1 exhibited an increased quantity of aluminum metaphosphate products compared with an α-alumina:H₃PO₄ ratio of 1:1 and a higher percentage of reaction (79%) compared with the reactions of an α-alumina:H₃PO₄ ratio of 1:1 or an alumina-gel:H₃PO₄ ratio of 1:1. The morphologies of aluminum triphosphate hydrate and aluminum metaphosphate product phases were observed using scanning electron microscopy.     

Thermal Expansion Behavior of Titanium-Doped La(Sr)CrO3 Solid Oxide Fuel Cell Interconnects

La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite has been designed as an interconnect material in high-temperature solid oxide fuel cells (SOFCs) because of its thermal expansion compatibility in both oxidizing and reducing atmospheres. La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ shows a single phase with a hexagonal unit cell of a = 5.459(1) Å, c = 13.507(2) Å, Z = 6 and a space group of R -3 C . Average linear thermal expansion coefficients of this material in the temperature range from 50° to 1000°C were 10.4 × 10⁻⁶/°C in air, 10.5 × 10⁻⁶/°C under a He–H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁵ atm at 1000°C), and 10.9 × 10⁻⁶/°C in a H₂ atmosphere (oxygen partial pressure of 4 × 10⁻¹⁹ atm at 1000°C). La_0.8Sr_0.2Cr_0.9Ti_0.1O₃ perovskite with a linear thermal expansion in both oxidizing and reducing environments is a promising candidate material for an SOFC interconnect. However, there still remains an air-sintering problem to be solved in using this material as an SOFC interconnect.