Structural, Morphological and Physical Properties of TiH2 Incorporated Hydroxyapatite

hydroxyapatite (HAp) was obtained from wet chemical facile method and was mixed with TiH 2 (5 to 20%). The mixes were shaped by pressing and samples were sintered at different temperatures from 900°C to 1200°C. The experimental result from XRD clearly reveals that the product composition leads to TCP and CaTiO 3 major phases. SEM results showed that grain size increased at 1200°C with increase in wt % of TiH 2 . The outcome of the sintering studies carried out shows that the maximum porosity was obtained at 5wt% of TiH 2 addition with the HAp at sintering temperatures of 900°C and 1000°C. The incidence of broad sintering reactions and phase dissociation of HAp leads to development of TCP – CaTiO 3 composites. Abstract Titanium incorporated hydroxyapatite preparation was endeavored using TiH 2 . Titanium has good mechanical properties, good biocompatibility and bioactivity. hydroxyapatite material prepared for orthopedic applications were reported to be better mechanical properties. Hydroxyapatite (HAp) was synthesized by wet chemical facile method and after calcination was mixed with TiH 2 (5 to 20%).The effect of sintering on phase formation , microstructure, density and porosity of Hap/TiH 2 was studied by sintering at temperatures from 900°C to 1200°C. The properties of the samples were characterized using X-ray diffraction technique (XRD), Scanning electron microscopy(SEM), Fourier transform spectroscopy (FT-IR), density and porosity. The results from studies showed the presence of β -tricalcium phosphate (β -TCP) and perovskite (CaTiO 3 ) as the major crystalline phases; while minor reaction products like α -TCP and TTCP were also recorded for samples with higher amount of TiH 2 irrespective of sintering temperatures. Morphology evaluation by SEM revealed the presence of CaTiO 3 needle structure at temperature till 1000°C, above which it appeared hexagonal due to crystal growth. Functional groups, density and porosity were also studied.


1.Introduction
Hydroxyapatite (HAp) is composed of a network of calcium orthophosphates and is widely used in biomedical applications like orthopedics and dentistry as it is similar to the mineral component of bone and teeth. HAp is water-insoluble,biodegradable and bioactive since after some time it is partially resorbed and replaced by natural bone [1]. HAp has been used in various bone repair and replacement applications in the form of dense or porous blocks, granules, powders, coatings or as a mineral component in a polymer composite [2]. HAp can be synthesized by different routes such as solid-state reaction, sol-gel process , hydrothermal process, micro emulsion technique , precipitation process, biomimetic process, etc. Among these methods, chemical precipitation is the widely used method as it is economical and provides pure product.
Natural bone and teeth are porous materials which have porosity in micrometer range.
Sponge bone has 50 to 90% porosity with pore diameter of roughly 1 µm [3,4]. Even the Haversian canals in cortical bone contains 3 to 12% porosity [5]. In teeth, the open porosity of the dental tissue ranges between 1.11% and 3.08% of its volume [6]. In bone tissue engineering, a scaffolding material is utilized either to actuate the development of bone from the encompassing tissue or to go about as a bearer or template for implanted bone cells or agents [7].
Porous structures and rough surfaces are essential for encouraging bone ingrowth and osteointegration, which makes dense HAp less resorbable and osteoconductive compared to porous HAp for the healing/filling of osseous defects. For obtaining cell migration and transportation of nutrients and metabolic wastes, highly porous architecture with interconnected porous network is important [8,9].
Porous material can be prepared by sacrificial template process [10,11,13] gel casting of foams [12],powder metallurgy [14], freeze drying [15] etc. All these methods result in porous structure with different pore characteristics. General pore creating materials are naphthalene, hydrogen peroxide and other similar materials which are easily vaporizable. In this work, TiH2 in various amounts is added to HAp and characterized to investigate its ability to act as pore former due to the presence of hydrogen group. Further, the incorporation of titanium ion with HAp to form composite is investigated.
Titanium and its alloys have been widely utilized as metallic implant materials because of the blend of advantageous properties like good mechanical properties, good biocompatibility and bioinertness. F.N. Oktar studied the mechanical properties of TiO2 (5 to 10wt %) added to HAp with different sintering temperatures between 1000 and 1300°C [17].
Research was earlier done on sintered density and microstructure modifications on the usage of TiH2 powders along with Ti. When TiH2 powder was used as an alternative starting material to Ti metal powder in titanium sponge preparation, it gives the benefit of reduced cost in titanium powder metallurgy since TiH2 is a transitional product in the hydrogenationdehydrogenate (HDH) operation [18][19]. TiH2 particles are most encouraging than the pure Ti Particles because of the sintering behavior of TiH2 [20]. Previously, TiH2 powders were used to enhance the foaming process of Titanium scaffolds in orthopedic applications [21].
The decomposition of the hydride is belated when TiH2 powder is pre-treated in atmospheric air condition at certain temperature [23]. The dehydrogenation of TiH2 occurs in a two-step process given as TiH2 → TiHx→ α-Ti, where 0.7<x<1.1 [24]. The coloration of TiH2 powder is the easiest way to estimate the stage of oxidation during the dehydrogenation of TiH2.
During heat treatment, the black powder turned to olive green at 400°C, purple at 450°C and blue between 500°C and 550°C [22].
The purpose of this novel work is to prepare composites with synthetic HAp and TiH2, and to analyze whether TiH2 had functioned as a pore former or an additive or both. 5 -20wt. % of TiH2 was added to HAp and sintered at 900°C -1200°C in pressureless sintering to study the effect of sintering temperature on phase formation, microstructure, functional group formation , density and porosity of samples with different TiH2 content.
Ammonia was added drop wise into the mixed solution with vigorous stirring until pH reached a value of around 10 to 11, and was continued for 2 -3 hours to allow the reaction to take place towards completion. A white precipitate was obtained and was aged for 12 hours at room temperature. The obtained precipitate was filtered, washed with deionized water for 3-4 times and dried at 110°C for 24 hours. The dried lumps were crushed using an agate mortar and calcined at 700°C. The powder was confirmed as pure hydroxyapatite by matching the XRD pattern obtained ( Fig.1)using D8 Advance (Bruker) analytical x-ray system with JCPDS file 09-0432.

Pellet Formation
TiH2 was added in different weight percentages of 5, 10, 15 and 20 to HAp, and mixed with pestle and mortar for homogenous mixing. Pellets of 10mm diameter were prepared using uniaxial press under 150 bars load. The as-mixed composite powders and pellets of HAp/TiH2 were dried at 110°C for 24 hours and firing was done in a muffle furnace at temperatures of 900°C, 1000°C, 1100°C and 1200°C with 3°C per minute heating rate and 2 hours soaking at the maximum temperature.

Sample characterization
Phase analysis and lattice parameters of sintered HAp/TiH2 powders were determined by X-ray diffraction (XRD) using Bruker D8 advance analytical x-ray system. The XRD patterns of the samples were obtained by using OriginPro software. An estimation of the crystallite size was obtained using the Debye-Scherrer equation (1) where, β is the peak width at half maximum intensity(FWHM) (in radians) K is the Scherrer constant dependent on crystal habit (0.9) λ is the wavelength of X-rays (1.5406Ǻ for CuKα radiation) The lattice constants of the HAp/TiH2 powders were determined by the following relation(2) The volume of the hexagonal unit cell of the materials was calculated according to the formula, where, V is the volume of unit cell (nm 3 ), a and c are the lattice constants.
Scanning electron microscopic (SEM) analysis (Quanta 200 FEG) was used for analyzing the size and morphology of sintered HAp/TiH2 powders. FTIR (PerkinElmer) was performed to assess the functional groups and chemical composition of the sintered HAp/TiH2 powders, for which the powders were mixed with KBr and pelletized, and spectra was obtained between 4000 -400 cm -1.
The density and porosity of the sintered HAp/TiH2 pellets were tested by Archimede's principle with water as fluid and calculated using the formula (4) and (5) respectively where, S is weight of the soaked piece (gm), D is weight of the dry piece (gm), I is weight of the immersed piece (gm).
The following formula was proposed to account for the decomposition of HAp when it is sintered to higher temperature (1350°C) [27].
Ca10(PO4)6(OH)2 → 3Ca3(PO4)2 + CaO + H2O↑ (7) According to equations (6)and (7), both the decomposition and dehydroxylation reactions include water vapour as a by-product, and the rates at which these reactions continue depend on the moisture in the furnace atmosphere. Thus, the secondary phase formation during sintering could be controlled by controlling the sintering atmosphere. High amount of humidity present in sintering atmosphere has the tendency to delay decomposition rate by inhibiting the dehydration of OHgroup from the HAp matrix. This can be attained by controlling the partial pressure of the atmosphere, as the saturated moisture content in the atmosphere would suppress the by-product of water vapour between the reactions of both the dehydroxylation and decomposition [28]. In general, sintering at elevated temperatures tends to take the OH -(hydroxide group) in the HAp matrix and concludes in the decomposition of HAp into α-TCP, β-TCP and TTCP [25,29]. traces which indicates the interaction between HAp and TiH2. According to the phase compositions, it is obvious that CaTiO3 is a product of reaction between TiO2 and CaO, which is one of the decomposition products of HAp (according to Eq. (7)). CaTiO3 was registered in the XRD pattern of composite powders (irrespective of TiH2 content) in Fig. 2, but there was no evidence of TiO2, suggesting high thermodynamic tendency of TiH2 to diffuse and then to react with HAp. However, the formation of CaTiO3 indicates that the reaction between HAp and TiH2 has taken place during thermal treatment. Hence, the interaction between TiH2 and HAp at higher temperature (˃900°C) is proposed as in equation (3).
2Ca10 (PO4)6(OH) 2 + 2TiH2+ 3O2→ 6Ca3(PO4)2 + 2CaTiO3 + 4H2O↑ Diffraction peaks from CaTiO3 were observed from 900°C onwards, and its intensity was found to increase with increasing percentage of TiH2 while that of apatite peaks decreased. The sharpness of CaTiO3 peaks increased with the sintering temperature hinting on the increased crystallinity. The HAp decomposition phase formed at a particular temperature was found to be influenced by the TiH2 content and the traces of pure HAp was found to remain undecomposed till 1200°C. The HAp decomposition phases were observed from 15wt% TiH2 at 900°C, and 5wt% TiH2 at 1000°C and the major phase was found to be β-TCP. Traces of α-TCP and TTCP formation at 900°C and 1000°C was found only on addition of 20wt% TiH2.
Contrary to the above observations, α-TCP formation had initiated from 5wt% TiH2 at 1100°C but the peaks became prominent only above 15wt% TiH2. The other decomposition phase obtained with α-TCP was β-TCP. TTCP formation was observed from 15wt% TiH2 at 1100°C. Samples sintered at 1200°C were observed to have CaTiO3 and β-TCP as their major phases at various TiH2 content. Additionally, minor peaks of α-TCP were found from 10wt% TiH2. TTCP peaks was absent at 1200°C.  Table 1 & 2. These values were found not to vary appreciably as a function of TiH2 content and heat treatment temperature.

FTIR Analysis
The FTIR spectrum of the samples heat treated at 900°C, 1000°C, 1100°C and 1200°C without and with different percentages of TiH2 are shown in Fig.5. Most bands characterize the phosphate group of HAp, especially at 570 cm -1 , 605 cm -1 , 1038-1049 cm -1 and 1119 -1120cm -1 as mentioned in Table.3. A sharp peak with weak intensity corresponding to OH − stretching vibration band is observed at 3571 cm −1 in pure HAp sample. This band is totally absent at higher TiH2 content or at higher temperature due to the dehydroxylation as a result of HAp decomposition . Weak bonds at 944cm -1 and 550 cm -1 related to the secondary β-TCP phase in HAp were observed in all HAp/TiH2 samples. The peak at 605 cm -1 are assigned to the asymmetric deformation of the PO4 3-ions (υ1). The peaks at 944 cm -1 and 962 cm -1 are relative to the symmetric stretching of the PO4 3-ions (υ3) and the peaks at 1038 -1090 cm -1 and 1119 -1120 cm -1 are relative to the asymmetric stretching of the PO4 3-ions (υ3). Furthermore, the relative intensity between the peaks at ~478 cm -1 and that at ~470 cm -1 reflects the degree of the dehydroxylation. The peak at 478 cm -1 is slightly stronger than the peak at 470 cm -1 . In addition to the above mentioned peaks, few other peaks are also detected at around 2350 and 2923 cm -1 which are due to KBr used for sample preparation [30].  from various HAp-TiH2 composites

Density and Porosity
Density of HAp/TiH2 composite was found to increase with increase in TiH2 weight percentage at 900°C and 1000°C. Density values decreased above 1000°C due to the decomposition of HAp, due to which the porosity of the composite has increased. As can be observed from the SEM images (Fig. 4) 6.e. shows the overall graph for the density and porosity values, and shows that increase in density decreases the porosity of sample. The correlation co-efficient calculated from the graph drawn between the overall density and porosity was 0.9225.

Conclusions
Hydroxyapatite was prepared by precipitation method and powders of hydroxyapatite with different TiH2 percentage were sintered in air atmosphere at the range of 900-1200°C for 2 h for simultaneously developing pore in the HAp and incorporate it with Titania. The experimental result from XRD clearly reveals that the product composition leads to TCP and CaTiO3 major phases. SEM results showed that grain size increased at 1200°C with increase in wt % of TiH2. The outcome of the sintering studies carried out shows that the maximum porosity was obtained at 5wt% of TiH2 addition to HAp at sintering temperatures of 900°C and 1000°C.
The incidence of broad sintering reactions and phase dissociation of HAp leads to development of TCP-CaTiO3 composites. The mechanical properties and bioactivity of the samples will be studied in future.  Density and porosity graph of HAp with different weight percentages of TiH2 (a) sintered at 900°C (b) sintered at 1000°C (c) sintered at 1100°C(d) sintered at 1200°C(e) Overall graph of density vs porosity.