Synthesis of hollow structural hydroxyapatite with different morphologies using calcium carbonate as hard template

Hydroxyapatite (HAp) with hollow structure was successfully synthesized by hydrothermal process of as-prepared calcium carbonate used as a hard template and calcium sources in a diammonium phosphate solution. Calcium carbonate was fabricated by precipitation, which possessed different morphologies such as balls, rods and blocks through regulating the amount of citric acid. The synthesized powders were characterized by X-ray diffraction (XRD), Fourier transform infrared spectrograph (FT-IR), field-emission scanning electron microscope (FESEM) and high-resolution transmission electron microscopy (HRTEM) and nitrogen adsorption–desorption. Results indicated that different morphologies calcium carbonate could convert to hollow structural HAp with the higher BET surface area and the mesopores. Hydrothermal temperature and hydrothermal time play a slight role on transition percentage. As hydrothermal temperature and hydrothermal time increased, the conversion rate of calcium carbonate to hydroxyapatite increased. The possible formation mechanism of hydroxyapatite was preliminarily investigated. The resultants of HAp are interesting materials for drug delivery and sustained-release.     

Synthesis and characterization of high purity hydroxyapatite nanorods by hydrothermal technique

High purity hydroxyapatite (HAp) nanorods were synthesized by hydrothermal technique using Ca(NO3)2 x 4H2O and (NH4)2HPO4 as starting materials in a hydrothermal reactor at 150-200 degrees C for 12-24 h with pH6 and pH9.5, respectively. The prepared HAp nanorods were characterized by XRD, FTIR, and TEM techniques. The XRD results confirmed the formation of pure phase of HAp at pH9.5. With increasing temperature and time, the crystallinity of the HAp was increased, showing the hexagonal structure of HAp with the lattice parameter a in a range of 1.144-1.148 nm and c of 0.723-0.724 nm. The crystalline sizes of the powders were found to be 44-85 nm as evaluated by the XRD line broadening technique. The chemical compositions of the HAp nanorods were characterized by FTIR spectroscopies. The peaks of the phosphate carbonate and hydroxyl vibration modes were observed in the FTIR spectra for all the samples. The morphology of the HAp was nanorods of diameter less than 100 nm, as revealed by TEM. Increasing the temperature and time resulted in the transition from polycrystalline to single crystalline phase of the HAp, as clearly confirmed by the analysis of TEM diffraction patterns.     

Biomimetic synthesis of hydroxyapatite/bacterial cellulose nanocomposites for biomedical applications

Hydroxyapatite (HAp) and bacterial cellulose (BC) are both excellent materials for use in biomaterial areas. The former has outstanding osteoconductivity and bioactivity and the latter is a high-strength nano-fibrous and extensively used biomaterial. In this work, the HAp/BC nanocomposites with a 3-dimensional (3-D) network were synthesized via a biological route by soaking both phosphorylated and unphosphorylated BCs in 1.5 simulated body fluid (SBF). Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) were employed to characterize the HAp/BC nanocomposites. SEM observations demonstrated that HAp crystals were uniformly formed on the phosphorylated BC fibers after soaking in 1.5 SBF whereas little HAp was observed on individual unphosphorylated BC fibers. Our experimental results suggested that the unphosphorylated BC did not induce HAp growth and that phosphorylation effectively triggered HAp formation on BC. Mechanisms were proposed for the explanation of the experimental observations. XRD and FTIR results revealed that the HAp crystals formed on the phosphorylated BC fibers were carbonate-containing with nano-sized crystallites and crystallinities less than 1%. These structural features were close to those of biological apatites.     

Dielectric properties of Na<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>K<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>NbO<Subscript>3</Subscript> in orthorhombic phase

Pellets of ceramic Na_1−xK_xNbO₃ (x = 0, 0.2 and 0.5), were prepared by conventional solid-state reaction method. Prepared samples were characterized using XRD and SEM. The frequency and temperature variation of dielectric constant, loss tangent and dielectric conductivity of prepared samples were measured in the frequency range from 10 KHz-1 MHz, and in the temperature range from 50–250°C for x = 0.2 and 0.5, and between 50 and 480°C for x = 0 compositions. It was observed that the dielectric constant and loss tangent decrease, and conductivity increases with increasing frequency. Near the transition temperature the material shows anomalous behaviour for the observed properties, and the peaks of dielectric constant and loss tangent were observed shifting towards lower temperature with increasing frequency.     

Development of calcium phosphate based apatite from hen’s eggshell

Stoichiometric hydroxyapatite with Ca/P molar ratio, 1.67, was synthesized using hen’s eggshell as calcium source and phosphoric acid by precipitation method. Conventional EDTA titration and gravimetric methods were adopted to estimate the amount of calcium and phosphorous, respectively. Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD) techniques were employed to investigate the formation of the HAP phase. Thermal analysis (TG-DTA) was carried out to investigate the thermal stability of HAP powder. FT-IR spectra show the characteristic peaks for phosphate and hydroxyl groups. XRD results reveal that the major characteristic peaks of HAP appear in the region of approximately 26°, 28°, 29°, 30–35°, 39°, 46°, 49° and 50° (2θ) and also indicate that there are no occurrences of secondary phases during HAP formation. TG-DTA result depicts that the synthesized HAP was stable up to 1300°C.     

Synthesis of mesoporous structured hydroxyapatite particles using yeast cells as the template

In this study, hydroxyapatite (HAp) particles with mesoporous structure have been synthesized from calcium hydroxide and di-ammonium hydrogen phosphate using yeast cells as the template. The characterization methods such as X-ray diffraction (XRD), Fourier transform infrared spectrograph (FTIR), N₂ adsorption–desorption isotherms (NADI), transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) were used for determination of the particles structure (particle size, structural evolution and morphology). The results show that HAp particles with mesoporous structure could be produced. The size of HAp particles was approximately hundreds of nanometer. The pore width of HAp particles was in the range of 2.0–40 nm and the maximum centered around 4.5 nm.     

Hydrothermal synthesis of porous triphasic hydroxyapatite/(alpha and beta) tricalcium phosphate

A novel, porous triphasic calcium phosphate composed of nonresorbable hydroxyapatite (HAp) and resorbable tricalcium phosphate (alpha- and beta-TCP) has been synthesized hydrothermally at a relatively low temperature. The calcium phosphate precursor for hydrothermal treatment was prepared by gel method in the presence of ascorbic acid. XRD, FT-IR, Raman analyses confirmed the presence of HAp/TCP. The surface area and average pore size of the samples were found to be 28 m2/g and 20 nm, respectively. The samples were found to be bioactive in simulated body fluid (SBF).     

Preparation of thermally stable nanocrystalline hydroxyapatite by hydrothermal method

Thermally stable hydroxyapatite (HAp) was synthesized by hydrothermal method in the presence of malic acid. X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FT-IR), Raman spectroscopy, scanning electron microscopy (SEM), differential thermal analysis (DTA), thermogravimetric analysis (TGA) was done on the synthesized powders. These analyses confirmed the sample to be free from impurities and other phases of calcium phosphates, and were of rhombus morphology along with nanosized particles. IR and Raman analyses indicated the adsorption of malic acid on HAp. Thermal stability of the synthesized HAp was confirmed by DTA and TGA. The synthesized powders were thermally stable upto 1,400 degrees C and showed no phase change. The proposed method might be useful for producing thermally stable HAp which is a necessity for high temperature coating applications.     

Hydrothermal microemulsion synthesis of stoichiometric single crystal hydroxyapatite nanorods with mono-dispersion and narrow-size distribution

The stoichiometric single crystal hydroxyapatite (HAp) nanorods with mono-dispersion and narrow-size distribution in diameter were successfully synthesized via a hydrothermal microemulsion method, and the as-synthesized powders were characterized by XRD, FETEM, FTIR and ICP-AES. In the present work, a novel technique of w/o microemulsions of CTAB/n-pentanol/n-hexane/water under hydrothermal condition to synthesize single crystal HAp nanorods with diameter 25–40 nm and length 55–350 nm, was described. The homogeneity in size distribution and shape of the HAp nanorods was probably attributed to the w/o nano-reactors and the soft template of the surfactants, and the high crystallization of the products was attributed to the hydrothermal treatment. The sintering ability of the nanorods and the mechanical properties of the fabricated HAp bioceramics were also preliminarily investigated. The results showed that the fabricated HAp bioceramics possessed high mechanical properties.     

Mechano-chemical synthesis and characterization of nanostructured β-TCP powder

Tricalcium phosphates (Ca₃(PO₄)₂: β-TCP) are known for their biodegradable characteristics and are essential components of natural bone. Unlike hydroxyapatite (HAp), β-TCP is not the stable phase at room temperature. It is normally obtained by solid-state synthesis at temperatures in excess of 600 °C where calcium deficient hydroxyapatite transforms to β-TCP. Low temperature approaches for synthesizing this phase could offer unique opportunities with regards to controlling the microstructure and its degradable characteristics. In this study, the possibility of synthesizing β-TCP directly by a mechano-chemical route has been investigated. The starting materials were mechanically milled for various time periods. The resultant calcium phosphate powder has been analyzed using XRD, FTIR, DTA/TGA and SEM.     

Wet Chemical Synthesis of Hydroxyapatite Particles from Nonstoichiometric Solutions

Hydroxyapatite [Ca₁₀(PO₄)₆(OH)₂; or HAp] particles with controlled stoichiometry were precipitated from solution by forced hydrolysis of mixtures of calcium and phosphate salts at a high pH and temperature. The mixing procedure determines the final stoichiometry of the precipitated HAp. The resulting panicles were characterized by FTIR, XRD, TEM, and light scattering and their chemical composition was determined by ICP, indicating that pure crystalline HAp with a narrow size distribution was indeed formed, despite some aggregation being detected by light scattering. The Ca/P ratio was found to depend on the specific preparation route.     

Studies on conductivity and dielectric properties of polyaniline-zinc sulphide composites

In the present paper, we report electrical conductivity and dielectric studies on the composites of conducting polyaniline (PANI) with crystalline semiconducting ZnS powder, wherein PANI has been taken as inclusion and ZnS crystallites as the host matrix. From the studies, it has been observed that the value of room temperature d.c. conductivity of the composites with volume fraction of PANI > 0.65 shows an unusual behaviour wherein, conductivity values of the composites exceed that of PANI itself with maximum value as high as 6 times that of PANI at the volume fraction of 0.85. A similar trend has also been observed for the real and imaginary parts of complex dielectric constant values of the composites. This unusual behaviour in the d.c. conductivity and dielectric properties has been attributed to the enhancement in the degree of crystallinity of PANI as a consequence of its interfacial interaction with ZnS matrix. The results of optical microscopy show coating of PANI all around the ZnS particles. The temperature dependent conductivity studies suggest the quasi one-dimensional VRH conduction in PANI as well as its composites with ZnS. FTIR and XRD studies have also been reported.     

Nanocrystalline hydroxyapatite bioceramic using microwave radiation: Synthesis and characterization

In this work, we synthesized bioactive hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, HAp) ceramic powder in the lower-end of nano-regime using microwave radiation, which offers several advantages. The powder was synthesized using calcium nitrate tetrahydrate and sodium phosphate dibasic anhydrous as the starting materials. EDTA served as the complex reagent. The pH of the final suspension was adjusted to 9 by adding ammonium hydroxide. Applied microwave power of 600 W, pH of the suspension, mole ratio of Ca/P in the staring chemicals, and the chelating effect of EDTA served as the factors in the synthesis of nanocrystalline HAp powder. The synthesized powder was studied using various characterizing techniques viz., XRD, SEM, HR-TEM, EDS, TG/DTA and FT-IR to determine powder morphology, particle-size, crystallinity, phases, elemental composition and thermal behavior. Results confirmed highly crystalline nano-powder (5–30 nm) with elemental composition of Ca and P in HAp phase and possessed mixed (elliptical and rod-shape) morphology. Using the Scherrer formula, the average crystallite size was found to be 12 nm. The FT-IR confirmed that the powder is of typical apatite structure. Thermal analysis showed a remarkably lower initial dehydroxylation temperature, compared to micron sized HAp, as reported in literature.     

Synthesis and magnetic study of Co-Al substituted calcium hexaferrite

A series of calcium substituted polycrystalline ferrite ceramics with magnetoplumbite structures were synthesized using perfect stoichiometric mixtures of oxides with chemical composition, CaAl_xCo_xFe_12−2xO₁₉ (x = 2−5), by standard ceramic technique. The variation in the values ofH _candM _s,which depends on the additive content and the temperature, was studied by means of a vibration magnetometer. The strong variation observed in coercivity, saturation magnetization and Curie temperature with chemical composition give rise to the possibility of controlling these properties and hence applying these compounds in the millimeter— microwave range.     

Enhancement in CO gas sensing properties of hydroxyapatite thick films: Effect of swift heavy ion irradiation

This paper investigates the effect of swift heavy ion (SHI) irradiation on surface morphology of Hydroxyapatite (HAp) thick films and modification in gas sensing characteristics. The HAp nanopowder is synthesized by wet chemical process and the thick films are prepared by screen printing technique. These films are irradiated with Ag⁷⁺ ions with energy of 100 MeV at different fluences ranging from 3 × 10¹⁰ to 3 × 10¹³ ions/cm². X-ray diffraction and atomic force microscopy tools are employed to examine the phase and surface modification in HAp thick films due to swift heavy ion irradiation. The ion irradiation study shows that crystallinity decreases and grain size changes with increase in ion fluence. A precise study on gas sensing is carried out to confirm operating temperature of HAp thick film sensor to detect CO gas. Saturation region of the film with increasing gas concentration and other parameters such as response and recovery time are also investigated from the point of view of using HAp films as a sensor device. SHI irradiated HAp thick film shows enhancement in the gas response and saturation limit for CO gas. Furthermore, the irradiated HAp film shows fast response and recovery time for CO gas. The study concludes that nanoceramic HAp thick film is an excellent CO gas sensor at an operating temperature of 195 °C.     

Adsorption and intercalation of anionic surfactants onto layered double hydroxides—XRD study

Layered double hydroxides (LDH) with brucite like structure was modified with various anionic surfactants containing sulfonate, carboxyl, phosphonate and sulfate end group through ion-exchange method. XRD reports indicated that the sulfonate group containing surfactants led to an adsorption process whereas the sulfate, carboxyl and phosphonate group containing surfactant led to an intercalation process. This can be evidenced from the change in basal spacing of LDH. The presence of anionic surfactants in the LDH was supported by FTIR spectroscopy. The FTIR spectrum indicated that complete removal of carbonate anion from the inter layer space of LDH is very difficult. The phosphonate intercalated HT showed less thermal stability than pristine LDH.     

Modelling of flame temperature of solution combustion synthesis of nanocrystalline calcium hydroxyapatite material and its parametric optimization

Hydroxyapatite (HAp), an important bio-ceramic was successfully synthesized by combustion in the aqueous system containing calcium nitrate-di-ammonium hydrogen orthophosphate-urea. The combustion flame temperature of solution combustion reaction depends on various process parameters, and it plays a significant role in the phase formation, phase stability and physical characteristics of calcium hydroxyapatite powder. In this work, an attempt has been made to evaluate the influence of each selected process parameters on the flame temperature as well as physical characteristics of powder, and to select an optimal parameters setting using Taguchi method. A regression model has also been developed to correlate the input parameters, viz. batch size, diluents, fuel to oxidizer ratio and initial furnace temperature, with flame temperature of the solution combustion reaction. The adequacy of the developed model has been checked using analysis of variance technique.     

Mechanical and photocatalytic properties of hydroxyapatite/titania nanocomposites prepared by combined high gravity and hydrothermal process

Hydroxyapatite/titania nanocomposites of different ratios have been successfully synthesized by combined high gravity and hydrothermal methods. SEM and TEM observations showed that small spheres of TiO₂, identified as anatase crystals of 10–15 nm, were deposited on HAp rod-like crystals. EDAX analysis confirmed the presence of Ca, P, Ti and O. X-ray diffraction patterns indicated the presence of hydroxyapatite and anatase phase. More number of anatase peaks appeared in the XRD patterns with higher colloidal concentration of TiO₂ in the HAp/TiO₂ compound. Mechanical stability of the HAp/TiO₂ nanocomposites was determined by reinforcing them with high molecular weight polyethylene (HMWPE) and the tensile strength of the samples was analyzed. Photocatalytic activity of the HAp/TiO₂ particles was examined by decomposition of methyl orange (MO). The results showed that photocatalytic properties of HAp/TiO₂ composites are more effective than that of individual HAp and TiO₂ which implied that the HAp improved the photocatalytic activity of well known photocatalyst TiO₂.     

Enhancement in CO gas sensing properties of hydroxyapatite thick films: Effect of swift heavy ion irradiation

This paper investigates the effect of swift heavy ion (SHI) irradiation on surface morphology of Hydroxyapatite (HAp) thick films and modification in gas sensing characteristics. The HAp nanopowder is synthesized by wet chemical process and the thick films are prepared by screen printing technique. These films are irradiated with Ag⁷⁺ ions with energy of 100 MeV at different fluences ranging from 3 × 10¹⁰ to 3 × 10¹³ ions/cm². X-ray diffraction and atomic force microscopy tools are employed to examine the phase and surface modification in HAp thick films due to swift heavy ion irradiation. The ion irradiation study shows that crystallinity decreases and grain size changes with increase in ion fluence. A precise study on gas sensing is carried out to confirm operating temperature of HAp thick film sensor to detect CO gas. Saturation region of the film with increasing gas concentration and other parameters such as response and recovery time are also investigated from the point of view of using HAp films as a sensor device. SHI irradiated HAp thick film shows enhancement in the gas response and saturation limit for CO gas. Furthermore, the irradiated HAp film shows fast response and recovery time for CO gas. The study concludes that nanoceramic HAp thick film is an excellent CO gas sensor at an operating temperature of 195 °C.     

Effect of urea on formation of hydroxyapatite through double-step hydrothermal processing

The effect of urea on the formation of hydroxyapatite (HAp) was studied by employing the double-step hydrothermal processing of a powder mixture of beta-tricalcium phosphate (β-TCP) and dicalcium phosphate dihydrate (DCPD). Co-existence of urea was found to sustain morphology of HAp crystals in the compacts under an initial concentration of 2 mol dm^-3 and less. Homogenous morphology of needle-like crystals was observed on the compacts carbonated owing to decomposition of urea. Carbonate ions (CO₃^2-) was found to be substituted in both the phosphate and hydroxide sites of HAp lattice. The synthesized HAp was calcium deficient, as it had a Ca/P atomic ratio of 1.62 and the phase was identified as calcium deficient hydroxyapatite (CDHA). The release of CO₃^2- ions from urea during the hydrothermal treatment determined the morphology of the CDHA in the compacts. The usage of urea in the morphological control of carbonate-substituted HAp (CHAp) employing the double-step hydrothermal method is established.