Low temperature consolidation of hydroxyapatite-reduced graphene oxide nano-structured powders

In this study, hydroxyapatite-reduced graphene oxide (HA-rGO) powders were first synthesized in situ using a hydrothermal method.

3 specific surface area (2630 m 2 g -1 ), and their unique mechanical properties play a reinforcing role in these nanocomposites [34][35][36][37][38][39]. Apart from the excellent mechanical properties of graphene such as elastic modulus of 1 TPa and high fracture strength of 130 GPa, good biocompatibility of this material has led to consideration in biomedicine applications such as drug delivery, orthopedics, and bioimaging [40][41][42][43]. Published reports show that the addition of graphene and its derivatives (reduced graphene oxide and graphene nanoribbons) to HA, which is performed in various methods such as hydrothermal process, has always improved the mechanical properties of HA [44][45][46].
In some applications, such as implants, synthesized powders need to be made into bulk shape.
Different methods such as hot pressing and spark plasma sintering have been used to consolidate these types of powders [47,48]. But in most techniques, it is used at high temperatures. That high temperature also increases the cost of production, as well as the risk of material destruction.
Recently, consolidation techniques at low temperatures have been very much considered. In this method, which is called cold sintering; it is used at temperatures below 300 ° C, with a pressure of 100 to 500 MPa. In the first instance, a liquid phase as a solvent and very little is added to the interface between particles [49][50][51]. A portion of the particle surface is dissolved in the liquid phase. The powders are then compressed under external pressure and compression is facilitated by the presence of a liquid phase as a lubricant. Dissolution occurs in the interface between particles and sharp edges of particles. It precipitates between particles and hollows. Finally, after applying the heat and pressure and removing the liquid phase, the particles are connected to each other. Important factors of cold sintering process are powder materials, solvents, and physical parameters, but the solvent selection is more important. In some cases, additional heat treatment 4 for annealing is used [52][53][54][55][56][57][58]. So far, very few reports have been published on cold sintering of hydroxyapatite containing powders [59][60][61].
In this study, the hybrid nanostructured powders were synthesized using a high-pressure hydrothermal method utilizing the hydrogen gas injection to increase the reduction rate of GO.
To investigate the final composite properties, a cold sintering method has been used. Sintered samples were subjected to a Vickers indentation technique for mechanical evaluation, X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR) analysis, and high-resolution transmission electron microscopy (HRTEM).

2-Experimental
The primary chemicals used in this study, along with their specifications, are listed in Table 1.
The initial solution (S1) was first prepared (DMF + DI water with a volume ratio of 20:80).
DMF was considered for better dispersion of GO in solvent [35,45].

2-1-Synthesis of powders
Schematic 1 shows the rGO-HA powders synthesis process. The amount of rGO in this study is considered to be 1.5% by weight, because according to previous studies [4], this amount has had the greatest effect on increasing fracture toughness of rGO-HA nanocomposites (The approximate amount of rGO was estimated by trial and error). Given the schematic 1, the following steps were performed in order.
i. The solution containing Ca +2 (4.7 grams of calcium nitrate tetrahydrate in 120 mL of S1) was added dropwise to a 20 mL stirred suspension of GO (HA/1.5% rGO) with stirring continued for 1 h (Schematic 1a, b).
ii. The solution containing phosphate ions (1.56 grams of diammonium hydrogenphosphate in 80 mL of S1) was dropwise added to the solution (Schematic 1c).
iii. The pH of the solutions was adjusted to >10 with ammonium solution.
iv. The resulting solution was poured into the Teflon (PTFE) vessel and transferred to the autoclave. The hydrothermal process was carried out for 5 h at 180 °C by injection of hydrogen gas at 10 bar (The volume of the PTFE container was 340 mL). v.
The powders were dried at oven for 12 h at 60 °C. vi.
The resulting powders were consolidated after drying and ball milling (250 rpm, 12 h).

2-2-Consolidation of powders
Chart 1 shows the steps of consolidation. Figure 1 shows the system used for sintering, and Vickers indentation evaluation of the samples. At cold sintering stage, the mixture (M1) used consists of GO, brushite, and DMF. The mixture of brushite and GO was prepared just like the powders synthesis step (Schematic 1c, at this stage of powder synthesis, a mixture of brushite and GO was synthesized). The use of brushite was due to the fact that this calcium phosphate is converted to HA under high pressure and temperature conditions [23]. The amounts of GObrushite powders were 5% by weight (DMF). The ratio of calcium to phosphate in this mixture was considered to be 1.67. A cold sintering method was chosen for sintering these powders Before performing the above steps, considering the study sources, the temperature, time and pressure applied were first evaluated and the best conditions were considered for cold sintering with respect to the relative density obtained for each sample (described later).

2-3-Vickers indentation
Instrumented microindentation experiments were conducted on the polished surfaces of 7 W t is the area below the load-displacement curve and W e the area below the unloading curve which corresponds to the elastic deformation. The energy W t is the total of elastic and plastic deformation (W e and W p respectively). λ is a dimensionless constant is close to 0.0498 for Vickers tip. C is the average crack length and P is the applied force. The use of experimental parameters is the major advantage of this method; it is easy to calculate when using instrumented indentation. and 60% relative humidity.
Raman spectroscopy (Renishaw inVia spectrometer) was used in the range of 300-3500 cm −1 , recording 5 times for 10 seconds of each accumulation, with a wavelength of 532 nm, green laser 8 line in a backscattering configuration using a microscope with a 100× objective, 100% power, and an acquisition time of 10 s, which had been excited from an argon ion laser.
HRTEM (TALOS F200A with a twin lens system, X-FEG electron source, Ceta 16M camera and a super-X EDS detector) was used to observe atomic structure of the samples and spatially resolved elemental analysis, with a spatial resolution higher than 2 nm. To study the atomic structure, fast fourier transform (FFT) and inverse fast fourier transform (IFFT) analysis were used. Imagej 1.52d and Diamond 3.2 softwares were used in this study.   As can be seen from the graphs, the best temperature was 200 °C (Figure 4c), the best holding time was 30 minutes (Figure 4b), and the best applied pressure was 500 MPa (Figure 4a) to reach the highest relative density. It should be noted that the solvent content in these samples was considered to be 20% by weight. According to studies, the famous GO peak is located at 2theta≈10. After reduction of GO to rGO, this peak is removed and a new one appears at 2theta≈26. Because of the amorphous structure of rGO, this peak is much weaker and wider than the HA (002) peak. Therefore, the rGO peak is covered by the HA (002) peak which is highly intensified due to its high crystallinity. Table 2 shows the specification of the main HA scatter planes obtained. According to the XRD pattern (002), (211), and (300) planes are the main growth planes in HA crystals where, (002) and (300) planes are perpendicular (Figure 5b). Increasing the amount of solvent has increased the intensity of the peaks in some directions and decreased in some directions. In the direction of the (002) plane, increasing the amount of solvent has reduced the peak intensity, but in case (211) and (300) planes the peak intensity has increased. Also, the peaks have been transmitted slightly to the left, which is probably due to increased pressure from the solvent steam. Comparing the peaks obtained, it is clear that the presence of rGO did not have much effect on the peaks movement [65,66,45,47].   are assembled together (Figure 7a) so that the HA particles are placed between them (Figures 7b,   7c). As it is known, the presence of this three-dimensional structure causes incomplete compression during sintering and, in any case, increases porosity and it is expected that by increasing the amount of rGO the porosity will increase equally as previously this issue was   (Figures 8a, 8b). The pore size distribution diagram drawn using the Washburn equation [67] (Figure 8c) shows that the size of the porosity in the rGO-HA sample has increased. The total porosities were 3.5% for HA and 5% for rGO-HA nanocomposite. Also, densitometry results ( Figure 8d) showed that the highest relative density was related to the III sample with 20% solution. compared to (II). Considering the same conditions for the preparation of samples, it is likely that another mechanism including a higher degree of GO reduction or higher crystallinity, and a stoichiometric most likely is responsible for this phenomenon. Also, according to these diagrams, the elastic work in (I) is greater than the other samples. Also, the plastics work is slightly higher in (I), but with a smaller ratio, which is obtained from the surface below the curves. In these diagrams, the transition to the left means the improvement of mechanical properties. In Figure 9b, the force-displacement curve shows that the Vickers indenter has hit a hole in its path. The part shown with the arrow shows the contact depth where the cavity is located. These changes are more evident in samples with more porosity. In some curves, these changes appear several times in a curve. These cases involve some errors in the calculations. The indentation analysis results (Table 3) show that the hardness, the Young's modulus of (II) and

3-Results and discussion
(III) samples are higher than that of (I). Also, (III) showed better mechanical properties than the (II). The reason for this increase in mechanical properties should be examined from two perspectives. First, increasing the hydrothermal pressure increases the crystallinity of the primary powder and improves the properties of the HA, and secondly, the presence of more hydrogen gas increases the reduction degree of GO and increases the mechanical properties of the graphene sheets [45,47]. The fracture toughness of this nanocomposite (III, by the Cold Sintering method) was approximately equivalent to the fracture toughness of the spark plasma sintered HA [68].    Figure 11 shows the interface analysis between the two phases after consolidation. In this figure, HA, graphene sheets, and three areas identified for analysis. Figure 11a shows the HA and rGO phases together. In Figure 11b, the d-spacing of graphene sheets is 0.34 nm, which shows that GO is well reduced. In Figure 11c, the d-spacing confirms the growth of (211) planes in HA. Figure 11d shows that the two phases are well connected. Previous researches have confirmed that there is a coherent interface between the two phases of HA and rGO [4,69,70]. to the fracture toughness of the spark plasma sintered HA. The interface between the two phases in this nanocomposite was coherent. The crack deflection and graphene bridging were among the mechanisms that increase the fracture toughness of these nanocomposites.