What is Aluminum Phosphate?
Aluminum phosphate is an inorganic compound composed of aluminum and phosphate ions. It has the chemical formula AlPO4 and exists in various crystalline and amorphous forms.
Structure and Properties of Aluminum Phosphate
The most common crystal structures are:
- Amorphous: It can exist in an amorphous state, lacking long-range order in its atomic arrangement.
- Tridymite-like: At lower temperatures, AlPO4 crystallizes in a tridymite-like structure, analogous to the low-temperature form of silica.
- Cristobalite-like: As the temperature increases, AlPO4 undergoes a phase transformation to a cristobalite-like structure, similar to the high-temperature form of silica.
Its crystals possess several inherent physicochemical properties that are crucial for their applications:
- Thermal Stability: It exhibits excellent thermal stability, with some amorphous compositions being metastable up to 1400°C. This property makes it suitable for high-temperature applications, such as refractory materials and thermal barrier coatings.
- Chemical Resistance: It is resistant to various chemicals, including acids and bases, making it suitable for applications in corrosive environments.
- Surface Properties: The surface properties, such as surface area and porosity, can be tailored by controlling the synthesis conditions and crystal structure. Amorphous aluminum phosphate gels can exhibit high surface areas (250-350 m²/g) and pore volumes (0.4-1.8 cc/g), making them suitable for catalysis and adsorption applications.
- Optical Properties: It has a wide bandgap of around 5.41 eV, making it a potential candidate for optoelectronic applications.
- Ion Exchange Capacity: The layered structure of certain phases, such as the berlinite-like structure, allows for ion exchange properties, enabling applications in water treatment and catalysis.
Benefits of Aluminum Phosphate
Improved Synthesis Methods
- Temperature-controlled precipitation: Settling it precipitate at 50-70°C improves product quality.
- Closed-system synthesis: Producing it within a closed system enhances process control and safety.
- Novel chemical routes: Environmentally-friendly synthesis by reacting it with alkaline metal aluminate solutions, generating nano-sized amorphous particles without co-products.
Enhanced Material Properties
- Amorphous structure: Amorphous aluminum phosphate exhibits a bulk density <2.30 g/cm³ and P/Al mole ratio >0.8, suitable for pigment applications.
- Thermal stability: Its coatings demonstrate high thermal durability and stability for substrate protection at elevated temperatures.
- Optical properties: Nano-sized amorphous particles exhibit homogeneous size and unique light backscattering properties.
Improved Functionality
- Protein adsorption: Its compounds with potassium/magnesium ions show excellent protein adsorption, useful for protein purification.
- Antioxidant properties: Temperature-sensitive solutions (3Al₂O₃/P₂O₅ = 1.2-1.5, M₂O/P₂O₅ = 0.02-0.15) are effective antioxidants for carbon materials.
- Inhibition of graphite oxidation: Aluminum phosphates (Al₂O₃·P₂O₅, Al₂O₃·3P₂O₅, Al₂O₃·9P₂O₅) increase the onset temperature for graphite oxidation by 75-100°C in pure oxygen.
Applications of Aluminum Phosphate
Coatings and Surface Treatments
It can be used to form amorphous, thermally stable coatings for protecting substrates from corrosion, oxidation, and harsh environments at elevated temperatures. These coatings exhibit high durability and transparency compared to silica or vitreous enamel coatings. Its coatings find applications in industrial and consumer products.
Pigments and Paints
Amorphous aluminum phosphate or polyphosphate particles with a high phosphorus to aluminum ratio (>0.8) and low bulk density (<2.3 g/cm³) can be used as pigments in paints. They serve as a substitute for titanium dioxide, providing opacity and hiding power.
Protein Purification and Adsorption
Its compounds exhibit excellent protein adsorption properties, making them useful as carriers for protein purification. Their high surface area and porous structure enable applications in adsorption and catalysis.
Vaccine Adjuvants
It is widely used as an adjuvant in various vaccines to enhance the immune response and improve efficacy. Controlling preparation conditions like pH, mixing speed, and reaction time can improve particle size uniformity for better adjuvant performance.
Adhesives and Cements
Condensed compositions find applications in inorganic adhesives, coatings, building materials, ceramics, and molding materials due to their binding and solidification properties.
Thermal Protection
Its coatings can protect high-silica reinforced fibers and fabrics from high-temperature environments, improving their mechanical strength and durability.
Application Cases
| Product/Project | Technical Outcomes | Application Scenarios |
|---|---|---|
| Aluminum Phosphate Coatings | Forming amorphous, thermally stable coatings that protect substrates from corrosion, oxidation, and harsh environments at elevated temperatures. These coatings exhibit high durability and transparency compared to silica or vitreous enamel coatings. | Industrial and consumer products requiring surface protection against corrosion and high temperatures. |
| Aluminum Polyphosphate Pigments | Providing opacity and hiding power as a substitute for titanium dioxide pigments in paints. Amorphous aluminum phosphate or polyphosphate particles with a high phosphorus to aluminum ratio (>0.8) and low bulk density (<2.3 g/cm³) are used. | Paints and coatings requiring opacifying pigments with high hiding power. |
| Protein Purification Media | Exhibiting excellent protein adsorption properties, making aluminum phosphate compounds useful as carriers for protein purification and separation. | Biotechnology and pharmaceutical industries for purifying and separating proteins. |
| Aluminum Phosphate Cements | Forming chemically bonded ceramics with high compressive strength, low porosity, and good acid resistance. These cements can be used for rapid repair and construction applications. | Rapid repair and construction applications requiring high compressive strength, low porosity, and acid resistance. |
| Aluminum Phosphate Catalysts | Acting as solid acid catalysts for various organic reactions, such as alkylation, isomerization, and esterification, due to their high surface area and tunable acidity. | Chemical industry for catalyzing organic reactions, particularly in the production of fine chemicals and petrochemicals. |
Latest Technical Innovations of Aluminum Phosphate
Synthesis and Preparation Methods
- Improved precipitation methods for producing it with increased particle size uniformity, achieved by controlling factors like the phosphate-to-aluminum ratio, mixing speed, stirring speed, reaction duration, and pH 7.
- Novel environmentally-friendly synthesis routes for amorphous alkaline metal-aluminum phosphates with nano-sized primary particles, involving mixing an acidic solution with a basic alkaline metal aluminate solution.
- Low-temperature and rapid production by adding an alkaline aluminum-containing solution to an acidic phosphate-containing solution with pH < 1.49, adjusting the final pH to 1.49-2.49.
Structural and Compositional Modifications
Carbon-coated lithium metal phosphates doped with 300-5000 ppm aluminum, exhibiting a BET surface area ≤ 15 m²/g, for improved low-temperature electrochemical performance as cathode materials in lithium-ion batteries.
Its thin films with a P/Al ratio of 1/2 evaluated as gate dielectrics for thin film transistors.
Amorphous aluminum phosphate gels with a P/Al atomic ratio of ~1, high pore volume (0.4-1.8 cc/g), and surface areas of 250-350 m²/g, suitable for catalyst support and adsorbent applications.
Novel Applications
- It coatings for sealing porous substrate surfaces, providing thermal durability and stability for protecting substrates from corrosion or oxidation at elevated temperatures.
- As a sealant for plasma-sprayed lamellar chromium oxide coatings.
- Lithium phosphates (Li3PO4, LiPON) as solid electrolytes in lithium-ion batteries, CO2 gas sensors, and optical humidity sensors, with potential for 3D structuring to increase energy storage capacity
Technical Challenges of Aluminum Phosphate
| Synthesis of Aluminum Phosphate with Uniform Particle Size | Developing improved precipitation methods for producing aluminum phosphate with increased particle size uniformity, achieved by controlling factors like the phosphate-to-aluminum ratio, mixing speed, stirring speed, reaction duration, and pH. |
| Novel Environmentally-Friendly Synthesis of Amorphous Nano-sized Aluminum Phosphates | Developing novel environmentally-friendly synthesis routes for amorphous alkaline metal-aluminum phosphates with nano-sized primary particles, involving mixing an acidic aluminum phosphate solution with a basic alkaline metal aluminate solution. |
| Low-Temperature and Rapid Production of Aluminum Phosphate | Enabling low-temperature and rapid production of aluminum phosphate by adding an alkaline aluminum-containing solution to an acidic phosphate-containing solution with pH < 1.49, adjusting the final pH to 1.49-2.49. |
| Carbon-Coated Aluminum-Doped Lithium Metal Phosphates for Improved Low-Temperature Performance | Developing carbon-coated lithium metal phosphates doped with 300-5000 ppm aluminum, exhibiting a BET surface area ≤ 15 m²/g, for improved low-temperature electrochemical performance as cathode materials in lithium-ion batteries. |
| Aluminum Phosphate Thin Films as Gate Dielectrics | Evaluating aluminum phosphate thin films with a P/Al ratio of 1/2 as gate dielectrics for thin film transistors. |
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