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titania composites prepared by microwave sintering
Materials Chemistry and Physics 241 (2020) 122340
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Structure, mechanical and bioactive properties of nanostructured hydroxyapatite/titania composites prepared by microwave sintering Hai-Long Yao a, *, Chao Yang a, Qu Yang b, Xiao-Zhen Hu c, Meng-Xian Zhang a, Xiao-Bo Bai a, Hong-Tao Wang a, Qing-Yu Chen a a
Jiangxi Province Engineering Research Center of Materials Surface Enhancing & Remanufacturing, School of Mechanical and Materials Engineering, Jiujiang University, Jiujiang, 332005, China School of Science, Jiujiang University, Jiujiang, 332005, China c School of Civil Engineering and City Construction, Jiujiang University, Jiujiang, 332005, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� HA/TiO2 powders are prepared by TBT hydrolysis in HA particles suspension. � Sintering behavior of HA/TiO2 compos ites are discussed for microwave sintering. � HA/TiO2 composite exhibit uniform Ti rich particles, fine grain and dense structure. � Microhardness and elastic modulus depend on sintering temperature and TiO2 content. � New apatite is formed on surfaces of HA/TiO2 composites after SBF immersion.
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydroxyapatite/titania composite Microwave sintering Sintering temperature Mechanical property Titania addition
In order to improve the properties of hydroxyapatite (HA), HA/TiO2 composites with different TiO2 addition contents were prepared by microwave sintering. Effects of sintering temperature and TiO2 addition on phase composition, microstructure, mechanical properties and bioactivity were investigated. HA/TiO2 mixtures were prepared by the hydrolysis of tetrabutyl titanate (TBT) in HA particles suspension, which presented high specific surface areas and amorphous TiO2 phase. Compared to no phase change for pure HA, TiO2 addition promoted the HA decomposition by their mutual chemical reactions depending on the sintering temperature and TiO2 content. The sintering behavior of HA/TiO2 composites was also discussed in term of sintering temperature and TiO2 addition content. Ti atoms were uniformly dispersed in the HA/TiO2 composites with the formation of gradual Ti rich particles. TiO2 addition enhanced the sintering ability of HA powders, and HA/TiO2 composites exhibited densifications and fine grains at elevated temperature due to the nanosized raw powders and limited atomic diffusions induced by short sintering durations and byproduct suppressions. The sintering temperature can play a great effect on the microhardness and elastic modulus. New apatite was formed on the surfaces of HA/TiO2 composites after immersion in simulated body fluid, which indicates high bioactivities for composites prepared by microwave sintering.
* Corresponding author. E-mail address: [email protected] (H.-L. Yao). https://doi.org/10.1016/j.matchemphys.2019.122340 Received 8 August 2019; Received in revised form 10 October 2019; Accepted 17 October 2019 Available online 22 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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high mechanical properties. Therefore, the sintering temperature and duration should be properly controlled for HA/TiO2 composites. Microwave sintering can also sinter compacts at a high temperature with a shot duration. Compared to those sintering approaches [9–15], microwave sintering presents a high degree of control over the micro structure, no thermal gradient, uniform microstructures of the materials [11]. Pure HA, HA/ZrO2 [15] and HA/TiO2 [7] composites have been sintered by microwave sintering. Compared to the conventional pres sureless sintering, the pure HA compact sintered by microwave sintering presents a higher fracture toughness and finer grains [10,16]. Those results can also be found for two-step microwave sintering compared to the conventional two-step sintering [11,17]. It also reported that similar strengths are achieved between the microwave and conventionally sintered HA/ZrO2 composites, although the microwave-sintered one presents a lower density [18]. Recently, HA/TiO2 composites with different TiO2 contents are also sintered by microwave sintering, resulting in a decreased hardness and enhanced elastic modulus [7]. As widely reported [3–7], both the sintering temperature and TiO2 addition content had important effects on the phase, microstructures and me chanical properties of the HA/TiO2 composites. However, the effect of both sintering temperature and addition content on HA/TiO2 compos ites has not been reported for microwave sintering. Thereby, it is necessary to investigate the cross influences of sintering temperature and TiO2 addition. In this study, HA/TiO2 mixed powders with different TiO2 contents were synthesized by in-situ TBT hydrolysis in the nanosized HA particles suspension. Specific surface area, phase structure and composition of HA/TiO2 mixed powders were investigated. HA/TiO2 composites were prepared by microwave sintering at different temperatures. Effects of sintering temperature and TiO2 contents on phase composition, micro structure, mechanical properties and sintering behavior of HA/TiO2 composites were investigated and discussed.
1. Introduction Hydroxyapatite (Ca10(PO4)6(OH)2, HA) has a similar chemical composition and crystal structure with the mineral phase of natural bone and teeth. It is considered as a particularly attractive material for human tissue implantations due to its favorable osteoconductive and bioactive properties [1,2]. Compared to the cortical bone, the low strength and susceptibility to brittle fracture hinder the applications of HA for load-bearing implants [1,2]. In order to improve mechanical property of HA, adding secondary reinforcements, obtaining fine grains and enhancing inter-particle bonding are the effective and widely applied strategies. Titania (TiO2) has been widely considered as a biomaterial due to its excellent biocompatibility and high stability, and also presents a high specific strength and fracture toughness. Due to those advantages, TiO2 is always added into a HA matrix to improve mechanical properties [3–7]. Distribution state of TiO2 phase in HA matrix should be consid ered for HA/TiO2 composites. Ball milling can mix together solid het erogeneous particles and decrease their grain sizes resulting in uniformly dispersed reinforcements [8]. Nevertheless, the contamina tion from milling vials and grinding balls is one of the major problems for mechanically milled/alloyed materials [8]. Compared to the ball milling, precipitation method, sol gel method, wet chemical method and mechano-chemical method are widely used to mix nanosized particles or particles with precursor solutions, resulting in uniform dispersed nanosized reinforcements. Thus, it is possible to mix nanosized HA particles with TiO2 precursor to prepare HA/TiO2 mixtures with uni formly dispersed reinforcements. In addition, possible chemical re actions should also be given attentions in HA/TiO2 composites. Most studies report that pure HA is stable at a high sintering temperature, whereas the TiO2 trends to enhance the HA decomposition and chemi cally react with HA at a low temperature [3–7]. Both the phase decomposition and chemical reaction are harmful for bioactivities and stabilities of the HA/TiO2 composites. Effective particles bonding and fine grains can further improve the mechanical properties of HA-based composites. Many studies report that conventional (one- and two-step) sintering and hot pressing can increase the mechanical properties of HA-based composites [9–11]. However, the conventional one-step sintering often results in grain coarsening and decomposition of the HA due to its relatively high temperature and long duration [9,10]. Although the two-step sintering method can inhibit the grain growth, it often comprises of a second sintering step with a very long period [11]. A high-frequency induction heating sintering [12], spark plasma sintering [5], electric field assisted sintering [13] and se lective laser sintering [14], can sinter compacts at a high temperature with a short duration, which successfully inhibit excessive grain growths and provide high mechanical properties. Thus, avoiding exposing the compacts to high temperatures for a long duration is an effective strat egy to obtain fine grains, effective inter-particle bonding and thereby
2. Experimental procedure 2.1. Preparation of HA/TiO2 composites Fig. 1 shows a schematic diagram for preparation procedure of HA/ TiO2 composites. Commercial nanosized HA powders (20 nm, Beijin Deke Daojin Science and Technology Co. Ltd.), acetic acid and ethanol were mixed together and stirred for about 1 h to form particles sus pension. After that, tetrabutyl titanate (C16H36O4Ti, TBT, 99%) was dropped into the above suspension. In order to decrease the hydrolysis rate of TBT at room temperature, distilled water of 320 ml was also dropped into the mixture at room temperature with continuous stirring. Then, the mixture was let to stand at room temperature around 20 � C for 5 days. Then, the mixture was filtered, washed with distilled water, and dried at 50 � C for 2 days. Different TBT contents were controlled to obtain 10 wt%, 20 wt% and 30 wt% TiO2 in the HA/TiO2 mixed
Fig. 1. Schematic diagram of HA/TiO2 composites preparation procedure. 2
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Fig. 2. Morphology, N2 adsorption-desorption curves, XRD patterns and FTIR of pure HA and HA/TiO2 mixed powders. (a) Morphology of HA powder; (b) Morphology of H10T powder; (c) Morphology of H20T powder; (d) Morphology of H30T powder; (e) N2 adsorption-desorption curves of both pure HA and HA/TiO2 mixed powders; (f) XRD patterns of TiO2, pure HA and HA/TiO2 mixed powders; (g) FTIR spectrum of pure HA and HA/TiO2 mixed powders.
powders, which were labeled as H10T, H20T and H30T. HA/TiO2 mixed powders were uniaxially pressed into pellets using cylindrical steel mold (Ф ¼ 30 mm, h ¼ 20 mm) at a pressure of 20 MPa for 5 min. Then the pre-pressed ones were further compacted by cold isostatic pressing at a pressure of 170 MPa for 10 min. Pure HA compacts were prepared for comparison. Those pellets were sintered by microwave sintering at different temperatures from 800 to 1100 � C with the interval of 100 � C in N2 at mosphere and the duration was fixed to 15 min. The heating rate was 60 � C/min and cooling rate was 20 � C/min.
Table 1 N2 adsorption-desorption data of both pure HA and HA/TiO2 mixture powders. BET surface area (m2/g) Pore volume (cm3/ g) Pore diameter (nm)
HA
H10T
H20T
H30T
72.9 � 6.4
135.2 � 9.5
186.4 � 12.’
213.9 � 17.3
0.27 � 0.04
0.36 � 0.07
0.45 � 0.05
0.41 � 0.11
14.7 � 2.3
10.6 � 1.9
9.6 � 1.7
7.6 � 2.3
Vickers hardness tester (HVS-1000, Shanghai, China) with the load of 50 gf and the dwell time of 20s, which were performed on polished surfaces after being mounted by resin. The Knoop indenter was used to determine the Young’s modulus (E). An indentation load of 50 gf with a dwell time of 20s was also used for the indentation tests. The hardness and Young’s modulus were averaging at least ten tests for each specimens, respectively.
2.2. Phase and microstructure characterization Phase of both powders and compacts for pure HA and HA/TiO2 composites were analyzed by X-ray diffraction (XRD, D8 Advance, Bruker, Germany). The XRD characterization was operated in the re flected model with Cu Ka (λ ¼ 1.5418 Å) radiation (35 kV and 35 mA) and diffracted beam monochromator with a scan rate of 0.2� /s over a 2θ range of 20–60� . Phase compositions of powders were characterized by Fourier transform infrared spectroscopy (FTIR, MX-1E, Nicolet, Amer ican) using KBr pellets technique. The resolution of infrared spectrum was 6 cm 1, the scan number was 3, and the scan range was 400–4000 cm 1. Scanning electron microscopy (SEM, VEGAII, Tescan, Czech Re public) was applied to characterize morphology of the raw composite powders and fractured compacts. Apparent porosities of different sam ples were measured by photoshop method with at least three SEM im ages. Specific surface area and pore volume of the samples were measured by the Brunauer-Emmett-Teller (BET) nitrogen adsorption method in a TriStar II 3020 surface area analyzer. The BET were aver aging at least three tests for each specimen.
2.5. SBF immersion In-vitro tests were carried out by immersing HA/TiO2 composites sintered at 1100 � C into Hanks’ simulated body fluid (SBF) at a tem perature of 37 � 1 � C for 14 days as described as our pervious study [19]. The volume of SBF followed the equation: Vs ¼ Sa/10 [20], where Vs is the volume of SBF (ml) and Sa is the apparent surface area of specimen (mm2). For every 2 days, the medium was changed to preserve its freshness. After the in vitro tests, the samples were washed using distilled water and dried in air. Then, surface morphologies were observed by SEM. 3. Results and discussion
2.3. Thermal analysis
3.1. Characterization of HA/TiO2 mixed powders
A simultaneous high temperature thermogravimetry (TG) technique was employed to investigate the thermal behavior of HA/TiO2 mixed powders. Mass changes in the HA-based samples were measured by a TG Setaram Setsys Evolution 17 analyzer, in the temperature range from 25 to 1200 � C, with a scanning rate of 10 � C/min in alumina crucibles under Ar flow. Error of TG measurement is �0.154%.
Fig. 2a–d shows the morphologies of both pure HA and HA/TiO2 mixed powders. Due to the TBT hydrolysis in nanosized HA particles suspension, the HA/TiO2 mixtures were similar with the HA powders exhibiting nanosized agglomerates. Fig. 2e shows N2 adsorptiondesorption curves of both pure HA and HA/TiO2 mixed powders. Table 1 summaries the specific surface areas, pore volume and diameter of pure HA and different HA/TiO2 mixed powders. It can be found that the specific surface area and pore volume were increased with the TiO2 content, while the pore size was decreased. This result indicates that the particle sizes of the HA/TiO2 mixed powders were much smaller than
2.4. Mechanical properties Hardness of pure HA, HA/TiO2 composites were characterized by a 3
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Fig. 3. XRD patterns of pure and HA/TiO2 composites sintered at different temperatures. (a) Pure HA; (b) H10T; (c) H20T; (d) H30T.
that of the HA powder [21]. Fig. 2f shows XRD patterns of the HA/TiO2 composite powders. The HA/TiO2 mixed powders were mainly comprised of HA, and the in tensities of HA main peaks were decreased with the increase of TiO2 content in HA/TiO2 mixtures. Nevertheless, there was no peak about the hydrolysis product of TBT in all the mixed powders. Some studies report that those hydrolysis products can be transformed into anatase TiO2 particles after hydrothermal treatment [8]. However, the XRD pattern shows that there was an amorphous phase for the hydrolysis product of pure TBT. The formation of amorphous TiO2 can be attributed to the low temperature for the TBT hydrolysis [22]. Although there was no peak for amorphous TiO2 phase, it is reasonable to consider that the decrease of HA peaks resulted from the TiO2 additions. Thus, the HA/TiO2 mixed powders were mainly comprised of HA phase and amorphous TiO2 phase. Fig. 2g shows HA related peaks, such as OH at 1630 and 3730 cm 1, PO34 at 602-605 cm 1 (PO34 , v4), 962-973 cm 1 (PO34 , v1), 10271042 cm 1, 1084-1090 cm 1 and 1422-1450 cm 1 (PO34 , v3) [23]. The peak at 3450 cm 1 resulted from absorbed water. It can be found that the intensities of PO34 peaks were decreased with the TiO2 addition. Although there was no peak about TiO2 phase in XRD patterns, Ti-O bond can be observed from the FTIR spectrum. It is reported that broad band peaks at 450-860 cm 1 are related to the Ti-O peaks [24,25], which were also overlapped with the PO34 peaks at 602-605 cm 1. It can be observed that the intensities and breadths of those peaks were improved from H10T to H30T, indicating the increase of Ti-O content in HA/TiO2 mixed powders.
3.2. Phase analysis and thermal behaviors of HA/TiO2 composites It is reported that pure HA is usually stable at an elevated tempera ture, and the pure HA starts to decompose as the sintering temperature up to 1200 � C [26]. Fig. 3a shows the pure HA was stable at sintering temperature up to 1100 � C, although peak widths were decreased with the sintering temperature indicating grain growths. At sintering tem perature of 800 � C, anatase TiO2 was formed in all the composites and peak intensities of TiO2 were increased with TiO2 content as shown in Fig. 3b–d. Conventionally, addition of TiO2 will affect the thermal sta bility of HA in the HA/TiO2 composites. Although there was no Ca3(PO4)2 phase in H10T at 800 � C, Ca3(PO4)2 phase was observed in H20T and H30T at 800 � C. It is worthy to note that the Ca3(PO4)2 phase was related to the mutual reaction between HA and TiO2 (Eq. 1–2) due to the stable pure HA, although the CaTiO3 phase was not detected due to its little amount. Those results confirmed that the mutual reaction between HA and TiO2 was prompted by the TiO2 content. With the in crease in sintering temperature of 900–1100 � C, the peak intensities of Ca3(PO4)2 and CaTiO3 phases were gradually enhanced, and the HA and TiO2 phases were gradually reduced in HA/TiO2 composites. For HA/TiO2 composites at 1000 � C, the HA was disappeared and completely transformed into Ca3(PO4)2 and CaTiO3, and a part of anatase TiO2 was transformed to the rutile TiO2. The residual TiO2 phase was retained in H30T at 1100 � C, which maybe attributed to the rapid heating rate and a shot duration for microwave sintering [8]. The presence of Ca3(PO4)2 and CaTiO3 in the samples reduces bio compatibilities of the composites and becomes unstable [6]. Thus, those 4
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Fig. 4. Thermogravimetric curves of pure HA and HA/TiO2 mixed powders.
byproduct phases should be considered responsible for applications of HA/TiO2 composites in biological environments. Ca10(PO4)6(OH)2 → Ca3(PO4)2 þ CaO þ H2O↑
(1)
Ca10(PO4)6(OH)2 þ TiO2 → 3Ca3(PO4)2 þ CaTiO3 þ H2O↑
(2)
Fig. 5. Fracture morphologies of H30T composites sintered at different temperatures.
Mutual reaction between HA and TiO2 is related to not only the sintering temperature and duration, but also the raw powders. It is re ported that there is no decomposition and reaction in HA/TiO2 at 900 � C [3,27], and the decomposition and reaction finished at 1100 � C by conventional sintering process with duration of 1 h [3]. Most HA was retained in the HA/TiO2 composite after sintering at 1200 � C by spark plasma sintering with duration of 5 min, although the mutual reaction also occurred [27]. There was HA phase reserved in the HA/TiO2 composites after microwave sintering at 1250 � C with a duration of 40 min [7]. Those results imply the short duration at a high temperature can preserve a part of HA phase in HA/TiO2 composites. In present study, the composites were sintered by microwave sintering with duration of 15 min. Theoretically, a part of HA should be reserved in the HA/TiO2 composites in present study. However, the HA phase was completely transformed to Ca3(PO4)2 and CaTO3 phases at 1000 � C. This phenomenon can be attributed to the contact states between TiO2 and HA powders and TiO2 addition content. The contact state of the two powders depends on powder morphology, size, mixing degree and effi cient particle packing [28,29]. In the reported literature [3,7,27], the HA/TiO2 mixtures are mainly prepared by mixing solid HA and TiO2 powders by mechanical methods. In present study, the TiO2 particles were formed by the TBT hydrolysis in the HA particles suspension, resulting in the formation of HA/TiO2 nano agglomerates as shown in Fig. 2. It is reported that nanosized TiO2 particles formed by the in-situ hydrothermal treatment can cover the HA particles [30,31]. The mixing degree and efficient particle packing by in-situ hydrolysis were greater than mixing solid particles. Consequently, the dissolution of Ca2þ ions from the HA and the incorporation of Ti4þ ions into HA favored [3], resulting in the enhancement of the HA dissolution into Ca3(PO4)2 and mutual reactions between HA and TiO2. Therefore, the sinterability of HA was improved by the nanosized TiO2 particles formed by in-situ hydrolysis. In order to further analyze the thermal behavior of HA/TiO2 com posites, the thermogravimetric analysis was compared for both HA and HA/TiO2 composites as shown in Fig. 4. It can be found that the weight loss for HA/TiO2 mixtures was increased with the content of TiO2 pre cursor. The first stage, from room temperature to 230 � C, was related to the evaporation of physically absorbed water. It is obviously the HA/ TiO2 mixtures presented higher weight loss than the pure HA. The
second stage, from 230 to 500 � C, can be mainly attributed to the combustion and carbonization of residual organic components and removal of chemically absorbed water [22]. In addition, it can also be attributed to the removal of OH caused by the calcinations of amor phous TiO2 phase [32]. The last visible weight loss takes place between 900 and 1000 � C, which is attributed to the partial decomposition of HA to tricalcium phosphate or mutual reactions between HA and TiO2 [33]. The weight loss was constant after the finish of decomposition and mutual reaction as the sintering temperature up to 1100 � C, which is consistent with the XRD patterns. 3.3. Microstructures of HA/TiO2 composites Fig. 5 shows fracture morphologies of H30T composites sintered at different temperatures. Evolutions of pore and grain size significantly depended on the sintering temperature. It can be found that H30T exhibited nanosized agglomerates at 800 � C without visible fuses. As up to 900 � C, a porous structure was formed with the apparent porosity of 10.4 � 0.7% and was composed of submicron grains. As higher than 1000 � C, the composites presented a significant densification with grain growths and pores became near-spherical shapes. The apparent porosity for 1000 � C and 1100 � C was 4.8 � 0.4% and 4.3 � 0.5%, respectively. In addition, the gradual particles were formed and uniformly dispersed at 1000 � C and 1100 � C (as labeled by arrows in Fig. 5c–d). Combining with the XRD results, those gradual particles can be mainly related to the CaTiO3 or TiO2 phase. Atomic distribution in the H30T at 1100 � C was investigated by EDS mapping as shown in Fig. 6. It can be found that Ti, P, Ca and O atoms were uniformly dispersed in the composite. The effect of TiO2 addition content on the sintering behavior of HA/ TiO2 composites was also investigated in term of 1100 � C. The pure HA exhibited a structure with the apparent porosity of 4.4 � 0.4% and fine grain sizes around 200 nm and irregular pores (Fig. 7a). After TiO2 addition, the HA/TiO2 composites show similar pores but larger grains. The apparent porosity for H10T, H20T and H30T was 5.4 � 0.6%, 2.7 � 0.2% and 4.3 � 0.5%, respectively. After the TBT hydrolysis in the HA particles suspension, HA particles were covered with TiO2 precursor resulting in enhancement of the mutual reaction between TiO2 and HA. The large grains can be attributed to the improved sinterabilities of HA/ 5
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Fig. 6. Atomic distributions in H30T sintered at 1100 � C.
Fig. 7b–d). Those results indicate that the TiO2 addition can improve the sinterabilities of the HA/TiO2 composites. The contents of different atoms in different HA/TiO2 composites of 1100 � C were listed in Table 2. Ti contents in the HA/TiO2 composites were increased with the TiO2 content in the raw powders. In addition, the Ca/P atomic ratio was 1.445, 1.435 and 1.538 for H10T, H20T and H30T respectively, which was less than the pure HA of 1.632. Those results were consistent with the XRD results, that Ca3(PO4)2 and CaTiO3 were the main phases in the HA/TiO2 composite at 1100 � C. In present study, it can be observed that both pure HA and HA/TiO2 composites exhibited fine grains compared to conventional sintering method [9,10], and was similar with microwave sintering [7] and spark plasma sintering processes [5]. It is reported that when the HA/TiO2 composites were prepared by microwave sintering at 1250 � C for 40 min, the grain sizes were around 500-600 nm [7]. This phenomena was similar with the HA/TiO2 composites prepared by spark plasma sintering processes [5]. Those fine grain sizes can be attributed to the raw powder and sintering process. One the one hand, both the HA and TiO2 particles in the composites were nanometers and the raw powders presented high specific surface areas. The chemical reaction and the HA decomposition were promoted, resulting in the formation of Ca3(PO4)2 and CaTiO3 phases at a low sintering temperature. Although the nano sized powders favored densifications and atomic diffusions, the CaTiO3 phase on the grain boundaries of Ca3(PO4)2 phase can effectively inhibit the grain growth. On the other hand, the sintering process presented a low sintering temperature and a short duration, which inhibits grain growths. This was similar with the first stage of the two-step sintering method [11], which sintered the specimens at a high temperature with a short duration. Therefore, the HA/TiO2 composites in present study exhibited dense microstructures and fine grains.
TiO2 mixtures due to their high specific surface areas. Compared to the pure HA, gradual particles less than 200 nm were formed and uniformly dispersed onto HA/TiO2 composites (as labeled by white arrows in
Fig. 7. Fracture morphologies of pure HA and HA/TiO2 composites sintered at 1100 � C. Table 2 Elements weight and atomic in the pure HA and HA/TiO2 composites at 1100 � C. Pure HA Element OK PK Ca K Ti K Ca/P
Wt.% 37.09 20.22 42.70 –
H10T At.% 57.44 16.17 26.39 – 1.632
Wt.% 41.34 18.09 33.84 6.72
H20T At.% 62.22 14.07 20.33 3.38 1.445
6
Wt.% 43.87 16.79 31.20 8.14
H30T At.% 64.79 12.81 18.39 4.02 1.435
Wt.% 44.28 13.23 25.89 16.60
At.% 66.10 10.20 15.42 8.27 1.538
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Fig. 8. Mechanical properties of pure HA and HA/TiO2 composites. (a) Microhardness; (b) Elastic modulus.
Fig. 9. Surface morphologies and EDS of pure HA and HA/TiO2 composites sintered at 1100 � C after SBF immersion for 14 days.
3.4. Mechanical properties of HA/TiO2 composites
depend on the sintering temperature and its content. HA/TiO2 com posites can show higher elastic modulus than the pure HA, while only the H20T and H30T at 1000 � C presented significantly higher micro hardness than the pure HA. The H30T at 1000 � C exhibited the highest microhardness and the highest elastic modulus. Those evolutions for mechanical properties can be attributed to the effects of both sintering temperature and TiO2 addition. On the one hand, the densification can increase the strengths, although the
HA is conventionally brittle and has a low strength, which limits its load-bearing applications. TiO2 is added into the HA matrix in order to enhance the mechanical properties of HA/TiO2 composites. Both microhardness and elastic modulus for specimens exhibited improve ments with the sintering temperature, except for the H30T at 1100 � C, as shown in Fig. 8. Nevertheless, the effects of TiO2 reinforcement may 7
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formation of soft TCP and grain coarsening derived by the sintering temperature will decrease strengths of the HA/TiO2 composites [8]. The formation of CaTiO3 and TiO2 phase at the Ca3(PO4)2 boundaries can effectively inhibit the grain growths [3–7]. After additions of TiO2 reinforcement, it is expected that the formation of CaTiO3 phase, TiO2 particles and densifications can favor the strengths of HA/TiO2 com posites. However, the TiO2 promoted the HA decomposition and the formation of soft Ca3(PO4)2 phase with the grain growths compared to the pure HA, which could decrease the reinforcing effect of TiO2 addi tion. Thereby, the H30T showed a decreased mcirohardness and elastic modulus at 1100 � C. This phenomenon is possibly attributed to the formation of the multi-phases in the composites and gain coarsening, which was widely reported by other studies [5,7,10,27]. Compared to the HA/TiO2 composites prepared by other studies [3–7], the HA/TiO2 composites in present study have a slightly lower microhardness and elastic modulus. This could be attributed to the low sintering tempera ture and short durations during microwave sintering, which results in insufficient particles bonding and low densifications. However, those HA/TiO2 composites exhibited comparable elastic modulus with natural bones [4,34], and the cancellous bone presents elastic modulus of 0.05–0.1 GPa [35]. It can expected that the HA/TiO2 composites pre pared by microwave sintering can be applied for the implant substitutes.
composites exhibited improvements in microhardness and elastic modulus due to the effect of both sintering temperature and TiO2 addition. New apatite was formed on the surfaces of HA/TiO2 compos ites after immersion for 14 days indicating high bioactivities for com posites prepared by microwave sintering. Acknowledgments The work was supported by Science Technology Project of Jiangxi Province (Grant numbers 20192BAB216004, 20192BAB206006, 20171BAB206007), National Natural Science Foundation of China (No. 51561013, 51861012), Science and Technology Planning Program of Jiangxi Provincial Education Department (No. GJJ161068, GJJ170946), Base and Talent/Outstanding Young Talent Program of Jiujiang Science and Technology (No. [2016]43(75)). Thanks Qu Yang for improving English presentations. References [1] G. Lewis, Nanostructured hydroxyapatite coating on bioalloy substrates: current status and future directions, J. Adv. Nanomater. 2 (2017) 18. [2] A. Sola, D. Bellucci, V. Cannillo, Functionally graded materials for orthopedic applications - an update on design and manufacturing, Biotechnol. Adv. 34 (2016) 504–531. [3] I. Kutbay, B. Yilmaz, Z. Evis, M. Usta, Effect of calcium fluoride on mechanical behavior and sinterability of nano-hydroxyapatite and titania composites, Ceram. Int. 40 (2014) 14817–14826. [4] E. Fidancevska, G. Ruseska, J. Bossert, Y.M. Lin, A.R. Boccaccini, Fabrication and characterization of porous bioceramic composites based on hydroxyapatite and titania, Mater. Chem. Phys. 103 (2007) 95–100. [5] W.X. Que, K.A. Khor, J.L. Xu, L.G. Yu, Hydroxyapatite/titania nanocomposites derived by combining high-energy ball milling with spark plasma sintering processes, J. Eur. Ceram. Soc. 28 (2008) 3083–3090. [6] M. Mahmoodi, P.M. Hashemi, R. Imani, Characterization of a novel nanobiomaterial fabricated from HA, TiO2 and Al2O3 powders: an in vitro study, Prog. Biomater. 3 (2014) 1–10. [7] J.Z. Jun, F. Nie, X.G. Huang, L. Wang, G.Z. Liu, J.P. Cheng, Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering, Ceram. Int. (2019), https://doi.org/10.1016/j.ceramint.2019.04.007. [8] A.E. Hannora, S. Ataya, Structure and compression strength of hydroxyapatite/ titania nanocomposites formed by high energy ball milling, J. Alloy. Comp. 658 (2016) 222–233. [9] K.C.B. Yeong, J. Wang, S.C. Ng, Fabricating densified hydroxyapatite ceramics from a precipitated precursor, Mater. Lett. 38 (1999) 0–213. [10] S. Ramesh, C.Y. Tan, S.B. Bhaduri, W.D. Teng, I. Sopyan, Densification behaviour of nanocrystalline hydroxyapatite bioceramics, J. Mater. Process. Technol. 206 (2008) 221–230. [11] D. Veljovic, E. Palcevskis, I. Zalite, R. Petrovic, D. Janackovic, Two-step microwave sintering-A promising technique for the processing of nanostructured bioceramics, Mater. Lett. 93 (2013) 251–253. [12] S.W. Kim, K.A. Khalil, S.L. Cockcroft, D. Hui, J.H. Lee, Sintering behavior and mechanical properties of HA-X% mol 3YSZ composites sintered by high frequency induction heated sintering, Compos. B Eng. 45 (2013) 1689–1693. [13] J. Yun, W. Qin, K. van Benthem, A.M. Thron, S. Kim, Y.H. Han, Nanovoids in dense hydroxyapatite ceramics after electric field assisted sintering, Adv. Appl. Ceram. 117 (2018) 376–382. [14] T. Kumaresan, R. Gandhinathan, M. Ramu, M. Ananthasubramanian, K. B. Pradheepa, Design, analysis and fabrication of polyamide/hydroxyapatite porous structured scaffold using selective laser sintering method for bio-medical applications, J. Mech. Sci. Technol. 30 (2016) 5305–5312. [15] D.W. Jang, T.H. Nguyen, S.K. Sarkar, B.T. Lee, Microwave sintering and in vitro study of defect-free stable porous multilayered HAp-ZrO2 artificial bone scaffold, Sci. Technol. Adv. Mater. 13 (2012), 035009. [16] M.G. Kutty, S.B. Bhaduri, H. Zhou, A. Yaghoubi, In situ measurement of shrinkage and temperature profile in microwave- and conventionally-sintered hydroxyapatite bioceramic, Mater. Lett. 161 (2015) 375–378. [17] K. Lin, L. Chen, J. Chang, Fabrication of dense hydroxyapatite nanobioceramics with enhanced mechanical properties via two-step sintering process, Int. J. Appl. Ceram. Technol. 9 (2012) 479–485. [18] D.J. Curran, T.J. Fleming, M.R. Towler, S. Hampshire, Mechanical properties of hydroxyapatite–zirconia compacts sintered by two different sintering methods, J. Mater. Sci. Mater. Med. 21 (2009) 1109–1120. [19] H.L. Yao, G.C. Ji, Q.Y. Chen, X.B. Bai, Y.L. Zou, H.T. Wang, Microstructures and properties of warm-sprayed carbonated hydroxyapatite coatings, J. Therm. Spray Technol. 27 (2018) 924–937. [20] T. Kokubo, H. Takadama, How useful is SBF in predicting in vivo bone bioactivity? Biomaterials 27 (2006) 2907–2915. [21] X.Y. Guo, P. Xiao, J. Liu, Z.J. Shen, Fabrication of nanostructured hydroxyapatite via hydrothermal synthesis and spark plasma sintering, J. Am. Ceram. Soc. 88 (2005) 1026–1029.
3.5. In-vitro tests of HA/TiO2 composites In order to investigate bioactivities of the HA/TiO2 composites, the composites with different TiO2 additions sintered at 1100 � C were immersed in the Hanks’ SBF with a period of 14 days. The surfaces of different samples after immersion showed significantly different from the fractured surfaces. From Fig. 9, uniform grains and pores were dis appeared; rough and particle agglomerates were observed for all the samples. EDS results show that the Ca/P atomic ratios for those ag glomerates were around 1.8 and significantly higher than that of sam ples before immersion, which were related to the new apatite phase. Formation of new agglomerates in Fig. 9 can be attributed to the dissolution and precipitation of Ca2þ and PO44 in the compacts and SBF solution [27]. On the one hand, the Ca2þ and PO44 are released from the HA or HA/TiO2 composites into the solution, which results in the super-saturation of Ca2þ and PO44 in the SBF. On other hand, the super-saturation of Ca2þ and PO44 in the SBF are deposited onto the surfaces of composites. It is reported that both the TiO2 and HA have a good biocompatibility by inducing apatite nucleation on the sample’s surface after being immersed in SBF [27,36]. Although the in-vitro test is considered to have some limitations, the apatite-forming ability in SBF is generally used as a bioactivity index and is often used to predict the bone-forming ability in vivo [20]. Therefore, the HA/TiO2 composites prepared by microwave sintering exhibited high bioactivities and can be used as substitutes for the bone defects in the future. 4. Conclusions HA/TiO2 composites with different TiO2 addition contents (10 wt%, 20 wt% and 30 wt%) were prepared by microwave sintering at different temperatures. HA/TiO2 mixtures were prepared by the TBT hydrolysis in the HA particles suspension, which exhibited amorphous TiO2 phase and high specific surface areas with TiO2 content. Compared to the phase stable of pure HA, TiO2 promoted the HA decomposition and mutual reactions during sintering, resulting in that main phases in HA/ TiO2 composites varied from HA and anatase TiO2 phases at 800 � C to Ca3(PO4)2, CaTiO3, even TiO2 phases at 1100 � C. The sintering behavior of HA/TiO2 composites was discussed in term of sintering temperature and TiO2 addition. Both HA and HA/TiO2 composites exhibited high densifications and fine grains at elevated temperature due to the nano sized raw and limited atomic diffusions induced by shot sintering du rations and byproduct suppressions. Gradual CaTiO3 particles were formed in HA/TiO2 composites sintered at 1000 and 1100 � C. HA/TiO2 8
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[22] K.C. Song, S.E. Pratsinis, The effect of alcohol solvents on the porosity and phase composition of titania, J. Colloid Interface Sci. 231 (2000) 289–298. [23] H.L. Yao, H.T. Wang, X.B. Bai, G.C. Ji, Q.Y. Chen, Improvement in mechanical properties of nano-structured HA/TiO2 multilayer coatings deposited by high velocity suspension flame spraying (HVSFS), Surf. Coat. Technol. 342 (2018) 94–104. [24] K.K.M. Miyayama, H. Yanagida, Engineering properties of single oxides, in: S. J. Schneider (Ed.), Engineered Materials Handbook, 4-Ceramic and Glasses, ASM International, Materials Park, OH, USA, 1991, pp. 748–757. [25] N. Binu, P.K. Jaseela, P. Pradeepan, Direct sunlight active Sm3þ doped TiO2 photocatalyst, Mater. Sci. Forum 855 (2016) 33–44. [26] D.S. Seo, K.H. Hwang, J.K. Lee, Nanostructured hydroxyapatite by microwave sintering, J. Nanosci. Nanotechnol. 8 (2008) 944–948. [27] W. Que, K.A. Khor, J.L. Xu, L.G. Yu, Effects of titania content and sintering temperature on structural, mechanical and bioactive behaviors of titania reinforced hydroxyapatite nanocomposites, Adv. Eng. Mater. 10 (2010) B53–B59. [28] J. Reed, Introduction to the Principles of Ceramic Processing, John Wiley And SonsInc., NewYork, 1988. [29] H.W. Kim, Y.J. Noh, Y.H. Koh, H.E. Kim, H.M. Kim, Effect of CaF2 on densification and properties of hydroxyapatite-zirconia composites for biomedical applications, Biomaterials 23 (2002) 4113–4121.
[30] B. Cheng, X.L. Chen, X.W. Wang, H. Qiu, S.H. Qi, Preparation and microwave absorbing performance of TiO2/NiFe2O4/hollow glass microsphere composite with core-shell structure, J. Mater. Sci. Mater. Electron. 28 (2017) 7575–7581. [31] Y. Yang, Q. Chang, Z. Hu, X. Zhang, A comparative study on the addition methods of TiO2 sintering aid to the properties of porous alumina membrane support, Membranes 8 (2018) 49. [32] A. Kurosaki, S. Okazaki, The effects of the addition of sulfate ions on the acidity of TiO2, Nippon Kagaku Kaushi (1976) 1816–1821. [33] M. Salarian, W.Z. Xu, Z. Wang, T.K. Sham, P.A. Charpentier, Hydroxyapatite-TiO2based nanocomposites synthesized in supercritical CO2 for bone tissue engineering: physical and mechanical properties, ACS Appl. Mater. Interfaces 6 (2014) 16918–16931. [34] H. Koelmans, J.J. Engelsman, P.S. Admiraal, Low-temperature phase transitions in β-Ca3(PO4)2 and related compounds, J. Phys. Chem. Solids 11 (1959) 172–173. [35] J.S. Temenoff, L. Lu, A.G. Mikos, Bone-tissue engineering using synthetic biodegradable polymer scaffolds, in: J. Davies (Ed.), Bone Engineering, 2000, p. 454. [36] L.G. Yu, K.A. Khor, H. Li, P. Cheang, Effect of spark plasma sintering on the microstructure and in vitro behavior of plasma sprayed HA coatings, Biomaterials 24 (2003) 2695–2705.
9
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Structure, mechanical and bioactive properties of nanostructured hydroxyapatite/titania composites prepared by microwave sintering Hai-Long Yao a, *, Chao Yang a, Qu Yang b, Xiao-Zhen Hu c, Meng-Xian Zhang a, Xiao-Bo Bai a, Hong-Tao Wang a, Qing-Yu Chen a a
Jiangxi Province Engineering Research Center of Materials Surface Enhancing & Remanufacturing, School of Mechanical and Materials Engineering, Jiujiang University, Jiujiang, 332005, China School of Science, Jiujiang University, Jiujiang, 332005, China c School of Civil Engineering and City Construction, Jiujiang University, Jiujiang, 332005, China b
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� HA/TiO2 powders are prepared by TBT hydrolysis in HA particles suspension. � Sintering behavior of HA/TiO2 compos ites are discussed for microwave sintering. � HA/TiO2 composite exhibit uniform Ti rich particles, fine grain and dense structure. � Microhardness and elastic modulus depend on sintering temperature and TiO2 content. � New apatite is formed on surfaces of HA/TiO2 composites after SBF immersion.
A R T I C L E I N F O
A B S T R A C T
Keywords: Hydroxyapatite/titania composite Microwave sintering Sintering temperature Mechanical property Titania addition
In order to improve the properties of hydroxyapatite (HA), HA/TiO2 composites with different TiO2 addition contents were prepared by microwave sintering. Effects of sintering temperature and TiO2 addition on phase composition, microstructure, mechanical properties and bioactivity were investigated. HA/TiO2 mixtures were prepared by the hydrolysis of tetrabutyl titanate (TBT) in HA particles suspension, which presented high specific surface areas and amorphous TiO2 phase. Compared to no phase change for pure HA, TiO2 addition promoted the HA decomposition by their mutual chemical reactions depending on the sintering temperature and TiO2 content. The sintering behavior of HA/TiO2 composites was also discussed in term of sintering temperature and TiO2 addition content. Ti atoms were uniformly dispersed in the HA/TiO2 composites with the formation of gradual Ti rich particles. TiO2 addition enhanced the sintering ability of HA powders, and HA/TiO2 composites exhibited densifications and fine grains at elevated temperature due to the nanosized raw powders and limited atomic diffusions induced by short sintering durations and byproduct suppressions. The sintering temperature can play a great effect on the microhardness and elastic modulus. New apatite was formed on the surfaces of HA/TiO2 composites after immersion in simulated body fluid, which indicates high bioactivities for composites prepared by microwave sintering.
* Corresponding author. E-mail address: [email protected] (H.-L. Yao). https://doi.org/10.1016/j.matchemphys.2019.122340 Received 8 August 2019; Received in revised form 10 October 2019; Accepted 17 October 2019 Available online 22 October 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.
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high mechanical properties. Therefore, the sintering temperature and duration should be properly controlled for HA/TiO2 composites. Microwave sintering can also sinter compacts at a high temperature with a shot duration. Compared to those sintering approaches [9–15], microwave sintering presents a high degree of control over the micro structure, no thermal gradient, uniform microstructures of the materials [11]. Pure HA, HA/ZrO2 [15] and HA/TiO2 [7] composites have been sintered by microwave sintering. Compared to the conventional pres sureless sintering, the pure HA compact sintered by microwave sintering presents a higher fracture toughness and finer grains [10,16]. Those results can also be found for two-step microwave sintering compared to the conventional two-step sintering [11,17]. It also reported that similar strengths are achieved between the microwave and conventionally sintered HA/ZrO2 composites, although the microwave-sintered one presents a lower density [18]. Recently, HA/TiO2 composites with different TiO2 contents are also sintered by microwave sintering, resulting in a decreased hardness and enhanced elastic modulus [7]. As widely reported [3–7], both the sintering temperature and TiO2 addition content had important effects on the phase, microstructures and me chanical properties of the HA/TiO2 composites. However, the effect of both sintering temperature and addition content on HA/TiO2 compos ites has not been reported for microwave sintering. Thereby, it is necessary to investigate the cross influences of sintering temperature and TiO2 addition. In this study, HA/TiO2 mixed powders with different TiO2 contents were synthesized by in-situ TBT hydrolysis in the nanosized HA particles suspension. Specific surface area, phase structure and composition of HA/TiO2 mixed powders were investigated. HA/TiO2 composites were prepared by microwave sintering at different temperatures. Effects of sintering temperature and TiO2 contents on phase composition, micro structure, mechanical properties and sintering behavior of HA/TiO2 composites were investigated and discussed.
1. Introduction Hydroxyapatite (Ca10(PO4)6(OH)2, HA) has a similar chemical composition and crystal structure with the mineral phase of natural bone and teeth. It is considered as a particularly attractive material for human tissue implantations due to its favorable osteoconductive and bioactive properties [1,2]. Compared to the cortical bone, the low strength and susceptibility to brittle fracture hinder the applications of HA for load-bearing implants [1,2]. In order to improve mechanical property of HA, adding secondary reinforcements, obtaining fine grains and enhancing inter-particle bonding are the effective and widely applied strategies. Titania (TiO2) has been widely considered as a biomaterial due to its excellent biocompatibility and high stability, and also presents a high specific strength and fracture toughness. Due to those advantages, TiO2 is always added into a HA matrix to improve mechanical properties [3–7]. Distribution state of TiO2 phase in HA matrix should be consid ered for HA/TiO2 composites. Ball milling can mix together solid het erogeneous particles and decrease their grain sizes resulting in uniformly dispersed reinforcements [8]. Nevertheless, the contamina tion from milling vials and grinding balls is one of the major problems for mechanically milled/alloyed materials [8]. Compared to the ball milling, precipitation method, sol gel method, wet chemical method and mechano-chemical method are widely used to mix nanosized particles or particles with precursor solutions, resulting in uniform dispersed nanosized reinforcements. Thus, it is possible to mix nanosized HA particles with TiO2 precursor to prepare HA/TiO2 mixtures with uni formly dispersed reinforcements. In addition, possible chemical re actions should also be given attentions in HA/TiO2 composites. Most studies report that pure HA is stable at a high sintering temperature, whereas the TiO2 trends to enhance the HA decomposition and chemi cally react with HA at a low temperature [3–7]. Both the phase decomposition and chemical reaction are harmful for bioactivities and stabilities of the HA/TiO2 composites. Effective particles bonding and fine grains can further improve the mechanical properties of HA-based composites. Many studies report that conventional (one- and two-step) sintering and hot pressing can increase the mechanical properties of HA-based composites [9–11]. However, the conventional one-step sintering often results in grain coarsening and decomposition of the HA due to its relatively high temperature and long duration [9,10]. Although the two-step sintering method can inhibit the grain growth, it often comprises of a second sintering step with a very long period [11]. A high-frequency induction heating sintering [12], spark plasma sintering [5], electric field assisted sintering [13] and se lective laser sintering [14], can sinter compacts at a high temperature with a short duration, which successfully inhibit excessive grain growths and provide high mechanical properties. Thus, avoiding exposing the compacts to high temperatures for a long duration is an effective strat egy to obtain fine grains, effective inter-particle bonding and thereby
2. Experimental procedure 2.1. Preparation of HA/TiO2 composites Fig. 1 shows a schematic diagram for preparation procedure of HA/ TiO2 composites. Commercial nanosized HA powders (20 nm, Beijin Deke Daojin Science and Technology Co. Ltd.), acetic acid and ethanol were mixed together and stirred for about 1 h to form particles sus pension. After that, tetrabutyl titanate (C16H36O4Ti, TBT, 99%) was dropped into the above suspension. In order to decrease the hydrolysis rate of TBT at room temperature, distilled water of 320 ml was also dropped into the mixture at room temperature with continuous stirring. Then, the mixture was let to stand at room temperature around 20 � C for 5 days. Then, the mixture was filtered, washed with distilled water, and dried at 50 � C for 2 days. Different TBT contents were controlled to obtain 10 wt%, 20 wt% and 30 wt% TiO2 in the HA/TiO2 mixed
Fig. 1. Schematic diagram of HA/TiO2 composites preparation procedure. 2
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Fig. 2. Morphology, N2 adsorption-desorption curves, XRD patterns and FTIR of pure HA and HA/TiO2 mixed powders. (a) Morphology of HA powder; (b) Morphology of H10T powder; (c) Morphology of H20T powder; (d) Morphology of H30T powder; (e) N2 adsorption-desorption curves of both pure HA and HA/TiO2 mixed powders; (f) XRD patterns of TiO2, pure HA and HA/TiO2 mixed powders; (g) FTIR spectrum of pure HA and HA/TiO2 mixed powders.
powders, which were labeled as H10T, H20T and H30T. HA/TiO2 mixed powders were uniaxially pressed into pellets using cylindrical steel mold (Ф ¼ 30 mm, h ¼ 20 mm) at a pressure of 20 MPa for 5 min. Then the pre-pressed ones were further compacted by cold isostatic pressing at a pressure of 170 MPa for 10 min. Pure HA compacts were prepared for comparison. Those pellets were sintered by microwave sintering at different temperatures from 800 to 1100 � C with the interval of 100 � C in N2 at mosphere and the duration was fixed to 15 min. The heating rate was 60 � C/min and cooling rate was 20 � C/min.
Table 1 N2 adsorption-desorption data of both pure HA and HA/TiO2 mixture powders. BET surface area (m2/g) Pore volume (cm3/ g) Pore diameter (nm)
HA
H10T
H20T
H30T
72.9 � 6.4
135.2 � 9.5
186.4 � 12.’
213.9 � 17.3
0.27 � 0.04
0.36 � 0.07
0.45 � 0.05
0.41 � 0.11
14.7 � 2.3
10.6 � 1.9
9.6 � 1.7
7.6 � 2.3
Vickers hardness tester (HVS-1000, Shanghai, China) with the load of 50 gf and the dwell time of 20s, which were performed on polished surfaces after being mounted by resin. The Knoop indenter was used to determine the Young’s modulus (E). An indentation load of 50 gf with a dwell time of 20s was also used for the indentation tests. The hardness and Young’s modulus were averaging at least ten tests for each specimens, respectively.
2.2. Phase and microstructure characterization Phase of both powders and compacts for pure HA and HA/TiO2 composites were analyzed by X-ray diffraction (XRD, D8 Advance, Bruker, Germany). The XRD characterization was operated in the re flected model with Cu Ka (λ ¼ 1.5418 Å) radiation (35 kV and 35 mA) and diffracted beam monochromator with a scan rate of 0.2� /s over a 2θ range of 20–60� . Phase compositions of powders were characterized by Fourier transform infrared spectroscopy (FTIR, MX-1E, Nicolet, Amer ican) using KBr pellets technique. The resolution of infrared spectrum was 6 cm 1, the scan number was 3, and the scan range was 400–4000 cm 1. Scanning electron microscopy (SEM, VEGAII, Tescan, Czech Re public) was applied to characterize morphology of the raw composite powders and fractured compacts. Apparent porosities of different sam ples were measured by photoshop method with at least three SEM im ages. Specific surface area and pore volume of the samples were measured by the Brunauer-Emmett-Teller (BET) nitrogen adsorption method in a TriStar II 3020 surface area analyzer. The BET were aver aging at least three tests for each specimen.
2.5. SBF immersion In-vitro tests were carried out by immersing HA/TiO2 composites sintered at 1100 � C into Hanks’ simulated body fluid (SBF) at a tem perature of 37 � 1 � C for 14 days as described as our pervious study [19]. The volume of SBF followed the equation: Vs ¼ Sa/10 [20], where Vs is the volume of SBF (ml) and Sa is the apparent surface area of specimen (mm2). For every 2 days, the medium was changed to preserve its freshness. After the in vitro tests, the samples were washed using distilled water and dried in air. Then, surface morphologies were observed by SEM. 3. Results and discussion
2.3. Thermal analysis
3.1. Characterization of HA/TiO2 mixed powders
A simultaneous high temperature thermogravimetry (TG) technique was employed to investigate the thermal behavior of HA/TiO2 mixed powders. Mass changes in the HA-based samples were measured by a TG Setaram Setsys Evolution 17 analyzer, in the temperature range from 25 to 1200 � C, with a scanning rate of 10 � C/min in alumina crucibles under Ar flow. Error of TG measurement is �0.154%.
Fig. 2a–d shows the morphologies of both pure HA and HA/TiO2 mixed powders. Due to the TBT hydrolysis in nanosized HA particles suspension, the HA/TiO2 mixtures were similar with the HA powders exhibiting nanosized agglomerates. Fig. 2e shows N2 adsorptiondesorption curves of both pure HA and HA/TiO2 mixed powders. Table 1 summaries the specific surface areas, pore volume and diameter of pure HA and different HA/TiO2 mixed powders. It can be found that the specific surface area and pore volume were increased with the TiO2 content, while the pore size was decreased. This result indicates that the particle sizes of the HA/TiO2 mixed powders were much smaller than
2.4. Mechanical properties Hardness of pure HA, HA/TiO2 composites were characterized by a 3
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Fig. 3. XRD patterns of pure and HA/TiO2 composites sintered at different temperatures. (a) Pure HA; (b) H10T; (c) H20T; (d) H30T.
that of the HA powder [21]. Fig. 2f shows XRD patterns of the HA/TiO2 composite powders. The HA/TiO2 mixed powders were mainly comprised of HA, and the in tensities of HA main peaks were decreased with the increase of TiO2 content in HA/TiO2 mixtures. Nevertheless, there was no peak about the hydrolysis product of TBT in all the mixed powders. Some studies report that those hydrolysis products can be transformed into anatase TiO2 particles after hydrothermal treatment [8]. However, the XRD pattern shows that there was an amorphous phase for the hydrolysis product of pure TBT. The formation of amorphous TiO2 can be attributed to the low temperature for the TBT hydrolysis [22]. Although there was no peak for amorphous TiO2 phase, it is reasonable to consider that the decrease of HA peaks resulted from the TiO2 additions. Thus, the HA/TiO2 mixed powders were mainly comprised of HA phase and amorphous TiO2 phase. Fig. 2g shows HA related peaks, such as OH at 1630 and 3730 cm 1, PO34 at 602-605 cm 1 (PO34 , v4), 962-973 cm 1 (PO34 , v1), 10271042 cm 1, 1084-1090 cm 1 and 1422-1450 cm 1 (PO34 , v3) [23]. The peak at 3450 cm 1 resulted from absorbed water. It can be found that the intensities of PO34 peaks were decreased with the TiO2 addition. Although there was no peak about TiO2 phase in XRD patterns, Ti-O bond can be observed from the FTIR spectrum. It is reported that broad band peaks at 450-860 cm 1 are related to the Ti-O peaks [24,25], which were also overlapped with the PO34 peaks at 602-605 cm 1. It can be observed that the intensities and breadths of those peaks were improved from H10T to H30T, indicating the increase of Ti-O content in HA/TiO2 mixed powders.
3.2. Phase analysis and thermal behaviors of HA/TiO2 composites It is reported that pure HA is usually stable at an elevated tempera ture, and the pure HA starts to decompose as the sintering temperature up to 1200 � C [26]. Fig. 3a shows the pure HA was stable at sintering temperature up to 1100 � C, although peak widths were decreased with the sintering temperature indicating grain growths. At sintering tem perature of 800 � C, anatase TiO2 was formed in all the composites and peak intensities of TiO2 were increased with TiO2 content as shown in Fig. 3b–d. Conventionally, addition of TiO2 will affect the thermal sta bility of HA in the HA/TiO2 composites. Although there was no Ca3(PO4)2 phase in H10T at 800 � C, Ca3(PO4)2 phase was observed in H20T and H30T at 800 � C. It is worthy to note that the Ca3(PO4)2 phase was related to the mutual reaction between HA and TiO2 (Eq. 1–2) due to the stable pure HA, although the CaTiO3 phase was not detected due to its little amount. Those results confirmed that the mutual reaction between HA and TiO2 was prompted by the TiO2 content. With the in crease in sintering temperature of 900–1100 � C, the peak intensities of Ca3(PO4)2 and CaTiO3 phases were gradually enhanced, and the HA and TiO2 phases were gradually reduced in HA/TiO2 composites. For HA/TiO2 composites at 1000 � C, the HA was disappeared and completely transformed into Ca3(PO4)2 and CaTiO3, and a part of anatase TiO2 was transformed to the rutile TiO2. The residual TiO2 phase was retained in H30T at 1100 � C, which maybe attributed to the rapid heating rate and a shot duration for microwave sintering [8]. The presence of Ca3(PO4)2 and CaTiO3 in the samples reduces bio compatibilities of the composites and becomes unstable [6]. Thus, those 4
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Fig. 4. Thermogravimetric curves of pure HA and HA/TiO2 mixed powders.
byproduct phases should be considered responsible for applications of HA/TiO2 composites in biological environments. Ca10(PO4)6(OH)2 → Ca3(PO4)2 þ CaO þ H2O↑
(1)
Ca10(PO4)6(OH)2 þ TiO2 → 3Ca3(PO4)2 þ CaTiO3 þ H2O↑
(2)
Fig. 5. Fracture morphologies of H30T composites sintered at different temperatures.
Mutual reaction between HA and TiO2 is related to not only the sintering temperature and duration, but also the raw powders. It is re ported that there is no decomposition and reaction in HA/TiO2 at 900 � C [3,27], and the decomposition and reaction finished at 1100 � C by conventional sintering process with duration of 1 h [3]. Most HA was retained in the HA/TiO2 composite after sintering at 1200 � C by spark plasma sintering with duration of 5 min, although the mutual reaction also occurred [27]. There was HA phase reserved in the HA/TiO2 composites after microwave sintering at 1250 � C with a duration of 40 min [7]. Those results imply the short duration at a high temperature can preserve a part of HA phase in HA/TiO2 composites. In present study, the composites were sintered by microwave sintering with duration of 15 min. Theoretically, a part of HA should be reserved in the HA/TiO2 composites in present study. However, the HA phase was completely transformed to Ca3(PO4)2 and CaTO3 phases at 1000 � C. This phenomenon can be attributed to the contact states between TiO2 and HA powders and TiO2 addition content. The contact state of the two powders depends on powder morphology, size, mixing degree and effi cient particle packing [28,29]. In the reported literature [3,7,27], the HA/TiO2 mixtures are mainly prepared by mixing solid HA and TiO2 powders by mechanical methods. In present study, the TiO2 particles were formed by the TBT hydrolysis in the HA particles suspension, resulting in the formation of HA/TiO2 nano agglomerates as shown in Fig. 2. It is reported that nanosized TiO2 particles formed by the in-situ hydrothermal treatment can cover the HA particles [30,31]. The mixing degree and efficient particle packing by in-situ hydrolysis were greater than mixing solid particles. Consequently, the dissolution of Ca2þ ions from the HA and the incorporation of Ti4þ ions into HA favored [3], resulting in the enhancement of the HA dissolution into Ca3(PO4)2 and mutual reactions between HA and TiO2. Therefore, the sinterability of HA was improved by the nanosized TiO2 particles formed by in-situ hydrolysis. In order to further analyze the thermal behavior of HA/TiO2 com posites, the thermogravimetric analysis was compared for both HA and HA/TiO2 composites as shown in Fig. 4. It can be found that the weight loss for HA/TiO2 mixtures was increased with the content of TiO2 pre cursor. The first stage, from room temperature to 230 � C, was related to the evaporation of physically absorbed water. It is obviously the HA/ TiO2 mixtures presented higher weight loss than the pure HA. The
second stage, from 230 to 500 � C, can be mainly attributed to the combustion and carbonization of residual organic components and removal of chemically absorbed water [22]. In addition, it can also be attributed to the removal of OH caused by the calcinations of amor phous TiO2 phase [32]. The last visible weight loss takes place between 900 and 1000 � C, which is attributed to the partial decomposition of HA to tricalcium phosphate or mutual reactions between HA and TiO2 [33]. The weight loss was constant after the finish of decomposition and mutual reaction as the sintering temperature up to 1100 � C, which is consistent with the XRD patterns. 3.3. Microstructures of HA/TiO2 composites Fig. 5 shows fracture morphologies of H30T composites sintered at different temperatures. Evolutions of pore and grain size significantly depended on the sintering temperature. It can be found that H30T exhibited nanosized agglomerates at 800 � C without visible fuses. As up to 900 � C, a porous structure was formed with the apparent porosity of 10.4 � 0.7% and was composed of submicron grains. As higher than 1000 � C, the composites presented a significant densification with grain growths and pores became near-spherical shapes. The apparent porosity for 1000 � C and 1100 � C was 4.8 � 0.4% and 4.3 � 0.5%, respectively. In addition, the gradual particles were formed and uniformly dispersed at 1000 � C and 1100 � C (as labeled by arrows in Fig. 5c–d). Combining with the XRD results, those gradual particles can be mainly related to the CaTiO3 or TiO2 phase. Atomic distribution in the H30T at 1100 � C was investigated by EDS mapping as shown in Fig. 6. It can be found that Ti, P, Ca and O atoms were uniformly dispersed in the composite. The effect of TiO2 addition content on the sintering behavior of HA/ TiO2 composites was also investigated in term of 1100 � C. The pure HA exhibited a structure with the apparent porosity of 4.4 � 0.4% and fine grain sizes around 200 nm and irregular pores (Fig. 7a). After TiO2 addition, the HA/TiO2 composites show similar pores but larger grains. The apparent porosity for H10T, H20T and H30T was 5.4 � 0.6%, 2.7 � 0.2% and 4.3 � 0.5%, respectively. After the TBT hydrolysis in the HA particles suspension, HA particles were covered with TiO2 precursor resulting in enhancement of the mutual reaction between TiO2 and HA. The large grains can be attributed to the improved sinterabilities of HA/ 5
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Fig. 6. Atomic distributions in H30T sintered at 1100 � C.
Fig. 7b–d). Those results indicate that the TiO2 addition can improve the sinterabilities of the HA/TiO2 composites. The contents of different atoms in different HA/TiO2 composites of 1100 � C were listed in Table 2. Ti contents in the HA/TiO2 composites were increased with the TiO2 content in the raw powders. In addition, the Ca/P atomic ratio was 1.445, 1.435 and 1.538 for H10T, H20T and H30T respectively, which was less than the pure HA of 1.632. Those results were consistent with the XRD results, that Ca3(PO4)2 and CaTiO3 were the main phases in the HA/TiO2 composite at 1100 � C. In present study, it can be observed that both pure HA and HA/TiO2 composites exhibited fine grains compared to conventional sintering method [9,10], and was similar with microwave sintering [7] and spark plasma sintering processes [5]. It is reported that when the HA/TiO2 composites were prepared by microwave sintering at 1250 � C for 40 min, the grain sizes were around 500-600 nm [7]. This phenomena was similar with the HA/TiO2 composites prepared by spark plasma sintering processes [5]. Those fine grain sizes can be attributed to the raw powder and sintering process. One the one hand, both the HA and TiO2 particles in the composites were nanometers and the raw powders presented high specific surface areas. The chemical reaction and the HA decomposition were promoted, resulting in the formation of Ca3(PO4)2 and CaTiO3 phases at a low sintering temperature. Although the nano sized powders favored densifications and atomic diffusions, the CaTiO3 phase on the grain boundaries of Ca3(PO4)2 phase can effectively inhibit the grain growth. On the other hand, the sintering process presented a low sintering temperature and a short duration, which inhibits grain growths. This was similar with the first stage of the two-step sintering method [11], which sintered the specimens at a high temperature with a short duration. Therefore, the HA/TiO2 composites in present study exhibited dense microstructures and fine grains.
TiO2 mixtures due to their high specific surface areas. Compared to the pure HA, gradual particles less than 200 nm were formed and uniformly dispersed onto HA/TiO2 composites (as labeled by white arrows in
Fig. 7. Fracture morphologies of pure HA and HA/TiO2 composites sintered at 1100 � C. Table 2 Elements weight and atomic in the pure HA and HA/TiO2 composites at 1100 � C. Pure HA Element OK PK Ca K Ti K Ca/P
Wt.% 37.09 20.22 42.70 –
H10T At.% 57.44 16.17 26.39 – 1.632
Wt.% 41.34 18.09 33.84 6.72
H20T At.% 62.22 14.07 20.33 3.38 1.445
6
Wt.% 43.87 16.79 31.20 8.14
H30T At.% 64.79 12.81 18.39 4.02 1.435
Wt.% 44.28 13.23 25.89 16.60
At.% 66.10 10.20 15.42 8.27 1.538
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Fig. 8. Mechanical properties of pure HA and HA/TiO2 composites. (a) Microhardness; (b) Elastic modulus.
Fig. 9. Surface morphologies and EDS of pure HA and HA/TiO2 composites sintered at 1100 � C after SBF immersion for 14 days.
3.4. Mechanical properties of HA/TiO2 composites
depend on the sintering temperature and its content. HA/TiO2 com posites can show higher elastic modulus than the pure HA, while only the H20T and H30T at 1000 � C presented significantly higher micro hardness than the pure HA. The H30T at 1000 � C exhibited the highest microhardness and the highest elastic modulus. Those evolutions for mechanical properties can be attributed to the effects of both sintering temperature and TiO2 addition. On the one hand, the densification can increase the strengths, although the
HA is conventionally brittle and has a low strength, which limits its load-bearing applications. TiO2 is added into the HA matrix in order to enhance the mechanical properties of HA/TiO2 composites. Both microhardness and elastic modulus for specimens exhibited improve ments with the sintering temperature, except for the H30T at 1100 � C, as shown in Fig. 8. Nevertheless, the effects of TiO2 reinforcement may 7
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formation of soft TCP and grain coarsening derived by the sintering temperature will decrease strengths of the HA/TiO2 composites [8]. The formation of CaTiO3 and TiO2 phase at the Ca3(PO4)2 boundaries can effectively inhibit the grain growths [3–7]. After additions of TiO2 reinforcement, it is expected that the formation of CaTiO3 phase, TiO2 particles and densifications can favor the strengths of HA/TiO2 com posites. However, the TiO2 promoted the HA decomposition and the formation of soft Ca3(PO4)2 phase with the grain growths compared to the pure HA, which could decrease the reinforcing effect of TiO2 addi tion. Thereby, the H30T showed a decreased mcirohardness and elastic modulus at 1100 � C. This phenomenon is possibly attributed to the formation of the multi-phases in the composites and gain coarsening, which was widely reported by other studies [5,7,10,27]. Compared to the HA/TiO2 composites prepared by other studies [3–7], the HA/TiO2 composites in present study have a slightly lower microhardness and elastic modulus. This could be attributed to the low sintering tempera ture and short durations during microwave sintering, which results in insufficient particles bonding and low densifications. However, those HA/TiO2 composites exhibited comparable elastic modulus with natural bones [4,34], and the cancellous bone presents elastic modulus of 0.05–0.1 GPa [35]. It can expected that the HA/TiO2 composites pre pared by microwave sintering can be applied for the implant substitutes.
composites exhibited improvements in microhardness and elastic modulus due to the effect of both sintering temperature and TiO2 addition. New apatite was formed on the surfaces of HA/TiO2 compos ites after immersion for 14 days indicating high bioactivities for com posites prepared by microwave sintering. Acknowledgments The work was supported by Science Technology Project of Jiangxi Province (Grant numbers 20192BAB216004, 20192BAB206006, 20171BAB206007), National Natural Science Foundation of China (No. 51561013, 51861012), Science and Technology Planning Program of Jiangxi Provincial Education Department (No. GJJ161068, GJJ170946), Base and Talent/Outstanding Young Talent Program of Jiujiang Science and Technology (No. [2016]43(75)). Thanks Qu Yang for improving English presentations. References [1] G. Lewis, Nanostructured hydroxyapatite coating on bioalloy substrates: current status and future directions, J. Adv. Nanomater. 2 (2017) 18. [2] A. Sola, D. Bellucci, V. Cannillo, Functionally graded materials for orthopedic applications - an update on design and manufacturing, Biotechnol. Adv. 34 (2016) 504–531. [3] I. Kutbay, B. Yilmaz, Z. Evis, M. Usta, Effect of calcium fluoride on mechanical behavior and sinterability of nano-hydroxyapatite and titania composites, Ceram. Int. 40 (2014) 14817–14826. [4] E. Fidancevska, G. Ruseska, J. Bossert, Y.M. Lin, A.R. Boccaccini, Fabrication and characterization of porous bioceramic composites based on hydroxyapatite and titania, Mater. Chem. Phys. 103 (2007) 95–100. [5] W.X. Que, K.A. Khor, J.L. Xu, L.G. Yu, Hydroxyapatite/titania nanocomposites derived by combining high-energy ball milling with spark plasma sintering processes, J. Eur. Ceram. Soc. 28 (2008) 3083–3090. [6] M. Mahmoodi, P.M. Hashemi, R. Imani, Characterization of a novel nanobiomaterial fabricated from HA, TiO2 and Al2O3 powders: an in vitro study, Prog. Biomater. 3 (2014) 1–10. [7] J.Z. Jun, F. Nie, X.G. Huang, L. Wang, G.Z. Liu, J.P. Cheng, Effect of TiO2 doping on densification and mechanical properties of hydroxyapatite by microwave sintering, Ceram. Int. (2019), https://doi.org/10.1016/j.ceramint.2019.04.007. [8] A.E. Hannora, S. Ataya, Structure and compression strength of hydroxyapatite/ titania nanocomposites formed by high energy ball milling, J. Alloy. Comp. 658 (2016) 222–233. [9] K.C.B. Yeong, J. Wang, S.C. Ng, Fabricating densified hydroxyapatite ceramics from a precipitated precursor, Mater. Lett. 38 (1999) 0–213. [10] S. Ramesh, C.Y. Tan, S.B. Bhaduri, W.D. Teng, I. Sopyan, Densification behaviour of nanocrystalline hydroxyapatite bioceramics, J. Mater. Process. Technol. 206 (2008) 221–230. [11] D. Veljovic, E. Palcevskis, I. Zalite, R. Petrovic, D. Janackovic, Two-step microwave sintering-A promising technique for the processing of nanostructured bioceramics, Mater. Lett. 93 (2013) 251–253. [12] S.W. Kim, K.A. Khalil, S.L. Cockcroft, D. Hui, J.H. Lee, Sintering behavior and mechanical properties of HA-X% mol 3YSZ composites sintered by high frequency induction heated sintering, Compos. B Eng. 45 (2013) 1689–1693. [13] J. Yun, W. Qin, K. van Benthem, A.M. Thron, S. Kim, Y.H. Han, Nanovoids in dense hydroxyapatite ceramics after electric field assisted sintering, Adv. Appl. Ceram. 117 (2018) 376–382. [14] T. Kumaresan, R. Gandhinathan, M. Ramu, M. Ananthasubramanian, K. B. Pradheepa, Design, analysis and fabrication of polyamide/hydroxyapatite porous structured scaffold using selective laser sintering method for bio-medical applications, J. Mech. Sci. Technol. 30 (2016) 5305–5312. [15] D.W. Jang, T.H. Nguyen, S.K. Sarkar, B.T. Lee, Microwave sintering and in vitro study of defect-free stable porous multilayered HAp-ZrO2 artificial bone scaffold, Sci. Technol. Adv. Mater. 13 (2012), 035009. [16] M.G. Kutty, S.B. Bhaduri, H. Zhou, A. Yaghoubi, In situ measurement of shrinkage and temperature profile in microwave- and conventionally-sintered hydroxyapatite bioceramic, Mater. Lett. 161 (2015) 375–378. [17] K. Lin, L. Chen, J. Chang, Fabrication of dense hydroxyapatite nanobioceramics with enhanced mechanical properties via two-step sintering process, Int. J. Appl. Ceram. Technol. 9 (2012) 479–485. [18] D.J. Curran, T.J. Fleming, M.R. Towler, S. Hampshire, Mechanical properties of hydroxyapatite–zirconia compacts sintered by two different sintering methods, J. Mater. Sci. Mater. Med. 21 (2009) 1109–1120. [19] H.L. Yao, G.C. Ji, Q.Y. Chen, X.B. Bai, Y.L. Zou, H.T. Wang, Microstructures and properties of warm-sprayed carbonated hydroxyapatite coatings, J. Therm. Spray Technol. 27 (2018) 924–937. [20] T. Kokubo, H. Takadama, How useful is SBF in predicting in vivo bone bioactivity? Biomaterials 27 (2006) 2907–2915. [21] X.Y. Guo, P. Xiao, J. Liu, Z.J. Shen, Fabrication of nanostructured hydroxyapatite via hydrothermal synthesis and spark plasma sintering, J. Am. Ceram. Soc. 88 (2005) 1026–1029.
3.5. In-vitro tests of HA/TiO2 composites In order to investigate bioactivities of the HA/TiO2 composites, the composites with different TiO2 additions sintered at 1100 � C were immersed in the Hanks’ SBF with a period of 14 days. The surfaces of different samples after immersion showed significantly different from the fractured surfaces. From Fig. 9, uniform grains and pores were dis appeared; rough and particle agglomerates were observed for all the samples. EDS results show that the Ca/P atomic ratios for those ag glomerates were around 1.8 and significantly higher than that of sam ples before immersion, which were related to the new apatite phase. Formation of new agglomerates in Fig. 9 can be attributed to the dissolution and precipitation of Ca2þ and PO44 in the compacts and SBF solution [27]. On the one hand, the Ca2þ and PO44 are released from the HA or HA/TiO2 composites into the solution, which results in the super-saturation of Ca2þ and PO44 in the SBF. On other hand, the super-saturation of Ca2þ and PO44 in the SBF are deposited onto the surfaces of composites. It is reported that both the TiO2 and HA have a good biocompatibility by inducing apatite nucleation on the sample’s surface after being immersed in SBF [27,36]. Although the in-vitro test is considered to have some limitations, the apatite-forming ability in SBF is generally used as a bioactivity index and is often used to predict the bone-forming ability in vivo [20]. Therefore, the HA/TiO2 composites prepared by microwave sintering exhibited high bioactivities and can be used as substitutes for the bone defects in the future. 4. Conclusions HA/TiO2 composites with different TiO2 addition contents (10 wt%, 20 wt% and 30 wt%) were prepared by microwave sintering at different temperatures. HA/TiO2 mixtures were prepared by the TBT hydrolysis in the HA particles suspension, which exhibited amorphous TiO2 phase and high specific surface areas with TiO2 content. Compared to the phase stable of pure HA, TiO2 promoted the HA decomposition and mutual reactions during sintering, resulting in that main phases in HA/ TiO2 composites varied from HA and anatase TiO2 phases at 800 � C to Ca3(PO4)2, CaTiO3, even TiO2 phases at 1100 � C. The sintering behavior of HA/TiO2 composites was discussed in term of sintering temperature and TiO2 addition. Both HA and HA/TiO2 composites exhibited high densifications and fine grains at elevated temperature due to the nano sized raw and limited atomic diffusions induced by shot sintering du rations and byproduct suppressions. Gradual CaTiO3 particles were formed in HA/TiO2 composites sintered at 1000 and 1100 � C. HA/TiO2 8
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