Effect of initial pH on formation of hollow calcium carbonate particles by continuous CO2 gas bubbling into CaCl2 aqueous solution

Hollow calcium carbonate (CaCO₃) particles are synthesized by continuous bubbling CO₂ gas into CaCl₂ aqueous solution. In order to study the formation kinetics of the hollow CaCO₃ particles, the transmitted light strength of the CaCl₂ solution during the reaction was measured automatically with an on-line monitoring. It was verified that the pH for the formation of the hollow particles depended on the reaction temperature. Higher initial pH of the CaCl₂ solution resulted in the higher ratio of hollow particles as compared to that obtained at lower pH.     

Assemblage of nano-calcium carbonate particles on palmitic acid template

As research continues, the control on the polymorph and morphology of calcium carbonate (CaCO₃) becomes a hot topic because its application is limited by these parameters. The assemblage of nano-CaCO₃ particles on palmitic acid template was successfully realized in the carbonation of calcium hydroxide (Ca(OH)₂) slurry. The polymorph and morphology of CaCO₃ particles were investigated by XRD and TEM, respectively, and the interaction between palmitic acid and CaCO₃ particles was studied by FTIR. It reveals that the amount of palmitic acid does not change the crystalline phase of CaCO₃ particles. However, the morphology of CaCO₃, from nano-cubic single particle to micro rod-like aggregation, can be controlled by varying the amount of palmitic acid. The assemblage of CaCO₃ particles is discussed from the view of the deformation of the micelle when it is absorbed on the Ca(OH)₂ particles and of the aggregation of extremely small CaCO₃ particles under the driving force of high surface energy.     

Synthesis and photocatalytic oxidation properties of titania hollow spheres

The hollow spheres of anatase TiO₂ with higher photocatalytic activity have been fabricated by spherical CaCO₃ nanoparticles as a template, and titanium sulfate (Ti(SO₄)₂) as a precursor, and the CaCO₃ templates were dissolved subsequently in dilute HNO₃ solution. The TiO₂ hollow spheres samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and N₂ adsorption-desorption isotherms. The characterization results indicate that as prepared TiO₂ hollow spheres sample was transformed to anatase phase in calcined at 400 °C, and the anatase TiO₂ hollow spheres have a higher specific surface area and show much better photocatalytic activity than commercial P25 in the photodegradation of Rhodamine B under the UV irradiation.     

Size-controllable synthesis of calcium carbonate nanoparticles using aqueous foam films as templates

A new method for the synthesis of calcium carbonate nanoparticles using two types of foam which are separately stabilized by surfactants with opposite change has been described here. In detail, one type of foam is formed by the aqueous solution of CaCl₂ which contains the anionic surfactant alkyl polyoxyethylene alcohol sulfate sodium (AES) and the other is formed by the aqueous solution of Na₂CO₃ and the cationic surfactant alkyl polyoxyethylene quaternary ammonium chloride (AEAC). Two types of foam contact each other in a specially designed apparatus after draining completely, then Ca²⁺ and CO₃²⁻ entrapped by the surfactant layers at the thin borders between the foam bubbles react and result in the generation of CaCO₃ nanoparticles. TEM determination showed that perfect mono-dispersed spherical nanoparticles were obtained and the size of particles can be conveniently controlled by changing the concentration of CaCl₂ and Na₂CO₃. Also, the mechanism leading to the synthesis of CaCO₃ nanoparticles was discussed in detail.     

Synthesis of ultra-small hollow silica nanoparticles using the prepared amorphous calcium carbonate in one-pot process

The ultra-small hollow silica nanoparticles were synthesized using the prepared amorphous calcium carbonate (ACC) particles as a template. The ACC particles were firstly prepared by carbonation method, which procedure was conducted in the methanol solvent to form the Ca(OCH₃)₂ layers on the ACC particles. An effect of methanol concentration on the morphology of ACC particles was also investigated. The prepared ACC particles were directly coated by silica through adding tetraethoxysilane (TEOS) into the methanol solvent. Hence, the ACC-silica core-shell particles were obtained since the ACC particles have a positive charge and interact with hydrolyzed TEOS. The ACC particles could be stabilized through the reaction between methanol and calcium ions when the methanol concentration was increased over than 40?vol%.     

Dynamic behaviour of the CO<Subscript>2</Subscript> bubble in a bubble column bioreactor for microalgal cultivation

Carbon dioxide (CO₂) gas is a major carbon source for microalgal cultivation. It is usually sparged into photobioreactors in the form of bubbles. The behaviour of the bubbles significantly affects mass transfer, distribution and microalgae growth. In this study, the dynamic behaviour of the CO₂ bubbles was compared between a microalgal suspension and pure water. These investigations were carried out via visual methods. The movement and distribution of microalgae at the gas–liquid interface were observed. The effects of gas flow velocity, CO₂ concentration and capillary orifice size were analysed. The results indicated that much of the microalgal cells adsorbed onto the surface of the CO₂ bubbles in the microalgal suspension, when compared with that in pure water. This resulted in an easier detachment of the bubbles in the microalgal suspension. The growth status of the bubbles were divided into two states according to changes in the Eötvös number and the behaviour of the CO₂ bubbles as influenced by gas flow velocity: steady and unsteady state. The critical gas velocity between the two states was achieved. The CO₂ bubble rising trajectory can be divided into three main phases: the vertical acceleration phase, the transition phase, and the oscillatory rising phase. During the oscillatory rising phase, the amplitude of the bubble rising trajectory was approximately two times greater than the bubble diameter. In addition, the wavelength of the bubble rising trajectory was approximately 16–18 times the bubble diameter in the microalgal suspension. A smaller capillary orifice size and larger CO₂ concentration led to a decrease in the bubble detachment diameter, an increase in velocity and an enlargement in the zone of bubble influence in the horizontal direction. These are advantageous for CO₂ transportation. These findings are beneficial for optimizing the design and operation of microalgal photobioreactors.     

Synthesis method for silica needle-shaped nano-hollow structure

Silica needle-shaped nano-hollow structure with high specific surface area was prepared by a sol-gel approach using needle-shaped calcium carbonate (CaCO₃) nanoparticles as novel template and sodium silicate as the silica source, followed by washing in acidic solution to remove the CaCO₃ sacrificial template. The sample was characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), energy disperse spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) and BET. The results show that regular uniform silica nano-hollow tubes are obtained: the length of the hollow tubes ranges from 200 nm to 300 nm with a inside diameter about 20 nm, the thickness of the tubes is around 10 nm, the specific surface area of the sample is up to 321.4 m²/g, the average pore diameter is 63.83 Å and the total pore volume is 0.5128 cm³/g.     

Controlled synthesis of hollow calcite microspheres modulated by polyacrylic acid and sodium dodecyl sulfonate

Hollow CaCO₃ microspheres were successfully synthesized through the precipitation reaction of Na₂CO₃ with CaCl₂ in the presence of polyacrylic acid (PAA) and sodium dodecyl sulfonate (SDS) at 80 °C. The concentration of SDS is an important factor to control the synthesis of hollow CaCO₃ microspheres. X-ray diffraction confirmed that the hollow CaCO₃ microsphere consists of calcite crystals. The “pearl-necklace model” of PAA/SDS micellar aggregates serves as the spherical templates to generate hollow microspheres of CaCO₃ crystals in the precipitation system.     

Using saponified olive oil to make cost effective calcium carbonate particles superhydrophobic

The main goal of this paper is to show that superhydrophobic particles of calcium carbonate can be synthesized at low cost from harmless biocompatible natural products like baking soda (NaHCO₃) and olive oil. Calcium carbonate particles were prepared by precipitating solutions containing CO₃^2? and Ca²⁺ ions at two different temperatures 30 °C and 80 °C. These particles were transformed into hydrophobic ones using soap made from olive oil from the region of Kabylie (Algeria). The olive oil was first saponified. Adjusted amounts of this soap were added to the sodium carbonate solution before the precipitation reaction. Several techniques were used to analyze the products of the reaction. Typical polymorphs of CaCO₃ (calcite, aragonite and vaterite) were identified by quantitative analysis of X-ray powder diffraction patterns, giving their crystallite size and atomic fractions. Scanning electron microscopy was used to probe the morphology of the CaCO₃ particles. FT-IR measurements indicated the presence of aliphatic chains in the hydrophobic particles, thereby proving that the calcium carbonate particles were functionalized by soap. The particles which formed were found to be superhydrophobic with a contact angle of up to 175°. Evidence of the efficiency of these particles in oil spill recovery is given.     

Effect of calcium carbonate particle size on formation and morphology of calcium hexaboride powder synthesized from condensed boric acid-poly(vinyl alcohol) product

Calcium hexaboride (CaB₆) powder was synthesized by carbothermal reduction via the transient boron carbide (B₄C) formation starting from a condensed boric acid (H₃BO₃)-poly(vinyl alcohol) (PVA) product. The effect of the size of calcium carbonate (CaCO₃) particles on the formation behavior of CaB₆ and the obtained particle morphology was investigated in this study. CaB₆ powder was prepared using CaCO₃ powders with microsize or nanosize particles. The CaB₆ formation reaction was accelerated at lower temperature and shorter heat treatment time when using nanosize CaCO₃ particles. The complete formation of CaB₆ was achieved at 1400?°C for 3?h in an Ar flow. Furthermore, CaB₆ powder with nanosize particles was obtained. The precursor powder obtained using nanosize CaCO₃ particles transiently formed fine B₄C and calcium borate particles, which are reactive species of CaB₆, leading to the facile formation of fine CaB₆ particles.     

Characterization of gas components and deposits in bubbles in silicate glasses prepared with sodium sulphate

The gas contents and surface deposits in bubbles that were formed at various temperatures in sodium calcium silicate glasses using sodium sulphate as a starting material were investigated by means of Raman microprobe and scanning microscopic techniques. Either elemental sulphur or sodium sulphate could be readily detected in the deposits, depending on treatment conditions for the glass. Higher amounts of elemental sulphur were found as deposits on bubble surfaces for glasses prepared using both carbon and sodium sulphate. SO₂ or CO₂ could be detected as gas components in various bubbles.     

The inducing effect of lecithin liposome organic template on the nucleation and crystal growth of calcium carbonate

Lecithin liposome was employed as an organic template to control the nucleation and growth of calcium carbonate. The specific interaction of lecithin liposome and calcium ions was investigated by measuring the Zeta potential of the lecithin liposome compound with Ca²⁺. The morphology and polymorphs of the CaCO₃ synthesized in lecithin liposome suspensions of different concentrations were studied. SEM and TEM images showed that spherical particles of CaCO₃, which were synthesized under the effect of lecithin liposome, composed of nano-particles in a hierarchical structure. The polymorph of the calcium carbonate was also modified by lecithin liposome. XRD results indicated lecithin liposome can induce the polymorph of vaterite of calcium carbonate as well.     

Attrition rate of CO2 adsorbent in bubbling fluidized beds

Particle attrition induced by bubbles in a bubbling fluidized bed was investigated with CO₂ adsorbent particles (0.128 mm in diameter, 1770 kg/m³ in apparent density). The theoretical relationship between the rate of attrition by gas jets on the perforated plate distributor (R_a,j) and the rate of attrition by bubbles (R_a,b) in the bed was revealed that the rate constant of attrition by bubbles (K_a,b) was the product of the rate constant of attrition by gas jets (C_a) and dimensionless particle diameter (d_pbc). An attrition tube (0.035 m-i.d.) using the perforated-plate distributor designed for reducing the attrition by gas jets was employed as the fluidized bed, and the air as the fluidizing gas. The mode of attrition by bubbles was identified as abrasion. The rate of attrition by bubbles (R_a,b) was linearly proportional to the power given to the bed solids by bubbles. The top size of the fine particles formed by attrition (d_pm,ab) increased exponentially with an increase of bed mass and gas velocity. The effects of temperature, pressure, and area of internal surface contacting particle bed on the R_a,b and d_pm,ab were negligible under the tested condition. Empirical relationships on R_a,b and d_pm,ab were proposed based on the experimental data. When both jet and bubble attrition were significant, there existed the static bed heights that gave respectively the minimum attrition rate and the minimum of the top size of fine particles formed by attrition. Each optimal static bed heights increased with an increase of the orifice jet velocity of the perforated plate distributor.     

Preparation of CaO as OLED getter material through control of crystal growth of CaCO3 by block copolymers in aqueous solution

As the starting materials of organic light-emitting diode (OLED) getter, calcium carbonate (CaCO₃) particles with various shapes and crystal structures have been successfully prepared with additives (L64 or PEGPG), which contain blocks of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO). These CaCO₃ particles were calcinated into highly crystalline calcium oxide (CaO) nanoparticles with high capacity of water adsorption up to 14.23 wt.%. The CaCO₃ and CaO particles prepared at various conditions were characterized using the field emission scanning electron microscopy (FE-SEM), Fourier transform infrared microscopy (FT-IR), X-ray powder diffraction (XRD), and dynamic vapor sorption (DVS) method.     

Using sucrose to prepare submicrometric CaCO3 vaterite particles stable in natural rubber

Vaterite with submicrometer particles was successfully synthesized by adding sucrose at certain amounts (x = 0–50 %) to the aqueous solutions of ammonium carbonate ((NH₄)₂CO₃) and calcium chloride (CaCl₂) precursors prior to the precipitation. Typical polymorphic phases and morphological shapes of CaCO₃ particles were characterized by XRD, FTIR, Raman spectroscopy, SEM, and TEM techniques. It is evident that when the sucrose concentration is increased, the average particle size of CaCO₃ is decreased which was reduced from 0.81 µm at x = 0 % to 0.42 µm at x = 50 %. Simultaneously, the % quantitative of the vaterite phase was increased from 92.52 % at x = 0 % to 99.10 % at x = 50 %. In addition, the regular round shape of the vaterite particles was evidenced by SEM and TEM images. Generally, commercial CaCO₃ which is a calcite polymorph with cubic form was used as an extender without reinforcing efficiency in rubber material. It is therefore challenging to incorporate the obtained round shape vaterite in natural rubber (NR) latex and it is interesting to find a stable vaterite CaCO₃ polymorph in the NR, evidenced by XRD and FTIR results. The negative charges on the surface of NR particles were proposed to play an important role to stabilize the vaterite particles.     

Synthesis and characterization of cellulose acetate–calcium carbonate hybrid nanocomposite

A hybrid nanocomposite composed of calcium carbonate (CaCO₃) and cellulose acetate (CA) was fabricated by bubbling CO₂ gas into the mixture of CA and Ca(OH)₂ solution. Cellulose acetate–calcium carbonate (CA–CC) nanocomposite was characterized by spectral, thermal and optical methods. FTIR and XRD analysis confirmed the formation of the hybrid nanocomposite and XRD confirmed the formation of CaCO₃ with calcite polymorph. Thermal analysis showed CA–CC nanocomposite has better thermal stability than pristine CA. The CaCO₃ nanoparticles were in sphere shape with 100–1000 nm diameter.     

Fabrication of porous low crystalline calcite block by carbonation of calcium hydroxide compact

Calcium carbonate (CaCO₃) has been widely used as a bone substitute material because of its excellent tissue response and good resorbability. In this experimental study, we propose a new method obtaining porous CaCO₃ monolith for an artificial bone substitute. In the method, calcium hydroxide compacts were exposed to carbon dioxide saturated with water vapor at room temperature. Carbonation completed within 3 days and calcite was the only product. The mechanical strength of CaCO₃ monolith increased with carbonation period and molding pressure. Development of mechanical strength proceeded through two steps; the first rapid increase by bonding with calcite layer formed at the surface of calcium hydroxide particles and the latter increase by the full conversion of calcium hydroxide to calcite. The latter process was thought to be controlled by the diffusion of CO₂ through micropores in the surface calcite layer. Porosity of calcite blocks thus prepared had 36.8–48.1% depending on molding pressure between 1 MPa and 5 MPa. We concluded that the present method may be useful for the preparation of bone substitutes or the preparation of source material for bone substitutes since this method succeeded in fabricating a low-crystalline, and thus a highly reactive, porous calcite block.     

Effect of a new functional double-hydrophilic block copolymer PAAL on the morphology of calcium carbonate particles

Calcium carbonate (CaCO₃) particles with various shapes were prepared by the reaction of sodium carbonate with calcium chloride in the presence of a new functional double-hydrophilic block copolymer poly (acrylic acid)-block-(acrylic hydroxy lactide) (PAAL) at room temperature. The as-prepared products were characterized with scanning electron microscopy and X-ray diffraction. The effects of pH, concentration of PAAL and CaCO₃ on the crystal form and morphologies of the as-prepared CaCO₃ were investigated. The results show that pH, concentration of PAAL and CaCO₃ are important parameters for the control of morphologies of CaCO₃. Depending on the experimental conditions, various morphologies of CaCO₃, such as plate-like aggregates, poly-nucleated spheres, ellipsoids, monodispersed spheres, rhombohedras, etc., can be obtained. Especially, the optimal experimental conditions for the production of monodispersed spherical CaCO₃ particles were determined.     

Controlled synthesis of calcium carbonate nanocrystals with multi-morphologies in different bicontinuous microemulsions

The nucleation and growth of calcium carbonate nanocrystals were studied in two types of bicontinuous microemulsions, consisted of P-octyl polyethylene glycol phenylether (OP)/n-amyl alcohol/cyclohexane/water, and the above microemulsion containing dl-aspartic acid (dl-Asp). The produced CaCO₃ nanocrystals were characterized by transmission electron microscopy (TEM), Fourier transform infrared spectrometer (FT-IR) and X-ray diffraction (XRD). The results indicated that OP and dl-Asp used as soft template could control synthesis of CaCO₃ nanocrystals well. The various shapes of CaCO₃ nanocrystals, such as solid sphere, network, whisker, rod, and hollow sphere were successfully prepared by altering the concentration of the reactants, adding dl-Asp, and adjusting the pH values of dl-Asp/CaCl₂ aqueous solution. Sole calcite phase was obtained in OP bicontinuous microemulsion. The presence of dl-Asp was helpful for the formation of thermodynamically unstable vaterite phase. In OP/dl-Asp bicontinuous microemulsion, the higher pH value of dl-Asp/CaCl₂ aqueous solution is, the more vaterite will be formed. A nucleation-limited growth and limited aggregation (NGA) model was used to explain growth mechanism of the network-like calcium carbonate in this study.     

Preparation and characterization of hollow hydroxyapatite microspheres by spray drying method

Hollow hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) microspheres were prepared using a simple spray drying method. The incorporation of ammonium bicarbonate could produce carbon dioxide and ammonia gas bubbles during the spraying, and thus created a hollow inner structure in the resultant microspheres. The hollow microspheres prepared using different amounts of ammonium bicarbonate were also characterized. These microspheres were composed of nanoparticles with an average crystallite size of 15 nm. A high surface area (80 m²/g) and porosity of the microspheres could be achieved when the concentration of ammonium bicarbonate was about 5 wt.%. Fourier transform infrared results showed that CO₃^2? was incorporated into the HA microspheres. These hollow microspheres have many potential uses such as injectable drug-delivery carriers.