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.     

Proton-Conductive Membranes Doped with Orthophosphoric Acid Based on Inorganic–Organic Hybrid Materials

A series of proton-conductive inorganic–organic hybrid membranes doped with phosphoric acid (H₃PO₄) have been prepared by the sol–gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-aminopropyltriethoxysilane (APTES), and tetraethoxysilane (TEOS) as precursors. High proton conductivity of 3.0 × 10⁻³ S/cm with a composition of 50TEOS–30GPTMS–20APTES–50H₃PO₄ was obtained at 120°C under 50% relative humidity (RH). The differential thermal analysis curve showed that thermal stability of membrane is significantly enhanced by the presence of an SiO₂ framework up to 250°C. X-ray ray diffraction revealed that the gels were amorphous. Infrared spectra showed a good complexation of H₃PO₄ in the matrix. The porous hybrid membrane, characterized by scanning electron microscopy, shows humidity-dependent conduction, and the conductivity under 75% relative humidity was significantly improved by addition of APTES due to the increase in the concentration of the defective site in the hybrid matrix.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Dehydration Behavior of Aluminum Sulfate Hydrates

An exothermic transition is observed near 400°CC on thermal dehydration of highly crystalline AI₂(SO₄)3.16H₂O, Al₂(S0₄)₃ 14H₂O, and Al₂(S0₄)₃ 9H₂O when the early stages of heating are carried out in vacuum. Amorphous or partially crystalline hydrates do not show the exotherm. No systematic relation is apparent between the decomposition behavior and the pore volume distribution of the various anhydrous A1₂(SO₄)₃ products.     

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.     

Thermochemistry of the Aluminas and Aluminum Trihalides

Consistent standard free energies of formation of gibbsite, bayerite, boehmite, and diaspore; their respective transition aluminas, α-Al₂O₃; and AlF₃, AlCl₃, H₂O₃ HF, and HCl are compiled from 298.16 to 2100 K from literature review, computations, and estimates. Significant adjustments and additions to earlier compilations are included. Revised analysis is made of the gibbsite-to-α-A1₂O₃ transition series and of reactions of appropriate aluminas with HF and HCl, comparing them with experimental data. These updated Δ G ° tables should also yield accurate ΔG° data for many other alumina reactions, e.g. with SiO₂, M₂O, MO, etc.     

Synthesis of Hydrated Aluminum Sulfate from Kaolin by Microwave Extraction

The feasibility of extracting alumina from kaolin via a microwave extraction process was investigated by comparing reaction times, reaction temperatures, and acid concentrations under microwave treatment with the same factors under conventional thermal extraction. The maximum amount of alumina extracted from kaolin under conventional processing at 90°C for 240 min with 1 M H₂SO₄ was 99.9%; the same amount of alumina was extracted under microwave processing at 90°C for 120 min with 1 M H₂SO₄.     

Temperature Dependence of Hardness of Alumina-Based Ceramics

Hardness was measured as a function of temperature (20° to 1000°C) for several Al₂O₃ ceramics, including single-crystal sapphire and polycrystalline aluminas containing different amounts of second phase. Hardness decreased steadily with increasing temperature for all materials tested, in accordance with a semiempirical relation of the form H=H₀ (1 – T/T₀). This behavior conformed with a thermally activated slip process, limited by Peierls stresses. At lower temperatures, the hardness values for debased aluminas were less (smaller H₀) than for the pure materials, consistent with a reduction in shear modulus resulting from the "soft" phase. However, at higher temperatures the hardness values for all the aluminas converged (identical T₀, i.e., material-invariant activation energy). The latter behavior indicated that the temperature dependence of the indentation deformation was controlled predominantly by the Al₂O₃ component.     

Stoichiometry of Fluorotopaz and of Mullite Made from Fluorotopaz

Mixtures of alumina and silica containing 68–78 wt% alumina react above 600°C with 0.5 mol of SiF₄ per mole of alumina to form fluorotopaz. High-resolution X-ray powder diffraction data were used to determine very accurate cell parameters of fluorotopaz as a function of 1 wt% compositional increments. The samples containing 69–76 wt% A1₂O₃ were found to have a linear cell parameter relationship. Compositions outside that range show discontinuities in the cell parameters, indicating solid solution behavior between 69 and 76 wt% alumina. Within this range the composition of fluorotopaz may be written 2A1₂O₃·_xSiO₂· SiF₄ where 1.07 x 1.53. Pyrolysis of all compositions of fluorotopaz solid solution yields mullite whiskers containing 76.1 wt% alumina (65.2 mol%).     

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.     

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.     

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.     

Low-Temperature Synthesis of Hexagonal Anorthite via Hydrothermal Processing

Hexagonal anorthite (CaAl₂Si₂O₈) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca₃Al₂O₆·6H₂O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO₂. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO₂ substitution has been shown to reduce the conversion of hydrogarnet to Ca₄Al₆O₁₃·3H₂O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl₂Si₂O₈ as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°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.