Ammonium Aluminum Carbonate Hydroxide Research Articles

PH-dependent formation of AACH fibers with tunable diameters and their in situ transformation to alumina nanocrystals with mesoporous structure

Through a simple hydrothermal and thermal decomposition process of the ammonium aluminum carbonate hydroxide (denoted as AACH) precursors, we prepared mesoporous γ-Al2O3 fibers with tunable diameters by manipulating the amount of urea (pH-adjuster) in the reaction mixture (i.e., pH control). The experimental results show that the diameter of the obtained AACH fibers decreased obviously when the pH values of the reaction mixtures changed from 2.5 to 5.0. The as-formed precursors could transform to the morphology-remained cubic γ-Al2O3 with a slight shrinkage in diameter of the fibers after an annealing process. N2 adsorption–desorption experiment indicates that the as-synthesized alumina microfibers and nanofibers have large surface area (176 and 230m2/g, respectively) and narrow pore-size distributions. Both the γ-Al2O3 microfibers and nanofibers are powerful in the removal of Congo red pollutant from waste water.

Preparation of BaMgAl10O17:Eu2+ phosphor with small particle size by co-precipitation method

A BaMgAl10O17:Eu2+ phosphor with fine particle size was synthesized by a chemical co-precipitation method. Co-precipitation of Ba2+, Al3+, Mg2+, and Eu2+ was achieved under precisely controlled precipitation conditions. The Al3+ ions were precipitated as ammonium aluminum carbonate hydroxide (AACH). Phase transition and particle growth processes of the precipitate mixture during calcining process were investigated, and the formation mechanism of the BaMgAl10O17 phase was discussed. The results show that the formation temperature of the BaMgAl10O17 phase in the co-precipitation mixture was significantly lower than that of a high temperature solid state reaction method. The BaMgAl10O17 phase was formed from the reaction between the BaAl2O4 phase and γ-Al2O3 phase; no α-Al2O3 phase appeared during the entire process. Well-dispersed BaMgAl10O17:Eu2+ phosphor powder in the form of hexagonal flakes with small particle size (1–2μm) could be prepared by this method.

The Effect of Different Solvent on the Microstructure of Gibbsite during Hydrothermal Treatment

Phase structure and micromorphology of gibbsite after hydrothermal treated in different solvents were studied. Results showed that there was a relationship between micromorphology and phase structure of as-synthesized product. The cubic slice-like boehmite microstructure can be obtained in water. The 3D flowerlike boehmite microstructure can be obtained in isopropanol or sodium carbonate solution. The global ammonium aluminum carbonate hydroxide (AACH) architectures assembled by the lathes can be obtained in urea solvent. Gibbsite can transform to boehmite after 12h hydrothermal treatment in water. In isopropanol or sodium carbonate solution, this time decreased to 6h. In urea solution, Aluminum hydroxide turned from gibbsite to the mixture of boehmite, gibbsite and AACH. Finally, AACH is synthesized. This procedure needs 24h.

Synthesis of Gallium-Aluminum Dawsonites and their Crystal Structures

Solid solutions of dawsonites, ammonium aluminum carbonate hydroxide–ammonium gallium carbonate hydroxide, were obtained by the addition of aqueous solutions of mixtures of aluminum and gallium nitrates to an excess ammonium carbonate solution. The dawsonite solid solutions were calcined at 700°C to give γ-Ga2O3–Al2O3 solid solutions. The structures of these two types of solid solution were discussed on the basis of their X-ray diffraction patterns and X-ray absorption fine structure spectra.

Preparation of micrometer-sized α-Al 2O 3 platelets by thermal decomposition of AACH

Micrometer-sized α-Al 2O 3 platelets with hexagonal shape were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) using AlF 3 as an additive. The precursor and the calcined product were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and differential scanning calorimetry. The α-Al 2O 3 platelets with the size of 2–3 μm were obtained by calcining AACH at 1200 °C with 5 wt.% AlF 3. The morphology modification is attributed to the various growth rates along different crystal orientations due to the adsorption and uneven distribution of AlF 3. A step growth mode is responsible for the formation of the platelets.

PEG-directed hydrothermal synthesis of alumina nanorods with mesoporous structure via AACH nanorod precursors

Al2O3 nanorods with mesoporous structures are successfully synthesized from a hydrothermal and thermal decomposition process via the ammonium aluminum carbonate hydroxide (denoted as AACH) precursors. TEM images show that the average diameter of Al2O3 nanorods is about 60 nm, and the length is around 1–2 μm. The experimental results show that well-crystallized mesopores with hierarchically distributed pore sizes are embedded in the Al2O3 nanorods. The N2 adsorption–desorption experiment indicates that the as-synthesized alumina nanorods have large surface area (ca. 176 m2/g) and narrow pore-size distributions. At the same time, the as-prepared Al2O3 nanorods exhibit strong photoluminescent properties at room temperature. A plausible surfactant-induced nanorod formation mechanism using the poly ethylene glycols as the template agent for the nanorod assembly is also proposed.

Synthesis and sintering of pre-mullite powders obtained via carbonate precipitation

Ultrafine pre-mullite powders, which yield mullite at high temperatures, have been prepared from colloidal silica and aluminium nitrate via carbonate coprecipitation and followed by calcination. The chemical and structural evolutions of the as-prepared precipitation powder during thermal treatment were studied and the sinterability of pre-mullite powders were investigated. The as-prepared powders are comprised of ammonium aluminum carbonate hydroxide and amorphous silica, which convert to mullite via the Al–Si spinel phase at 1250 °C. Calcination of the as-prepared powders at 1000 °C gives a very active powder which can be reactively sintered to 98.2% theoretical density at 1550 °C. The sintered body possesses a relatively uniform chemical composition with Al 2O 3/SiO 2 mole ratio of 1.48 and exhibits a very fine interlocking equiaxed and polygonal grain morphology with grain size of 100–200 nm.

Synthesis of Ga2O3–Al2O3 Catalysts by a Coprecipitation Method for CH4-SCR of NO

Synthesis of the γ-Ga2O3–Al2O3 solid solutions by the coprecipitation method was examined. Precipitates were obtained by adding at once an aqueous solution of aluminum and gallium nitrates to an excess base solution (reverse strike precipitation). The precipitates were calcined at 700 °C to give γ-Ga2O3–Al2O3 solid solutions. When an ammonium carbonate solution was used as the precipitant, ammonium aluminum carbonate hydroxide (ammonium dawsonite, AACH)–ammonium gallium carbonate hydroxide (AGCH) solid solutions were obtained. Calcination of the AACH–AGCH solid solutions at 700 °C gave the γ-Ga2O3–Al2O3 solid solutions which exhibited high activity for selective catalytic reduction of NO with methane.

PEG-directed hydrothermal synthesis of multilayered alumina microfibers with mesoporous structures

Novel superstructures of multilayered Al 2O 3 microfibers with mesoporous structures are successfully synthesized from a hydrothermal preparation and thermal decomposition of a layered precursor of ammonium aluminum carbonate hydroxide (denoted as AACH). Nanolayer-based AACH microfibers are self-assembled by the hydrothermal process by using aluminum nitrate nonahydrate and urea as the starting materials. The corresponding multilayered Al 2O 3 microfibers with mesoporous structures are obtained by the thermal decomposition AACH precursor at 1173 K. SEM images show that the average diameter of Al 2O 3 microfibers is about 300–500 nm, and the length is around 5–10 μm. Studies also show that well-crystallized pores are embedded in the multilayer of the Al 2O 3 microfibers. The N 2 adsorption–desorption experiment show that the as-synthesized alumina microfibers have large surface area (ca. 358 m 2/g) and narrow pore-size distributions. A plausible layer-by-layer self-assembly mechanism using in situ formed AACH microfibers as the precursor for this multilayered alumina microfibers is also proposed.

Preparation of ultrafine α-Al2O3 using precipitation-azeotropic distillation method

Ammonium aluminum carbonate hydroxide (AACH) was prepared by a precipitation-azeotropic distillation method, which uses aluminum sulfate as the Al source and ammonium carbonate as the precipitant. Then, AACH was calcined into ultrafine α-Al2O3 powder. The factors that influence the dispersion property of ultrafine α-Al2O3 powder are discussed in this paper, such as the methods of adding materials, surfactant, and drying methods. The changes of the structure and property of ultrafine alumina in the thermal treatment process are also studied. The morphological structure and properties of AACH are characterized by DTA/TGA, SEM, XRD, and ICP measurements. The results show that ultrafine α-Al2O3 powder with a uniform particle size and well-distributed property can be synthesized only after aluminum sulfate atomizes into ammonium carbonate, proper amount of PEG1000 is added as the dispersant, and the product is treated by azeotropic distillation. The phase transformation of alumina during the calcination process can be described as amorphous Al2O3→γ-Al2O3→ϑ-Al2O3→α-Al2O3. The crystal grain size and density of ultrafine alumina powder increase with the increase of the calcination temperature. After AACH has been calcined at 1200°C for 2 h, the ultrafine α-Al2O3 with uniform particle size, spherical shape, and more than 99.97% purity is obtained and its powder is well dispersed.

Study on the morphology of α-Al2O3 precursor prepared by precipitation method

Abstract Three typical types of precursors with obviously different morphology were synthesized when preparing α-Al2O3 powder from aluminum nitrate and ammonium acid carbonate solution using a precipitation method. The phases of the precursor samples were attributed to be an amorphous structure, ammonium aluminum carbonate hydroxide (AACH) and an undetermined phase. Two kinds of phase transition phenomenon were found among these three phases. The α-Al2O3 powder samples with different morphology and particle size were produced after baking the three types of precursors at 1200 °C for 1 h. The acicular α-Al2O3 with particle length of 1–2 μm and diameter of 100–200 nm was obtained by baking the AACH precipitate as precursor.

Effect of Magnesium Addition on the Phase Transformation of α-Alumina Prepared from Route of Ammonium Aluminum Carbonate Hydroxide

Effect of magnesium addition on rapid transformation of α-alumina prepared from route of ammonium aluminum carbonate hydroxide during thermal heating and microwave radiation heating was investigated. The phase transformation and the final particle size of the transient alumina composite powder were significantly affected by amount of magnesium added in the aluminum precursor during the microwave radiation heating in various environmental atmospheres. Rapid transformation from γ- to α-phase was found in the magnesium added transient alumina by microwave-assisted transformation and nano-sized α-alumina was obtained. When the 3 wt% magnesium added ammonium aluminum carbonate hydroxide was heated by microwave radiation under nitrogen atmosphere, the transformation temperature from γ- to α-alumina was considerably lowered to 1000°C and the average particle size of 27.6 nm was attained for the α-alumina-spinel composite powder.

Thermal Decomposition Kinetics of Ammonium Aluminum Carbonate Hydroxide

The thermal decomposition of ammonium aluminum carbonate hydroxide was studied under non-isothermal conditions in air. The decomposition kinetics were evaluated from data of TG-DTA by means of the Kissinger equation and the Coats-Redfern equation. The values of the activation energy E, the preexponential factor A and the algebraic expression of integral G(α) functions of the thermal decomposition were calculated. The ammonium aluminum carbonate hydroxide (AACH) was characterized by X-ray diffraction, differential thermal analysis and thermogravimetric and field emission scanning electron microscopy.

Phase Transition Activity Characteristics of Nanosized AlOOH in the Hydrothermal Synthesis of Nanosized α-Al<sub>2</sub>O<sub>3</sub>

α-Al2O3 nanopowders were prepared by a novel synthesis process, using the nanosized α-Al2O3 obtained from pyrolyzing ammonium aluminum carbonate hydroxide as seeds and the self-dispersed nanosized AlOOH crystal powders as precursors. Based on their good self-dispersion in water, the α-Al2O3 seeds were dispersed evenly into the AlOOH sol by the new homodispersion mixing technique. This process enables the conversion of AlOOH to alumina at 190°C (hydrothermal temperature), in which the alumina is calcined to nanosized alpha-alumina having an average length to diameter ratio of 60nm:15nm at 930°C. In the synthesis reaction for transforming the AlOOH to alumina, the effect of superfine pulverization and self-dispersion of the precursors was studied.

Preparation of High-Purity Ultrafine α-Al<sub>2</sub>O<sub>3</sub> by Solid-State Reaction at Room Temperature

Ammonium aluminium carbonate hydroxide (AACH), with a small quantity of γ-AlOOH, was synthesized through solid-state reaction at room temperature using AlCl3·6H2O and NH4HCO3 as raw materials and polyethylene glycol (PEG-10000) as the dispersant. After calcined at 1100°C for 1.5h, α-Al2O3 powders with primary particle sizes of 20~30nm were obtained. The crystal phase, particle size and morphology of the high-purity ultrafine α-Al2O3 were characterized. The results showed that a small quantity of γ-AlOOH in the AACH decomposed and formed crystal seeds. The presence of crystal seeds reduced the nucleation activation energy and therefore reduced the phase transformation temperature.

Nanocrystalline Particle Coatings on α-Alumina Powders by a Carbonate Precipitation and Thermal-Assisted Combustion Route

We have suggested ultrafine particle coating processes for preparing nanocrystalline particle coated alpha-alumina powders by a carbonate precipitation and thermal-assisted combustion route, which is environmentally friendly. The nanometric ammonium aluminum carbonate hydroxide (AACH) as a precursor for coating of alumina was produced from precipitation reaction of ammonium aluminum sulfate and ammonium hydrogen carbonate. The synthetic crystalline size and morphology were greatly dependent on pH and temperature. By adding ammonium aluminum sulfate solution dispersed the alpha-alumina core particle in the ammonium hydrogen carbonate aqueous solution, nanometric AACH with a size of 5 nm was tightly bonded and uniformly coated on the core powder due to formation of surface complexes by the adsorption of carbonates, hydroxyl and ammonia groups on the surface of aluminum oxide. The synthetic precursor rapidly converted to amorphous- and y-alumina phase without significant change in the morphological features through decomposition of surface complexes and thermal-assisted phase transformation. As a result, the nanocrystalline polymorphic particle coated alpha-alumina core powders with highly uniform distribution were prepared from the route of carbonate precipitation and thermal-assisted combustion.

Artificial neural network analysis of preparation of nano α-Al2O3 powders by thermal decomposition of ammonium aluminium carbonate hydroxide

Nano α-Al2O3 powders were prepared by thermal decomposition of ammonium aluminium carbonate hydroxide (AACH). The effects of α-Al2O3 crystal seeds, reactant concentration, content of active agent (PEG), annealing temperature and ZnF2 addition on the preparation of nano α-Al2O3 powder were studied. The results showed that nano α-Al2O3 powder with apparent crystallite size ∼40 nm could be prepared, and the complete transformation temperature of α-Al2O3 could be reduced from 1250 to 1050°C as α-Al2O3 seeds and ZnF2 were added. The prepared nano α-Al2O3 powders were characterised by X-ray diffraction (XRD), differential thermal analysis and thermogravimetric (TG-DTA) and scanning electron microscopy (SEM). A back propagation (BP) artificial neural network (ANN) has been used to establish models between the relative content of the nano α-Al2O3 powders and the preparation parameters. The results of checking experiments were in accordance with the calculated values.

Reforming Dawsonite by Memory Effect of AACH-Derived Aluminas

Ammonium aluminum carbonate hydroxide (AACH), a material isostructural with the mineral dawsonite (NaAl(OH)2CO3), leads to finely dispersed and high-surface area Al2O3 upon thermal decomposition. This family of compounds exhibit memory effect, i.e., the dawsonite structure can be recovered upon treatment of the derived aluminas in (NH4)2CO3 solutions at 323 K. Treatment of the oxides in water or NH4OH, NH4Cl, and Na2CO3 solutions led to aluminum (oxi)hydroxides. The structural, morphological, and porous properties of the materials were investigated by ICP-OES, elemental analysis, XRD, FTIR and Raman spectroscopies, He pycnometry, TEM, gas (N2 and Ar) adsorption, and thermal analysis (TGA and DSC). The reconstruction of dawsonite in ammonium carbonate was complete for the solids calcined at 523 and 723 K, consisting of a highly amorphous and low-skeleton-density alumina phase with well-developed porosity. The recrystallization was incomplete upon formation of larger γ-Al2O3 crystals by calcination at 1073 K, and no reconstruction occurred when α-Al2O3 was obtained by calcination at 1473 K. Characterization of the reconstructed samples indicates the attainment of NH4-dawsonite with higher purity than in the parent material. This is due to the presence of amorphous Al-containing phase(s) in the as-synthesized sample coexisting with AACH. All the phases were selectively converted into dawsonite by thermal decomposition and (NH4)2CO3 treatment. The reformed samples consist of relatively large crystals with newly developed microporosity as compared to the parent material. These results extend the unique memory property of oxides derived from particular layered materials such as hydrotalcites to other families of mineral-like compounds.

Rapid Transformation of α-Alumina Nanocomposite Powders by Microwave Assisted Combustion

The microwave-assisted combustion synthesis as a route to obtain ultrafine α-alumina and magnesium aluminate composite powders starting from the ammonium aluminum carbonate hydroxide precursor was investigated. The synthetic temperature and the crystallite size of the α- alumina nanocomposite powder were significantly affected by the environmental atmosphere in the microwave assisted combustion. The α-alumina and spinel composite powder was obtained by the microwave-assisted combustion at temperature of 1000°C under H2/N2, but at 1150°C by the normal heat process under air. The rapid transformation from γ- to α-phase was achieved by microwave assisted nucleation at low temperature of 900°C under H2/N2 atmosphere. The least crystallite size of 26.2 nm was obtained under H2/N2 atmosphere at 1000°C for 10 min.

Fabrication and synthesis of α-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH)

An α-Al2O3 nanopowder was prepared via the thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). The AACH precursor, which had an average crystallite size of 5–8nm, was fabricated at a reaction temperature of 8°C with the pH of the aqueous ammonium hydrogen carbonate (AHC) precursor solution set to 10, which afforded the highest [NH4+][AlO(OH)n(SO4)−3−n/2]][HCO2−] ionic concentration. The AACH precursor was transformed to NH4Al(SO4)2, rhombohedral (Al2(SO4)3), amorphous-, θ-, and α-Al2O3 according to the heat-treatment temperature. A time-temperature-transformation (TTT) diagram for thermal decomposition in air was determined. The critical particle size of θ-Al2O3 in the θ→α phase transformation was ∼30nm. Homogeneous, spherical α-Al2O3 nanopowders with an average particle size of 70nm were prepared by firing the precursor crystallites at 1150°C for 3h in air.