Phase characterization and mechanical and flame‐retarding properties of nano‐CaSO4/polypropylene and nano‐Ca3(PO4)2/polypropylene composites

An in situ deposition approach was used for the synthesis of nano‐CaSO₄ and nano‐Ca₃(PO₄)₂. The nanosize particles were confirmed with an X‐ray diffraction technique. Composites of polypropylene (PP) with 0.1–0.5 wt % nano‐ or commercial CaSO₄ or nano‐Ca₃(PO₄)₂ were prepared. The transition from the α phase to the β phase was observed for 0.1–0.3 wt % nano‐CaSO₄/PP and nano‐Ca₃(PO₄)₂/PP composites. This was confirmed by Fourier transform infrared. A differential scanning calorimetry analysis was carried out to determine the thermal behavior of the nanocomposites with increasing amounts of the nano‐CaSO₄ and nano‐Ca₃(PO₄)₂ fillers. Increases in the tensile strength and Young's modulus were observed up to certain loading and were followed by a decrease in the tensile strength. A continuous decrease in the elongation at break (%) was also observed for commercial CaSO₄ and larger nano‐Ca₃(PO₄)₂. A decrease in the mechanical properties after a certain loading might have been due to the agglomeration and phase transition of PP in the composites. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 670–680, 2007     

New hyperdispersant agent for polypropylene/CaSO4 composites

A new hyperdispersant agent with Si? OH as an anchoring group and poly(butyl acrylate) as a solvatable chain was synthesized, and its effect on the properties of polypropylene (PP)/CaSO₄ composites was investigated. Fourier transform infrared spectroscopy results showed that the hyperdispersant agent reacted on the CaSO₄ surface and the modified CaSO₄ particles. The tensile strength and impact strength of the PP/CaSO₄ composites increased about 14 and 34%, respectively, versus that of PP/CaSO₄ (filled with the same unmodified fraction). According to surface analysis by scanning electron microscopy, the CaSO₄ particles were buried well in the PP matrix when CaSO₄ was coated with the hyperdispersant agent. CaSO₄ significantly increased the crystallization temperature and crystallization rate of PP by differential scanning calorimetry, but the addition of hyperdispersant‐agent‐modified CaSO₄ did not lead to the formation of crystalline PP through X‐ray diffraction. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nonisothermal Crystallization Modeling and Simulation of Polypropylene/Nano Ca3(PO4)2 Composites with Variation in Wt.% of Loading and Reduction in Nanosize

Nonisothermal crystallization kinetics of polypropylene (PP)/Ca₃(PO₄)₂ composites was studied using differential scanning calorimetry (DSC) for various nanosizes by employing Avrami and Ozawa's combined analysis. Parameters such as Avrami's exponent (n) and composite growth rate constant (Z_t) were determined, which characterize the system of different nanosize composites and virgin PP. The relative degree of crystallinity as a function of temperature for PP-nano Ca₃(PO₄)₂ composites at the same cooling rate and sigmoidal shape of curves indicates that there is a strong interaction between PP molecule and nanolayer, which leads to greater nucleation with reduction in nanosizes. A theoretical combination of kinetic equations is found to be suitable to describe the physical phenomena of a real system. The values of parameters n, Z_t, and predicted time t for crystallization at a single cooling rate were obtained from the mathematical model.     

Structural elucidation and iron oxidation states in situ formed β‐Ca3(PO4)2/α‐Fe2O3 composites

A detailed structural analysis on the in situ synthesized β‐Ca₃(PO₄)₂/α‐Fe₂O₃ composites is demonstrated. Compositional ratios, the influence and occupancy of iron at the β‐Ca₃(PO₄)₂ lattice, oxidation state of iron in the composites are derived from analytical techniques involving XRD, FT‐IR, Raman, refinement of the powder X‐ray diffraction and X‐ray photoelectron spectroscopy. Iron exists in the Fe³⁺ state throughout the investigated systems and favors its occupancy at the Ca²⁺(5) site of β‐Ca₃(PO₄)₂ until critical limit, and thereafter crystallizes as α‐Fe₂O₃ at ambient conditions. Fe³⁺ occupancy at the β‐Ca₃(PO₄)₂ lattice yields a Ca₉Fe(PO₄)₇ structure that is isostructural with its counterpart. A strong rise in the soft ferromagnetic behavior of β‐Ca₃(PO₄)₂/α‐Fe₂O₃ composites is obvious that depends on the content of α‐Fe₂O₃ in the composites. Overall, the diverse level of iron inclusions at the calcium phosphate system with a Ca/P ratio of 1.5 yields a structurally stable β‐Ca₃(PO₄)₂/α‐Fe₂O₃ composites with assorted compositional ratios.     

Formation of a new solid β tricalcium orthophosphate structure,from reaction between hydroxyapatite and ammonium sulfate

The reaction which occur during heating, from room temperature to 1100°C, of a mixture of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ [HAP] and ammonium sulfate (NH₄)₂SO₄ [AS] are studied. The formation of Ca₂(NH₄)₂(SO₄)₃, Ca₂P₂O₇ and Ca(PO₃)₂ is observed between 200°C and 300°C; at 400°C CaSO₄ appears. From 500 to 700°C, Ca(PO₃)₂ reacts with Ca SO₄ and with HAP and gives β-Ca₂P₂O₇. Lastly, from 700°C to 1000°C, β-Ca₂P₂O₇ reacts with HAP and with CaSO₄ and gives β-Ca₃(PO₄)₂ [β-TCP]; From 1000 to 1100°C, β-TCP and CaSO₄ react and form a sulfate ion containing calcium phosphosulfate, the structure of which is β-TCP     

Nonisothermal Crystallization Modeling and Simulation for Polypropylene/Nano CaSO4 Composites with Variations in Nanosizes and Wt.% of Loading

Avrami and Ozawa's combined analysis was employed to study the nonisothermal crystallization kinetics of Polypropylene (PP): CaSO₄ (of 12 and 22 nm) composites using a Differential Scanning Calorimeter (DSC). The parameters, such as Avrami's exponent (n) and growth rate constant (Z_t), that characterized the system of different nanosize composites and virgin PP, were determined. The relative degree of crystallinity as a function of temperature for PP/nano CaSO₄ composites at the same cooling rate and the Sigmoidal shape of curves indicate a strong interaction between PP molecules and the nanolayer, which leads to greater nucleation with a reduction in nanosizes. The theoretical combination of kinetic equations was found to be suitable to describe the physical phenomena of real system. The values of parameters n, Z_t and predicted time t for crystallization at a single cooling rate were obtained from the mathematical model.     

Structural and Crystal Chemical Investigation of Intermediate Phases in the System Ca2SiO4–Ca3(PO4)2–CaNaPO4

Two intermediate compounds of the system Ca₂SiO₄–Ca₃(PO₄)₂–CaNaPO₄ were synthesized by reaction sintering at 1600°C and analyzed structurally, chemically, and optically. The structure of Ca₇(PO₄)₂(SiO₄)₂ nagelschmidtite (space group P6₁, a = 10.7754(1) Å, c = 21.4166(3) Å) was determined by single crystal X‐ray analysis. Its unit cell can be interpreted as a supercell (≈ 2 × a, 3 × c) of the high‐temperature polymorph α‐Ca₂SiO₄. Evidence for pseudo‐hexagonal symmetry is shown. Using electron microprobe, the solid solution Ca_7?xNa_x(PO₄)_2+x(SiO₄)_2?x, (x ≤ 2), of nagelschmidtite was confirmed. Volume thermal expansion coefficients of Ca_6.8Na_0.2(PO₄)_2.2(SiO₄)_1.8 and Ca_5.4Na_1.5(PO₄)_3.7(SiO₄)_0.3 were determined using high‐temperature X‐ray powder diffraction, yielding mean α_V = 3.95 and 5.21 [×10^?5/°C], respectively. Ca₁₅(PO₄)₂(SiO₄)₆ is a distinct phase in the binary section Ca₂SiO₄–Ca₃(PO₄)₂ and was found to extend into the ternary space according to Ca_15?xNa_x(PO₄)_2+x(SiO₄)_6?x, (x ≤ 0.1). Quenching experiments of the latter allowed for structural analysis of a strongly disordered, defective high‐temperature polymorph of the α‐Ca₂SiO₄–α‐Ca₃(PO₄)₂ solid solution. Structural relations between nagelschmidtite, Ca₁₅(PO₄)₂(SiO₄)₆ and the end‐member compounds of the system are discussed.     

Polyamide Nanocomposites: Investigation of Mechanical,Thermal and Morphological Characteristics

In the present work, polyamide/CaSO₄ nanocomposites were prepared via melt intercalation on twin-screw extruder. Different particle sizes (23, 15, 10 nm) of CaSO₄ were synthesized by in situ deposition technique and its sizes and shape were confirmed on a transmission electron microscope (TEM). The TEM study showed that nano-CaSO₄ has a needlelike or fiberlike structure. Nano-CaSO₄ was added from 1 to 4 wt% in the polyamide. Properties such as Tensile strength, Elongation at Break, Young's Modulus, and hardness were studied. These results were then compared with commercial CaSO₄-filled polyamide composites. There was a propounding effect to be observed on properties of polyamide nanocomposites due to uniform dispersion of nano-CaSO₄ and commercial CaSO₄. The 4 wt% of 10 nm CaSO₄ shows 16% improvement in Tensile Strength compared to commercial CaSO₄ (11%) filled in polyamide composites, whereas, Elongation at Break decreases drastically in 10 nm CaSO₄-filled polyamide nanocomposites up to 22% compared to commercial CaSO₄-filled polyamide composites at 4 wt% loading (11%). Among these properties, Young's Modulus was found to be more effective in 4 wt% loading of 10 nm CaSO₄ and was recorded to be 66% more compared to commercial CaSO₄-filled-in polyamide composites (22%). Moreover, thermal properties such as thermal degradation and flammability were studied by TGA and flame testers. It was found that nano-CaSO₄ was thermally more stable compared to commercial CaSO₄-filled polyamide composites. Extent of dispersion of nano-CaSO₄ was studied along with micro cracks generated during tensile testing using an Atomic Force Microscope (AFM).     

Preparation,Characterization and Properties of Poly(vinyl chloride)/CaSO4 Nanocomposites

Poly(vinyl chloride)/CaSO₄ nanocomposites were prepared on Brabender plasticorder. Moreover, for comparison PVC/ commercial CaSO₄ composites was prepared. The amount of loadings of nano CaSO₄ and commercial CaSO₄ was in the range of 0–12 wt%. Nano CaSO₄ was synthesized by matrix-mediated growth technique. The size and shape of nano CaSO₄ were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). These PVC nanocomposites were then subjected to mechanical, thermal analysis on UTM, and TGA respectively. Morphology studies of PVC nanocomposites were done on scanning electron microscope (SEM). The morphology study showed that nano CaSO₄ get exfoliated and intercalated with the PVC matrix. Notched Charpy impact strengths of PVC nanocomposites was drastically improved compared to that of pristine PVC and commercial CaSO₄ composites. PVC/CaSO₄ nanocomposites showed higher values of tensile strength, elongation at break and impact strength than PVC/CaSO₄ (commercial) composites. Impact strengths also gives some drastic dependence on nano CaSO₄ content and give a maximum value at certain CaSO₄ loadings.     

Alkene selectivity enhancement in the oxidation of propane on calcium-based catalysts

The oxidation of propane has been investigated in the presence and absence of tetrachloromethane (TCM) on calcium hydroxyapatite (CaHAp), Ca₃(PO₄)₂, CaSO₄ and CaO at 723 K. In the absence of TCM, the conversion of C₃H₈ on CaHAp was 7.7–9.2% during 6 h on-stream while that on Ca₃(PO₄)₂, CaSO₄ and CaO was 0.6, 0 and 0.2–0.4%, respectively. The principal products on all catalysts in the absence of TCM were CO and CO₂ with small selectivities to C₃H₆ and C₂H₄ (both 5–6%) observed on CaHAp. Upon addition of TCM, the selectivity to C₃H₆ on all catalysts and the conversion of C₃H₈ on CaSO₄ increased while, with increasing time-on-stream, the changes in the conversion and selectivity were dependent upon the nature of the catalysts. XPS and XRD analyses provide evidence for the presence of chlorine in the surface and/or bulk of three of the catalysts, suggesting that chlorinated species on the solids play a role in the selectivity enhancement, but the absence of chlorine from the sulphate demonstrates the dissimilarities of the catalysts in their abilities to sorb and decompose TCM. This revised version was published online in July 2006 with corrections to the Cover Date.     

Production and study of in vitro behaviour of monolithic α-tricalcium phosphate based ceramics in the system Ca3(PO4)2–Ca2SiO4

In this work a new kind of α-tricalcium phosphate (α-Ca₃(PO₄)₂) doped with dicalcium silicate (Ca₂SiO₄) ceramic materials, with compositions lying in the field of the Ca₃(PO₄)₂ solid solution in the system Ca₃(PO₄)₂–Ca₂SiO₄, were obtained. The properties of the sintered ceramics were discussed in detail as well as some in vitro relevant properties for bone repairing. Crystalline α-Ca₃(PO₄)₂ solid solution (α-TCPss) was the only phase in the ceramics containing from 1 wt% to 4 wt% of Ca₂SiO₄. The release of ionic Si in simulated body fluid increased with the content of Ca₂SiO₄ and favoured α-Ca₃(PO₄)₂ surface transformation. In addition, cell attachment test showed that the α-TCPss supported the mesenchymal stem cells adhesion and spreading, and the cells established close contact with the ceramics after 24 h of culture. According to the results, the investigated α-TCPss ceramics possesses good bioactivity, biocompatibility and mechanical properties, and might be a promising bone implant material.     

Decomposition behavior of CaSO4 during potassium extraction from a potash feldspar-CaSO4 binary system by calcination

The extraction of potassium from a tablet mixture of K-feldspar ore and CaSO_4by roasting was studied with a focus on the effects of the decomposition behavior of CaSO_4on the potassium extraction process.The roasted slags were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),energy-dispersive X-ray spectroscopy,and thermogravimetric(TG)analysis.The XRD analysis revealed that hydrosoluble mischcrystal K_2Ca_2(SO_4)_3was obtained by ion exchange of Ca~(2+)in CaSO_4and K~+in KAlSi_3O_8.Meanwhile,the intermediate product,SiO_2,separated from KAl Si_3O_8and reacted with CaSO_4to decompose CaSO_4.The SEM results showed that some blowholes emerged on the surface of the CaSO_4particles when they reacted with SiO_2at 1200°C,which indicates that SO_2and O_2gases were released from CaSO_4.The TG curves displayed that pure CaSO_4could not be decomposed below 1200°C,while the mixture of K-feldspar ore and CaSO_4began to lose weight at 1000°C.The extraction rate of potassium and decomposition rate of CaSO_4were 62%and 44%,respectively,at a mass ratio of CaSO_4to K-feldspar ore of 3:1,temperature of 1200°C,tablet-forming pressure of6 MPa,and roasting time of 2 h.The decomposition of CaSO_4reduced the potassium extraction rate;therefore,the required amount of CaSO_4was more than the theoretical amount.However,excess CaSO_4was also undesirable for the potassium extraction reaction because a massive amount of SO_2and O_2gas were derived from the decomposition of CaSO_4,which provided poor contact between the reactants.The SO_2released from CaSO_4decomposition can be effectively recycled.     

Synthesis and luminescence properties of single‐component Ca5(PO4)3F:Dy3+, Eu3+ white‐emitting phosphors

A series of Ca₅(PO₄)₃F:Dy³⁺, Eu³⁺ phosphors was synthesized by a solid‐state reaction method. The XRD results show that all as‐prepared Ca₅(PO₄)₃F:Dy³⁺, Eu³⁺ samples match well with the standard Ca₅(PO₄)₃F structure and the doped Dy³⁺ and Eu³⁺ ions have no effect on the crystal structure. Under near‐ultraviolet excitation, Dy³⁺ doped Ca₅(PO₄)₃F phosphor shows blue (486 nm) and yellow (579 nm) emissions, which correspond to ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 transitions respectively. Eu³⁺ co‐doped Ca₅(PO₄)₃F:Dy³⁺ phosphor shows the additional red emission of Eu³⁺ at 631 nm, and an improved color rendering index. The chromaticity coordinates of Ca₅(PO₄)₃F:Dy³⁺, Eu³⁺ phosphors also indicate the excellent warm white emission characteristics and low correlated color temperature. Overall, these results suggest that the Ca₅(PO₄)₃F:Dy³⁺, Eu³⁺ phosphors have potential applications in warm white light‐emitting diodes as single‐component phosphor.     

Effect of filler treatments on rheological behavior of calcium carbonate and talc‐filled polypropylene hybrid composites

Commercial stearic acid treated calcium carbonate (CaCO₃) was used to make a comparative study on rheological behavior of the CaCO₃ and talc‐filled polypropylene (PP) hybrid composites with nontreated filler. Apparent shear viscosity and extrudate swell were investigated with variation of filler ratio and temperature with 30% by weight total of filler was used in PP composite. The Shimadzu capillary rheometer was used to evaluate shear viscosity and shear rate of the composite. It was found that the shear viscosities decrease with increasing shear rate. The apparent shear viscosity of the composite containing the stearic acid treated is slightly lower than untreated filler. Shear thickening behavior at higher shear rate has also shown by 15/15 treated composites at higher temperature about 220°C and investigation by SEM has proved that filler being densely packed at that condition. Treated composites also exhibit lower swelling ratio value than untreated composite, and swelling ratio also decreases linearly with increasing temperature and the die length–diameter ratio. It is believed that dispersion of filler play an important role not only on shear viscosity but also on swelling ratio of PP composite. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5421–5426, 2006     

Effect of A+ (A = Li,Na and K) co-doping on enhancing the luminescence of Ca5(PO4)2SiO4:Eu3+ red-emitting phosphors as charge compensator

A series of Ca_5-x(PO₄)₂SiO₄:xEu³⁺ red-emitting phosphors were synthesized through solid-state reaction, and alkali metal ions A⁺ (A = Li, Na and K) were co-doped in Ca₅(PO₄)₂SiO₄:Eu³⁺ to improve its luminescence property. The impacts of synthesis temperature, luminescence center Eu³⁺ concentration and charge compensator A⁺ on the structure and luminescence property of samples were studied in detail. X-ray diffraction results indicated that prepared Ca₅(PO₄)₂SiO₄:Eu³⁺, A⁺ had a standard Ca₅(PO₄)₂SiO₄ structure with space group P6₃/m. Under the excitation of 392 nm, Ca₅(PO₄)₂SiO₄:Eu³⁺ phosphors showed a red emission consisting of several emission peaks at 593 nm, 616 nm and 656 nm, relevant to ⁵D₀→⁷F₁, ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄ electron transitions of Eu³⁺ ions, respectively. Luminescence intensity and lifetime of Ca₅(PO₄)₂SiO₄:Eu³⁺ can be significantly enhanced through co-doping alkali metal ion A⁺, which play an important role as charge compensator. The results suggest that Ca₅(PO₄)₂SiO₄:Eu³⁺, A⁺ red phosphors with excellent luminescence property are expectantly served as red component for white light-emitting diodes excited by near-ultraviolet.     

The role of sulphates in cement clinkering reactions : Phase formation and melting in the system CaOCa2SiO4CaSO4K2SO4

The sulphate-rich portions of the system CaOCa₂SiO₄CaSO₄K₂SO₄ have been studied by the quenching technique, supplemented by DTA. CaO and C₃S are relatively insoluble in sulphate-rich liquids. These rapidly decompose C₂S. A quaternary eutectic point between Ca₂K₂(SO₄)₃, K₂SO₄, CaO and Ca₂SiO₄ occurs at 824°C: the composition of the liquid (in weight %) is CaO 1.67; SiO₂ 0.33; CaSO₄ 39.20; K₂SO₄ 58.80.     

Luminescence enhancement of single-component Ca19Zn2(PO4)14:Dy3+ white-emitting phosphor powders through partial substitution of PO43− with SiO44− and BO33-

A series of Ca₁₉Zn₂(PO₄)_14-yAG_y:Dy³⁺ (AG = BO₃^3?, SiO₄^4?) white-emitting phosphors with partial PO₄^3? substitution were synthesized through high-temperature solid-state reaction. The BO₃^3? and SiO₄^4? anionic groups were introduced into Ca₁₉Zn₂(PO₄)₁₄:Dy³⁺ for enhancing the luminescence of Dy³⁺. The X-ray diffraction results exhibited that all samples were assigned to the standard trigonal Ca₁₉Zn₂(PO₄)₁₄ structure with R3c (161) space group. The white emission spectrum of Dy³⁺-doped phosphors was composed of several characteristic peaks located at 484 nm, 575 nm and 665 nm, corresponding to ⁴F_9/2 → ⁶H_15/2, ⁶H_13/2 and ⁶H_11/2 electron transitions, respectively. After partial SiO₄^4? and BO₃^3? substituting PO₄^3?, the luminescence intensity of Ca_18.62Zn₂(PO₄)_13.75(SiO₄)_0.25:0.38Dy³⁺ and Ca_18.62Zn₂(PO₄)_13.65(BO₃)_0.35:0.38Dy³⁺ samples were higher by 1.68 and 1.49 times than that of Ca_18.62Zn₂(PO₄)₁₄:0.38Dy³⁺ sample, respectively, due to the charge compensation and crystal field environment effect. The actual w-LEDs fabricated with as-prepared Ca_18.62Zn₂(PO₄)_13.75(SiO₄)_0.25:0.38Dy³⁺ and Ca_18.62Zn₂(PO₄)_13.65(BO₃)_0.35:0.38Dy³⁺ phosphors showed excellent optical performances. All results indicated that the single component Ca_18.62Zn₂(PO₄)_14-yAG_y:0.38Dy³⁺ white-emitting phosphors could have a potential application in w-LEDs.     

Combustion and structure formation in the Ti-Ta-C-Ca<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript> system

Biocompatible composites (Ti, Ta)C _x + Ca₃(PO₄)₂ for deposition of nanofilms onto load-bearing implants by ion-plasma sputtering were prepared from Ti + Ta + C + Ca₃(PO₄)₂ mixtures by forced SHS compaction. The effect of Ta + C addition to green mixtures (characterized by parameter z) on the structure/phase formation in combustion products was explored. The addition of tantalum and carbon was found to have little or no influence on the burning velocity U and combustion temperature T _c. Two thermal spikes exhibited by thermograms were associated with the occurrence of two consecutive reactions leading to formation of titanium and tantalum carbides. With increasing z, the grain size of (Ti, Ta)C was found to diminish, its relative density to decrease, while the hardness to markedly grow.      

Phase transition and proton transport characteristics in CsH2PO4/SiO2 composites

The stabilization of superprotonic phase in neat CsH₂PO₄ and CsH₂PO₄/SiO₂ composites as well as the anomalous phase transformation with a large hysteresis was investigated through proton conductivity, thermal analysis and Raman spectroscopy. The reversibility of the transition to the superprotonic phase and the phase transformation between monoclinic phase and cubic phase in neat CsH₂PO₄ at around T_c = 230 °C was confirmed under humidified and sealed conditions. In CsH₂PO₄/SiO₂ composites, a large asymmetric thermal hysteresis in the conductivity appeared, i.e. significant supercooling in the superprotonic phase was induced in silica matrices. A differential thermal analysis revealed that the temperature of a reverse transition from the cubic phase (superprotonic phase) to the monoclinic phase decreased in the composites. This effect became significant with an increase in silica volume fraction. The stabilization of superprotonic phase (cubic phase) in the composites will be induced by shear elastic forces at the interface between CsH₂PO₄ and silica particles. The main origin of the anomalous asymmetric thermal hysteresis in proton conductivity is the phase stability arising from the shear elastic forces and a proton-conducting network in silica matrices.     

Phase transition mechanisms involved in the formation of structurally stable β-Ca3(PO4)2-α-Al2O3 composites

Composite powders comprising various proportions of β-Tricalcium phosphate [β-Ca₃(PO₄)₂] and α-Alumina (α-Al₂O₃) were synthesized by wet precipitation and then heat treated for drying and crystalline phase development. The phase formation mechanism was assessed through a set of characterization techniques including XRD, FT-IR and Raman spectra, and quantitative Rietveld refinement analysis. Al₂O₃ additions delayed the transformation kinetics from calcium deficient apatite to β-Ca₃(PO₄)₂ and preserved the thermal stability of β-Ca₃(PO₄)₂ − α-Al₂O₃ composites till 1400 °C. Such enhancement of thermal stability was due to the occupancy of Al³⁺ at both Ca²⁺(4) and Ca²⁺(5) lattice sites of β-Ca₃(PO₄)₂. Beyond the occupancy saturation limit for Al³⁺, the excess of aluminium crystallized as α-Al₂O₃. Morphological analysis revealed the growth of rod-like α-Al₂O₃ platelets on the surface of micron sized β-Ca₃(PO₄)₂ grains. The mechanical data obtained from indentation of bulk composites displayed enhanced hardness and Young’s modulus with increasing α-Al₂O₃ content in the composites.