Nano Pores Evolution in Hydroxyapatite Microsphere during Spark Plasma Sintering

Micron-spherical granules of hydroxyapatite (HAp) nanoparticles were prepared by powder granulation methods. Through subsequent sintering, porous HAp microspheres with tailored pore and grain framework structures were obtained. Detailed microstructure investigation by SEM and TEM revealed the correlation of the pore structure and the necking strength with the sintering profiles that determine the coalescence features of the nanoparticles. The partially sintered porous HAp microspheres containing more than 50% porosity consisting of pores and grains both in nano-scale are active in inducing the precipitation of HAp in simulated body fluid. The nano-porous HAp microspheres with an extensive surface and interconnecting pores thus demonstrate the potential of stimulating the formation of collagen and bone and the integration with the newly formed bones during physiological bone remodeling.


Introduction
Hydroxyapatite (HAp), has been used for a great number of biomedical applications such as bone substitute and dental treatments in the form of granules, blocks and porous implants [1][2][3][4].Porous spherical granules in micron scale have been recently researched because of the advantages such as high specific surface area, good flow ability, and stable physical and chemical properties and so on [5].Several manufacture methods, like spray drying [6], template based synthesis [7], and microemulsion [8] have been widely used, and these developments have led to a renewed interest of HA powders in the high technology field.Each of the techniques has its own merits, however, most of them involve multi-steps process, which may be complicated and uneconomical for use in industrial manufacture scale [9].It is hard to pick out the optimal manufacture method from them, but it is no doubt that spray drying combined with sintering process is the most convenient one to operate in the industrial field [10].Spark plasma sintering (SPS) has been proved to be one of the most efficient methods in producing ceramics composites [11].When compared with conventional sintering techniques, the higher heating rate and shorter heating time during SPS sintering would probably bring different sintering effects on the ceramics composites.
In recent years, studies have been focused not only on the shape of morphology, but more on the crystal size, particle size distribution, porosity, and crystallinity, etc., because both the mechanical performance and bioactivity of HAp depend strongly on them [5].Porosity offers more opportunities for interfacial bonds to develop between the HAp and the living tissues leading to enhance of the mechanical strength of the overall structure.However, lower mechanical strength of pure HAp has limited its use as a bone implant material because of conflicting requirements of porosity and strength.It is well known that sintering can help to enhance the attachment between grains, which will greatly improve the mechanical strength of spherical granules [12].In this paper, the SPS sintered HAp granules were studied to investigate the sintering effects on the microstructures, especially on the porosity of HAp microsphere, and also on bioactive properties.

Experiments
The starting powder used in this study was HAp powder granules named Nal 20375, with an approximate particle diameter of 10 nm aggregated in granules.The granules had been prepared earlier by spray drying with 3 to 30μm as granule size.Spark Plasma Sintering was performed in DR-SINTER SPS-2050.Six groups of samples were sintered and the sintering temperatures are 750 o C, 800 o C, 850 o C, 900 o C, 950 o C, 1000 o C respectively at atmospheric pressure.Before each sintering, the sample was kept in 600 o C for 3 min, and then the temperature increased at the rate of 50 o C/min to reach the desired temperature.They were kept at the desired temperatures for 5 min before the samples were cooled down naturally in atmosphere.The microscopic morphological features were observed by using a SEM (JEOL JSM-7401F).An accelerating voltage of 1.0kV was chosen without gentle beam.Powder XRD patterns were recorded on an X' Pert PRO PANalytical equipped with monochromatic of Cu/Co α1 radiation (40kV, 20mA) at the rate of 99.45 seconds per 0.0262606 degree over the range of 20-60 o (2θ).N 2 adsorption desorption measurement was done on a Micrometritics ASAP 2020 equipment.11 measuring points were chosen with the first 9 (start from 8 mmHg with intervals of 30 mmHg) for calculation of surface area and the last two points (above 740 mmHg) for pore volume measurement.The surface area was achieved as BET surface area and the pore volume is single point adsorption total pore volume.The simulated body fluid (SBF) test was carried out in The Kokubo solution [13] at 37 o C for 3 hours.

Results and Discussion
The SEM morphology of SPS sintered HAp microspheres under different temperatures are shown in Fig. 1.The regular round or oval shape granules can be seen which indicates that sintering will not destroy the structures of spray drying granules.The sizes of granules are ranged from 3 to 30μm.The granules with the regular shapes would have positive effect on the improvement of flow ability when used in the surgical operation.At the same time, the increased particle size and pore size can be observed as well as decreasing number of pores with increasing temperatures.It is clear that worm-shape particle morphology formed during the sintering process.The mean particle sizes and pore sizes are shown in Tab.I.More than 100 particles/ pores were measured to achieve the average values.
XRD patterns for samples sintered in different temperatures and the starting powder are shown in Fig. 2. When analyzing XRD results we could confirm the increasing crystal sizes, due to the decreasing peak widths, with increasing sintering temperature.This corresponds well with the SEM images.The decreased peak width also confirms an increased crystallinity of HAp with increasing sintering temperature.What also can be observed is that the intensity of the peaks increases when the peak widths become smaller.When increasing the sintering temperature to over 850°C, one can observe a shift of the peaks to higher angles, which probably corresponds to that the HAp lattice has contracted due to the dehydroxylation of HA.Dehydroxylation of HA results in the formation of oxyhydroxyapatite, as shown below [14,15]: Ca 10 (PO 4 ) 6 (OH) 2 → Ca 10 (PO 4 ) 6 (OH) (2-2x) + xH 2 O The XRD patterns for the samples were compared with reference data for single crystalline hydroxyapatite and ß-tricalcium phosphate.For temperatures of 900°C and higher, we could confirm phase changes in hydroxyapatite and ß-tricalcium phosphate phase was formed.No phase changes can be observed below 900°C.
The obtained values for surface area and pore volume are shown in Tab.II.We obtained both a reduced BET surface area and pore volume when increasing the sintering temperature.The surface area of the starting powder was in a very high leave reaching to about 138m 2 /g.With the increasing temperature, the specific surface area decreased dramatically, and finally down to nearly 3.6m 2 /g when temperature got to 1000 o C. It could be found that begin with 750 o C, there was a remarkable decrease in surface area.This might imply that at 750 o C, HAp crystals began to grow and were sintered together, which can also be proved from the SEM results in Fig. 1.The pore volume decreased from 0.5597 to 0.0032cm 3 /g when the temperature increased up to 1000 o C, which implied that the shrinkage of HAp did exist during sintering process, but did not result in a complete collapse of the pore system because the pore size kept in enlarging as shown in Tab.I.

Tab. II
Pore size distribution is also an important parameter influencing the bioactive property of HAp microspheres.The distributions of pore size at different sintering temperatures are shown in Fig. 3. Compared with the pore size data in Tab.II it is obviously that the pore size calculated by BET method is corresponding to the width of pore sizes measured in the SEM photos, which is determined by the calculation model used in the BET results analysis.Although the pore sizes have the trend to increase in dimension, but the distribution does not change distinctly during sintering process.This is an excellent property which means we can easily control the pores size and distribution by only controlling the sintering parameters.Another key factor that controls the pore growth is the pore shape.It is obviously that after sintering, the main shape of pore is continuous prolonged pores in which the length of pores is several times larger then the width of pores.Therefore, the L/W rate, which represents the rate of length to width of pores, can be used to indicate the pore shape.Although the pores grew in both length and width directions when increasing the sintering temperature, it is clear that the growth rates are different.From the results of L/W rate in Tab.II, the pore growth can be divided into two different steps.Before 850 o C, the growth rate in the length direction is faster than the width direction, which means the pores were prolonged in their shape.After that, the L/W rate began to decrease, indicating that the pores have the trend to change their morphology to round shape.
The shape evolution processes can be used to explain the mechanism of pore growth, and also the sintering mechanism, and finally influence on the properties of porous microspheres.There is an obvious pore enlarge process during the sintering below 850 o C, although the pore volume decreased.According to the BET calculated results, from the starting granule to 750 o C sintered samples, the pore size in width increased more than 5 times, which is incongruous with the common views in the theories of initial sintering, where based on the interparticle neck growth mechanism, only nearly 1.27 times increase could be observed which was observed in the microns grain systems [16].In this case, it seems that a new pore growth mechanism, in other words, grain growth mechanism, is dominant in this process when sintering the nano-size grains.Since the starting granule is made by nano-size particles with only 10 nm in diameter, the high surface energy of nano particles greatly lowered down the sintering temperature, which results in grain growth started from a relatively low temperature lever, in this case, only 750 o C, as shown in Fig. 4.Besides grain boundary motion, the nano-size grains have the potential to change their grain orientation and combine together to form new large grains during sintering process.This grain rotation process [17][18][19] is thermodynamically based on the needs to reduce the surface energy.Therefore, in the sintering process of nano porous Hap ceramics, the rotation of nano-sized grains results in the elongation of the pore prom, and easy to form the grains with wormy shape, which is consistent to the SEM results.As mentioned above, the pore size and distribution, as well as the pore shape, have great effects on the final bioactivity of porous HAp microspheres.The simulated body fluid test results are shown in Fig. 5.It is very clear small apatite particles were formed in all the sintered samples, which proves the activity of HAp in SBF.And the sizes of precipitated apatite particles increased with the increasing pore size of the HAp microspheres, but the quantities reduced.This should be reasoned by the reduction of nucleation site, and also can be explained by the formation of TCP phase, which delays the precipitation process by accelerating the rate of dissolution [20].As summary, the partial sintered high purity HAp microspheres, with high mechanical property and modest bioactivity, should be the best choice for the usage of physiological bone remodeling.

Conclusions
The nano porous HAp microspheres were prepared by an economical way.Compared with microspheres made by other method, it is no doubt that the mechanical property of spray dried granules can be greatly improved after SPS sintering, which is necessary in the medical application.The grain and pore growth were observed, which was proved to be the results of grain coalescence process during the sintering process.The sintered HAp microspheres are

Fig. 3 .
Fig. 3. Pore size distribution of HAp microsphere before and after sintering at different temperature (A) Before sintering; (B) 750 o C; (C) 850 o C; (D) 1000 o C

Fig. 4 .Fig. 5
Fig. 4. TEM images of HAp grains (A) Before sintering; (B) SPS sintered at 750 o C Tab.I The effect of sintering temperature on the particle and pore sizes