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Synthesis and mechanism of tetracalcium phosphate from nanocrystalline precursor.

1. Introduction

Monoclinic tetracalcium phosphate (CaO[(P[O.sub.4]).sub.2], TTCP), a kind of calcium phosphates which can be formed in the (CaO-P2O5) system at high temperature (>1300[degrees]C), is the only calcium phosphate phase with a Ca/P ratio greater than stoichiometric hydroxyapatite ([Ca.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2], HA). It shows moderate reactivity and solubility when combined with acidic calcium phosphates such as dicalcium phosphate anhydrous (CaHP[O.sub.4], DCPA, monetite) or dicalcium phosphate dihydrate (CaHP[O.sub.4] * 2[H.sub.2]O, DCPD, brushite). Therefore, TTCP has been widely used in self-setting calcium phosphate cements for bone regeneration, which consequently leads to the formation of HA in physiologic conditions [1-4].

The preparation of high-purity TTCP powders, an important raw material in the field of calcium phosphate bone cements, is the key to fabricate calcium phosphate bone cements (CPC) with desirable performance in vivo. Regarding the synthesis of TTCP, there are mainly two approaches. One is direct solid-state reaction and the other is wet process reaction. The solid-state reaction at high temperature has been widely used in the synthesis of TTCP [5-7]. This technique is usually based on the use of mixtures of calcium carbonate (CaC[O.sub.3]) and dicalcium phosphate anhydrate (CaHP[O.sub.4]) (Ca/P ratio of 2), followed by heating at 1450-1500[degrees]C for 6-12 h. However, the treatment time of this method is long and the obtained products are generally not pure, including other phases such as calcium phosphates and even calcium oxide [8-11]. Dai et al. [12] have prepared the HA and CaO doped TTCP by this method. On the other hand, wet process reaction mainly refers to coprecipitation-solid state reaction. The nanoscale hydroxyapatite-calcium carbonate powder mixture was firstly prepared by the coprecipitation, followed by calcination at 1500[degrees]C to obtain TTCP. In this way, high purity TTCP can be obtained at low temperature by this time-saving and facile method which most likely resulted from the small dimension effect and surface effect of precipitated nanoparticles. In this study, we present a modified method for the preparation of pure TTCP based on nanosized powder mixtures of the calcium carbonate and hydroxyapatite by wet process reaction and then calcinated at different temperature. The phase transition resulting from the synthesizing process of TTCP was investigated by TEM, DtA-TG, XRD, FT-IR, and RS. The reaction mechanism was explored, and the optimal temperature for synthesis of TTCP ceramics with high purity was obtained.

2. Materials and Methods

2.1. Preparation of Sample. Calcium nitrate (Ca [(N[O.sub.3]).sub.2] * 4[H.sub.2]O) (Shanghai ShanPu Chemical Reagent Co., Ltd., China), diammonium hydrogen phosphate ([(N[H.sub.4]).sub.2]HP[O.sub.4]) (Tianjin Dengke Chemical Reagent Co. Ltd., China), ammonium carbonate ([(N[H.sub.4]).sub.2]C[O.sub.3]) (Tianjin Kermel Chemical Reagent Co., Ltd., China), and all other reagents used in the present study were of analytical grade.

The [(N[H.sub.4]).sub.2]HP[O.sub.4] aqueous solution was slowly dropped into the stirred Ca[(N[O.sub.3]).sub.2] aqueous solution at water bath with the temperature of 60[degrees]C with a Ca/P molar ratio 2, and the pH for mature solution was adjusted to 11-12 by ammonium solution (26-28 vol%). After stirring for 2 h and precipitating for 24 h, pump filter was followed by washing 3 times with alcohol and drying in vacuum drier at 90[degrees] C for 24 h. After that the mixtures of nano-hydroxyapatite (n-HA) and nanocalcium carbonate (nano-CaC[O.sub.3]) were acquired. The dried mixture was calcinated at 850[degrees]C, 1150[degrees]C, 1200[degrees]C, 1250[degrees]C, 1300[degrees]C, 1350[degrees]C, 1400[degrees]C, 1450[degrees]C, and 1500[degrees]C in an alumina crucible with a heating rate of 10[degrees] C/min and kept for 8 h. The product was rapidly cooled to room temperature by means of a lifting device in the air (about cooling rate 150[degrees]C/min).

In addition, some mixing powder calcinated at 1400[degrees]C, 1450[degrees]C, and 1500[degrees]C was cooled to room temperature in the furnace (about cooling rate 1[degrees]C/min).

2.2. Characterization of Sample. The morphology and microstructure of the precipitate mixtures were examined by transmission electron microscopy (TEM, JEM-2100 (URP), Jeol, Japan).

Powder XRD was used for phase analysis of the samples. The analysis was carried out on an X-ray diffractometer (XRD, D8ADVANCE, Bruker, Germany) equipped with a monochromator in the diffracted beam. The diffraction spectra were recorded from 15[degrees] to 60[degrees] using CuK[alpha] (wavelength = 0.154056 nm, 40 mA, 40 kV) radiation with step size 0.05[degrees] and step duration 1 s.

DTA-TG was conducted on the as-prepared powders (10 mg) to ascertain the thermal stability and decomposition temperature. DTA-TG was performed using thermal analysis-thermogravimetry (DTA-TG, STA409PC/4/H, NETZSCH, Germany) at a heating rate of 10[degrees]C min-1 from 50 to 1500[degrees]C in a static air atmosphere.

For excitation of Raman spectra the 514.5 nm line (20 mW) of an [Ar.sup.+]-ion laser (inVia-Laser microscopic confocal Raman spectroscopy, Renishaw, UK) was used with resolution of 1 [cm.sup.-1].

Fourier transformed infrared (FTIR) spectra were recorded from KBr pellets by means of a Shimadzu-2000 FT-IR spectrometer (FT-IR, Shimadzu 2000, Shimadzu, Japan) with a spectral resolution of 2 [cm.sup.-1].

3. Results and Discussion

3.1. TEM Analysis. Figure 1(a) is the TEM images of synthesized nanocrystals and Figures 1(b) and 1(c) are the corresponding selected area electron diffraction analysis. It can be seen that the synthesized mixtures were nanosized rod-like crystals of calcium carbonate with size of 2-4 nm in diameter and 30-40 nm in length with an aspect ratio of ~12. Needle-like crystals of poorly crystallized hydroxyapatite were also observed with crystal size of 10-20 nm in diameter by 40-60 nm in length with an aspect ratio of ~3. n-HA and nano-CaC[O.sub.3] diffraction patterns were tested by selected area electron diffraction (SAED) as shown in Figures 1(b) and 1(c). Electron diffraction patterns were made of different radius of concentric circles, which clearly indicated that the substance was slightly crystallized crystal. The electron diffraction pattern composition was further determined by calculation of interplanar spacing based on

d = K/R, (1)

where d is the crystal interplanar distance, K is the camera constant, and R is radius of diffractions rings. By comparing the calculated interplanar distance with standard PDF CARDS, it was confirmed that the synthetic crystals were composed of n-HA and n-CC. The corresponding (HKL) value of diffraction rings has been marked in Figures 1(b) and 1(c).

3.2. Thermal Analysis. Figure 2 shows the TG-DTA diagram of synthesized samples. It was observed that the weight loss was about 5.5% when the temperature changed from room temperature to 230[degrees]C, mainly caused by the loss of adsorbed water in the sample. Lattice water of the samples was removed in the temperature range of 230-570[degrees]C and the weight loss was shown to be about 2.5%. CaC[O.sub.3] was decomposed (equation (2)) in the temperature range of 570-850[degrees]C and weight loss was about 4.59%. In the range of 850-1185[degrees]C, the nonstoichiometric HA was decomposed into [beta]-tricalcium phosphate ([beta]-[Ca.sub.3][(P[O.sub.4]).sub.2], [beta]-TCP) and HA (equation (3)) [13]. While the temperature exceeded 850[degrees]C, HA could react to form oxygen apatite ([Ca.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2-2x][O.sub.x][[].sub.x], OHA) (equation (4)) 13,14]. When reaction temperature was above 1050[degrees]C, [Ca.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2] could react to form [beta]-TCP and TTCP (equation (5)) [13]; while the temperature exceeded 1350[degrees]C, [beta]-TCP could transform into [alpha]-tricalcium phosphate ([alpha]-[Ca.sub.3][(P[O.sub.4]).sub.2], a-TCP) (equation (6)) [13]; meanwhile, the [beta]-TCP and HA were formed by the reaction as shown in (3), which could also react to form a-TCP and TTCP in the temperature range of 1185-1255[degrees]C (equation (7)) [13]. Moreover, when the enough CaO existed in the calcination samples, the final products TTCP could be formed by reaction shown in (8) and (9) [11,13]. Consider the following:

CaC[O.sub.3] [right arrow] CaO + C[O.sub.2] (2)

nonstoichiometric HA

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

and stoichiometric HA

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

[alpha]-[Ca.sub.3] [(P[O.sub.4]).sub.2] + CaO [right arrow] [Ca.sub.4]O [(P[O.sub.4]).sub.2] (8)

[beta]-Ca3 [(P[O.sub.4]).sub.2] + CaO [right arrow] [Ca.sub.4]O [(P[O.sub.4]).sub.2] (9)

3.3. XRD Analysis. Figure 3 shows the XRD patterns of the air-cooled quenching products sintered at different temperatures. It can be seen from Figures 3(a) and 3(b) that coprecipitation products were mainly composed of HA and calcium carbonate (CaC[O.sub.3]), as evidenced by the characteristic peaks of HA (JCPDS-09-0432) at 2[theta] = 25.9[degrees](002), 31.9[degrees](211), 32.3[degrees](112), 32.9[degrees](300), 39.9[degrees](310) and the characteristic peak of CaC[O.sub.3] (JCPDS-29-0305) (2[theta] = 29.4[degrees](104)) appearing in the XRD pattern of the samples. Moreover, the broad diffraction peaks revealed that both HA and CaC[O.sub.3] were poorly crystallized. After the calcination of coprecipitation products at 850[degrees]C, 1100[degrees]C, and 1150[degrees]C for 8h (Figure 3(c)), respectively, mainly HA and calcium oxide (CaO, JCPDS-37-1497) remained. The absence of TTCP characteristic peaks indicated that the synthesis reaction of TTCP does not happen in this temperature range. The presence of characteristic peaks of the HA, TTCP, and CaO after calcination at 1200[degrees]C for 8h showed that the calcination temperature between 1150[degrees]C and 1200[degrees]C resulted into the formation of TTCP phase, which corresponded to the results of DTA analysis, namely, crystal transition of TTCP about 1185[degrees]C. Figure 3(d) shows the XRD patterns of the products calcinated at 1180[degrees] C, 1185[degrees] C, and 1190[degrees]C for 8h. When the temperature was varied from 1180 to 1185[degrees] C, the peak intensity of CaO at 2[theta] = 374[degrees] (200) and HA at 2[theta] = 25.4[degrees] decreased. Such phenomena maybe due to the formation of a little amount of TTCP, causing an absence of characteristic peak of TTCP.

If the coprecipitation mixture was calcinated at 1250[degrees]C to 1500[degrees]C for 8h followed by air-cooling, the characteristic peaks of the resultant products were at 21.8[degrees](121), 25.4[degrees](200), 25.7[degrees](130), 28.0[degrees](211), 28.3[degrees](211), 29.3[degrees](032), 29.8[degrees](040), 30.9[degrees] (-103), 31.2[degrees](221), 31.9[degrees] (-132), 32.1[degrees](113), 32.4[degrees] (-212), and 32.9[degrees](212), which agreed with the JCPDS pattern number 25-1137 of TTCP. Meanwhile, the disappearance of CaO characteristic peaks showed that the high-purity TTCP can be prepared with the temperature rising to more than 1250[degrees]C.

Figure 4 displays XRD patterns of samples by different cooling methods after the high temperature calcination. It was obvious that the furnace-cooled samples exhibited characteristic peak of calcium oxide at ca. 37.5[degrees] [14]; the other characteristic peaks at 25.9[degrees], 31.8[degrees], 32.2[degrees], 32.9[degrees], 34.1[degrees], 39.8[degrees], 46.7[degrees], 48.1[degrees], 49.2[degrees], and 49.5[degrees] can be related to the peaks of HA phase. However, the characteristic peaks of TTCP and HA at 20 = 31.8[degrees], 32.2[degrees], 32.9[degrees], 32.2[degrees], 46.7[degrees], and 48.1[degrees] are relatively similar, which makes it quite difficult to identify the phase composition of this mixture, thereby confirming the previous conclusion that with the furnace cooling method pure TTCP cannot be obtained [15].

3.4. FT-IR Analysis. The fundamental vibration modes for the isolated P[O.sub.4.sup.3-] ion are designated as [v.sub.1], [v.sub.2], [v.sub.3], and [v.sub.4]. Vibrations from the symmetric stretching motion of the PO bonds corresponded to [v.sub.3], while the bending vibrations corresponded to [v.sub.2]. The wavenumber of asymmetric stretching vibrations was represented as [v.sub.3] and the wavenumber of O-P-O asymmetric bending vibrations was represented as [v.sub.4] [16,17].

Figure 5 shows the FT-IR spectra of the calcinated and air-cooled products. The peaks at 603 [cm.sup.-1] and 567 [cm.sup.-1] ([v.sub.4]) were assigned to the asymmetric bending vibration of P[O.sub.4] group in HA, the peaks at 1037 [cm.sup.-1] and 1092 [cm.sup.-1] ([v.sub.3]) was related to the antisymmetry stretching vibration of P[O.sub.4.sup.3-] group, and and the peak at 962 [cm.sup.-1] ([v.sub.1]) represented the symmetry stretching vibration of P[O.sub.4.sup.3-] group. The stretching and bending vibrations of O[H.sup.-] groups in HA appeared at about 3572 [cm.sup.-1] and 635 [cm.sup.-1]. Thus, the characteristics vibrations of P[O.sub.4.sup.3-] and O[H.sup.-] indicated that HA existed in the synthetic sample. Peaks at 1637 [cm.sup.-1] and 3431 [cm.sup.-1] belonged to bending vibration and stretching vibration of surface adsorbed water in the synthetic sample, respectively. Peaks at 1386 [cm.sup.-1] belonged to antisymmetric stretching vibration of N[O.sub.3.sup.-] in the residual samples. The antisymmetric stretching vibration and out-of-plane bending vibration of C[O.sub.3]2- group in CaC[O.sub.3] were at 857 [cm.sup.-1] and 1481[cm.sup.-1]. The peak at 3644 [cm.sup.-1] was related to stretching vibration of OH- group in calcium hydroxide [15], which existed in the samples calcinated from temperature between 850[degrees]C and 1200[degrees]C for 8 h. This fact was evidenced by the OH band (at 3644 [cm.sup.-1]), which was derived from Ca[(OH).sub.2] formed through absorption of water by CaO [18]. The disappearance of peak at 3644 [cm.sup.-1] and bending vibration of OH- in HA at 635 [cm.sup.-1] at the temperature above 1200[degrees]C indicated that calcium oxide and HA were obtained; thus they were consumed during reaction, which was too little to be detected. This phenomenon was consistent with characteristic diffraction peaks of calcium oxide in Figure 3, in which samples were calcinated from 850[degrees]C to 1200[degrees]C, respectively.

The peak at 3672 [cm.sup.-1] was derived from the stretching vibration of O[H.sup.-] group in HA. This peak existed in these samples calcinated from 850[degrees]C to 1400[degrees]C for 8h. However, the peak intensity was gradually weakened and disappeared when the calcination temperature went up to 1450[degrees]C. This indicated that the calcinated samples contained a some of HA under 1450[degrees]C, which could be decomposed when calcination temperature was above 1450[degrees]C. When calcination temperature went up to 1450[degrees]C and 1500[degrees]C, the characteristic peaks of the TTCP at 454 [cm.sup.-1], 470 [cm.sup.-1], 501[cm.sup.-1], 570 [cm.sup.-1], 593 [cm.sup.-1], 601[cm.sup.-1], 621[cm.sup.-1], and 930-1100 [cm.sup.-1] appeared, corresponding to the vibration mode showed in Table 1.

Figure 6 displays FT-IR spectra of the calcination sample by different cooling methods. The three quenched samples scarcely contained OH- stretching vibration, and only the quenched samples calcinated at 1400[degrees]C exhibited very weak OH- stretching vibration at ca. 3572 [cm.sup.-1]; these furnace cooling samples exhibited sharp peaks of OH- stretching vibration in Ca[(OH).sub.2] at 3643 [cm.sup.-1]; sharp peaks of OH-stretching vibration in HA at 3572 [cm.sup.-1] and 635 [cm.sup.-1], and asymmetric stretching vibration of C[O.sub.3.sup.2-] in CaO, the calcium carbonate was derived from reaction of Ca[(OH).sub.2] and C[O.sub.2] in the air. This result revealed that TTCP was decomposed into HA and CaO during the furnace cooling.

3.5. RS Analysis. Figure 7 shows the Raman spectra of synthetic samples by air-cooled quenching at various temperatures. As can be seen from Figures 7(b) and 7(g), the samples have the symmetric bending vibrations [v.sub.2] of P[O.sub.4.sup.3-] at 429 [cm.sup.-1], the asymmetric bending vibrations [v.sub.4] of P[O.sub.4.sup.3-] at 586 [cm.sup.-1], the symmetric stretching vibrations [v.sub.1] of P[O.sub.4.sup.3-] about at 960 [cm.sup.-1], and the symmetric stretching vibrations of C[O.sub.3]2- at 1084 [cm.sup.-1]. Obviously, with calcination temperatures increasing from 1200[degrees]C to 1500[degrees]C, the characteristic peaks of P[O.sub.4.sup.3-] at 407 [cm.sup.-1], 448 [cm.sup.-1], 480 [cm.sup.-1], 586 [cm.sup.-1], 958 [cm.sup.-1], 1042 [cm.sup.-1], 1075 [cm.sup.-1], and 1095 [cm.sup.-1] of P[O.sub.4.sup.3-] were observed in the spectra of the calcinations samples. The O[H.sup.-]stretching vibration at about 3570 [cm.sup.-1] disappeared gradually, and the HA in the samples completely transformed into TTCP with the increase of calcination temperatures.

Figure 7(g) shows the Raman spectra of the samples calcinated from 850[degrees]C to 1200[degrees]C. The OH- Raman peaks from HA and Ca[(OH).sub.2] were clearly observed at 3571 [cm.sup.-1] and 3619 [cm.sup.-1], respectively. However, the two Raman peaks were gradually weakened with the temperature increasing from 850[degrees]C to 1200[degrees]C; finally the Raman peak at 3619 [cm.sup.-1] disappeared and only a strong OH- Raman peak of HA at 3571 [cm.sup.-1] was retained when calcinated at 1250[degrees]C. The peak at 3571[cm.sup.-1] also gradually decreased with the increasing temperature, which was almost invisible at 1400[degrees]C, and completely disappeared when the temperature reached 1450[degrees]C. The results suggested the formation of TTCP resulting from the reaction between CaO and HA with the calcination temperature increased, as given by the following equations:

2CaO + [Ca.sub.10] [(P[O.sub.4]).sub.6] [(OH).sub.2] [right arrow] 3[Ca.sub.4]O [(P[O.sub.4]).sub.2] + [H.sub.2]O (10)

Equation (2) represented the existing reaction at 850[degrees]C, in which the appearance of the OH- Raman peak in Ca[(OH).sub.2] was observed, which resulted from the reaction of calcium oxide and the water in the air. Equation (10) represented the reaction occurring when calcination temperature was up to 1150[degrees]C and above 1150[degrees]C, which was evidenced by the change of Raman peak of the sample. As can be seen from Figure 7(d), the symmetric bending vibration peaks [v.sub.2] of P[O.sub.4.sup.3-] appeared at 389 [cm.sup.-1] and 407 [cm.sup.-1], and the original characteristic peak at 429 [cm.sup.-1] was weakened at 1200[degrees]C. The Raman peak at 429 [cm.sup.-1] disappeared with the increase of temperature. Meanwhile, the Raman spectrum showed that the asymmetric bending vibrations peaks [v.sub.4] of P[O.sub.4.sup.3-] at 578 [cm.sup.-1],589 [cm.sup.-1], and 606 [cm.sup.-1] were splitted into six peaks at 555 [cm.sup.-1], 565 [cm.sup.-1], 576 [cm.sup.-1], 597 [cm.sup.-1], 608 [cm.sup.-1], and 616 [cm.sup.-1]. As can be seen from Figure 7(e), the symmetric stretching vibration of P[O.sub.4.sup.3-] ([v.sub.1]) appeared at 946 [cm.sup.-1] when it is at 1150[degrees]C, then was splitted into four intense distinct peaks at 941[cm.sup.-1], 948 [cm.sup.-1], 957 [cm.sup.-1] and 963 [cm.sup.-1] when it was increased to 1200[degrees]C. Meanwhile, the appearance of lattice vibration Raman peak of TTCP at about 326 [cm.sup.-1] in the calcination samples was confirmed. The reaction showed as (10) did not cease until it was up to 1450[degrees]C, which can be demonstrated from the disappearance of OH- characteristic Raman peak at 1450[degrees]C. In addition, the DTA curve in Figure 2 showed that the endothermic peaks of crystal structure transition began to occur at 1185[degrees]C in the sample, and it can be confirmed that the initial reaction temperature of (2) was between 1150 and 1200[degrees]C.

Figure 8 is the Raman spectra patterns of synthetic samples calcinated at 1500[degrees]C, 1450[degrees]C, and 1400[degrees]C and then cooled by two different methods, and the changes of Raman peaks are listed in Table 2. The Raman test showed that the furnace cooling samples led to the presence of the characteristic peak of O[H.sup.-] in HA at about 3573 [cm.sup.-1] and characteristic peak of O[H.sup.-] in Ca[(OH).sub.2] at about 3619 [cm.sup.-1]. This result revealed that the sample was a mixture of HA and CaO. Posset et al. [17] presented that the Raman peaks of P[O.sub.4.sup.3-] symmetric stretching vibration [v.sub.1] were its characteristic peak. This peak, splitting or not, gave a major evidence for the difference between TTCP and HA. Only an intense distinct Raman peak in the range between 960 and 940 [cm.sup.-1] and the spliting peak in the samples by furnace cooling could not be observed, indicating that TTCP can't be obtained by this cooling method. While the air-quenching samples exhibited four intense distinct peaks at this range, which demonstrated that TTCP was only product by air quenching.

3.6. Discussion. At present, common methods for the synthesis of TTCP are limited to solid-state reactions at high temperatures, based on powder mixtures of CaC[O.sub.3] and dicalcium phosphate anhydrate (CaHP[O.sub.4]) 19], in which the particle size of powder is in the micron level, with calcination temperature higher than 1450[degrees] C. Though the reaction time was long enough for 6-12 h, even up to 15-24 h [9, 12, 20], the products may still contain HA or OHA. Sargin et al. [15] reported several attempts to lower the required processing temperature for the TTCP synthesis using different combinations of various starting products. By varying the process temperatures, they showed that HA or TCP appeared as intermediate products at lower temperatures in all reactions. Although this reaction enabled the formation of TTCP even at 1200[degrees] C, which was much lower than previous report, the purity was lower than that in the product obtained at 1350[degrees]C. Jalota et al. [6] investigated N[H.sub.4][H.sub.2]P[O.sub.4] and Ca(CH3COO)2-[H.sub.2]O powders to further reduce the temperature required for the preparation of phase-pure TTCP. However, the method was suitable only for the synthesis of HA-seeded TTCP powders, which were very sensitive and easily transformed into HA by mechanochemical activation [21]. Romeo and Fanovich [7] studied a solid-state reaction of CaC[O.sub.3] and [(N[H.sub.4]).sub.2]HP[O.sub.4] powders mixed with a Ca/P ratio of 2 and ball-milled prior to thermal treatment. The smaller crystallite size of the milled powders increased the reactivity and significantly improved the purity of the final product.

It is well known that the driving force of calcination process comes from the decrease of systematic free energy, including increase in particles bonding areas and the total specific surface area, decrease in surface free energy, void volume and area reduction, the lattice distortion's elimination, and so forth. Due to the surface and interface effect, small size effect of nanoparticles, the driving force of grain growth is very big, decades or even one hundred times that of ordinary powder calcination process. So, this research shows that it is easy to obtain pure TTCP by using the wet method for synthesizing the mixture of nanoscale HA and calcium carbonate, followed by high temperature treatment.

According to [20], when the temperature was increased, the dehydration or decomposition of the prior formed HA will occur to form weak crystal oxyapatite ([Ca.sub.10][(P[O.sub.4]).sub.6][(OH).sub.2-2x][O.sub.x][[].sub.x], OHA) (equations (4), (11)), x = 0 - l, and [] is oxygen vacancy [22]. As the thermodynamic metastable equilibrium phase [14], [Ca.sub.l0] (P[O.sub.4]).sub.6]O (x = l, OA) was less stable, easy to decompose into tricalcium phosphate (Ca3[(P[O.sub.4]).sub.2], TCP) and CaO (equation (12)). When CaO is relatively excessive in the reaction system, TCP reacted with CaO rapidly with the extension of heat preservation time and TTCP was obtained (equation (9)). In this study, the synthetic mixture of nanoscale HA and CC crystals were calcinated at different temperature. Only characteristic diffraction peaks of HA and CaO were obtained at temperature lower than 1150[degrees] C as shown in Figure 3, in which the characteristic peak of TTCP appeared at 1200[degrees]C. Meantime, the intensity of characteristic peaks from HA and CaO decreased; the characteristic peaks of CaO disappeared at 1250[degrees]C, which might not be detected in the XRD pattern due to the sensitivity limitations of the instruments. However, as seen in Figure 7(g), the strong Raman peaks of the O[H.sup.-] in HA showed that it still contained HA at 1250[degrees]C. When the calcination temperature was higher than 1250[degrees] C, the Raman peak intensities of O[H.sup.-] in HA gradually decreased, which however can be still distinguished at 1400[degrees]C. The Raman peak of OH- disappeared at 1450[degrees]C and pure TTCP could be obtained in this case. The results showed that there was no [alpha]-TCP or [beta]-TCP in this experimental process, except for the intermediate products of HA and CaO. When the temperature is higher than 1150[degrees]C, the reaction between HA and CaO occurred directly and TTCP was formed (equation (10)):

[Ca.sub.10] [(P[O.sub.4]).sub.6] [(OH).sub.2] [right arrow] [Ca.sub.10] [(P[O.sub.4]).sub.6] O + [H.sub.2]O (11)

[Ca.sub.10] [(P[O.sub.4]).sub.6] O [right arrow] 3[Ca.sub.3] [(P[O.sub.4]).sub.2] + CaO (12)

In the preparation process of TTCP by high temperature solid-state reaction, cooling methods are of great importance. After heating, the samples must be rapidly quenched to room temperature in order to avoid the formation of undesired secondary phases such as HA, CaO, CaC[O.sub.3], and [beta]-TCP. Bohner [23] pointed out that TTCP could be obtained by a solid-state reaction at high temperatures (typically 1400[degrees]C), usually between equimolar quantities of DCPD and CaC[O.sub.3]. This reaction should be carried out in a dry atmosphere of vacuum, or with rapid cooling to prevent TTCP to uptake of water from the environment which might lead to the formation of HA. Guo et al. [11, 20] investigated the effect of cooling rate on the purity of TTCP prepared by high temperature solid-state reaction. The results indicated that the purity of TTCP depended decisively on the cooling rate after TTCP was sintered at 1500[degrees]C for 15 h. Quenching in air was propitious for the formation of pure TTCP, while cooling in furnace will cause the decomposition of TTCP and less TTCP obtained. It showed that cooling rate was a key to the synthesis of TTCP by solid-state reaction.

4. Conclusion

The high-purity TTCP was prepared via coprecipitation-solid state reaction and calcination of n-HA and nano-CaC[O.sub.3]. In contrast, mixtures with good homogenization and weak crystallinity of nanocrystals were obtained using wet method reaction. In the latter preparation method, nano-CaC[O.sub.3] and n-HA with high activity can be prepared after coprecipitation. The enhanced reactivity of the mixtures was related to both the loss of crystallinity of the reactants and the generation of defects at the surface of nanocrystals. It was established that the mixture of TTCP, HA, and CaO can be obtained by 8 h of heat treatment at temperature above 1200[degrees]C, and the pure TTCP can be obtained if the temperature was above or equal to 1450[degrees] C.

http://dx.doi.org/10.1155/2014/840102

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (no. U1304820), the Education Department of Henan Province Basic Research Program (no. 13A430331), and Doctoral Program of Henan Polytechnic University (B2009-36).

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Jianguo Liao, (1) Xingze Duan, (1) Yanqun Li, (1) Caifeng Zheng, (2) Zhengpeng Yang, (1) Aiguo Zhou, (1) and Dinghua Zou (1)

(1) School of Materials Science and Engineering, Cultivating Base for Key Laboratory of Environment-Friendly Inorganic Materials in University of Henan Province, Henan Polytechnic University, Jiaozuo 454000, China

(2) Carbon/Carbon Composites Technology Research Center, Northwestern Polytechnical University, Xi'an 710072, China

Correspondence should be addressed to Jianguo Liao; liaojianguo10@hpu.edu.cn

Received 23 July 2014; Revised 17 October 2014; Accepted 17 October 2014; Published 11 November 2014

Academic Editor: Christian Brosseau

TABLE 1: Vibrational wave numbers [[cm.sup.-1]] of samples.

                               Raman                     FT-IR
                          [[lambda].sub.0]                KBr
                             = 514.5 nm

1400~1500[degrees]C   327
x8h                   390, 408, 448, 481         454, 470, 501
                      555, 565, 577, 598, 608,
                      616                        570, 593, 621

                      941, 948, 957, 963         938, 958

                      992,1010,1027,1046,        985, 1009, 1031, 1045,
                      1075, 1095, 1122, 1135     1,063,107,410,941,100

850-1150[degrees]C    430, 445                   431, 471
x8h                   579, 590, 607              604
                                                 634
                                                 871

                      961                        961

                      1028,1046,1075,1123        1025, 1091
                      1085
                                                 1412, 1452, 1467
                      3572                       3572
                      3619                       3643

                      Assignments

1400~1500[degrees]C   Lattice modes
x8h                   [v.sub.2], P[O.sub.4.sup.3-] bands in TTCP,
                      symmetric  bending vibrations mode
                      [v.sub.4], P[O.sub.4.sup.3-] bands in TTCP,
                      asymmetric bending vibrations mode
                      [v.sub.1], P[O.sub.4.sup.3-] bands in TTCP,
                      symmetric  stretching mode
                      [v.sub.3], P[O.sub.4.sup.3-] bands in TTCP,
                      asymmetric stretching mode

850-1150[degrees]C    [v.sub.2], P[O.sub.4.sup.3-] bands in HA
x8h                   [v.sub.4], P[O.sub.4.sup.3-] bands in HA
                      Vibrational mode of O[H.sup.-]
                      Out-of-plane bending vibrations of
                      C[O.sub.3.sup.2-]
                      [v.sub.2], P[O.sub.4.sup.3-] bands in HA

                      [v.sub.3], P[O.sub.4.sup.3-] bands in HA
                      Symmetric stretching of C[O.sub.3.sup.2-]
                      Asymmetric stretching of C[O.sub.3.sup.2-]
                      O[H.sup.-]stretching vibration (in HA)
                      O[H.sup.-]stretching vibration (in
                      Ca[(OH).sub.2])

TABLE 2: Raman shift [[cm.sup.-1]] of the calcinations samples by
different cooling methods.

                   1400~1500[degrees]C x8 h,      1400~1500[degrees]C
                   air quenching                  x8 h, furnace
                                                  cooling

Raman              327
[[lambda].sub.0]   390, 408, 448, 481             427, 444
= 514.5 nm
                   555, 565, 577, 598, 608, 616   575, 588, 605

                   941, 948, 957, 963             959

                   992, 1010, 1027, 1046, 1075,   1026, 1040, 1072,
                   1095, 1122, 1135               1121
                                                  3573
                                                  3619

                   Assignments

Raman              Lattice modes (TTCP)
[[lambda].sub.0]   [v.sub.2], P[O.sub.4.sup.3-] bands, symmetric
= 514.5 nm         bending vibrations mode
                   [v.sub.4], P[O.sub.4.sup.3-] bands, asymmetric
                   bending vibrations mode
                   [v.sub.1], P[O.sub.4.sup.3-] bands, symmetric
                   stretching mode
                   [v.sub.3], P[O.sub.4.sup.3-] bands, asymmetric
                   stretching mode
                   O[H.sup.-] stretching vibration (in HA)
                   O[H.sup.-] stretching vibration (in Ca[(OH).sub.2])
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Title Annotation:Research Article
Author:Liao, Jianguo; Duan, Xingze; Li, Yanqun; Zheng, Caifeng; Yang, Zhengpeng; Zhou, Aiguo; Zou, Dinghua
Publication:Journal of Nanomaterials
Date:Jan 1, 2014
Words:6273
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