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In this study, we show the synthesis of reduced graphene oxide/hydroxyapatite (rGO/HA) composites using a hydrothermal autoclave with argon-15% hydrogen gas injection. This both increases the hydrothermal pressure and uses hydrogen as a reductive agent in the process. The synthesized powders were then consolidated with spark plasma sintering method. The analysis of the consolidated samples included Vickers Indentation technique and cell viability. The results showed that injected gases in the autoclave produced powders with a higher crystallinity compared to synthesis without the gases. Also, hydrogen gas led to increased reduction of GO. The microscopic analysis confirmed existing graphene sheets with folding and wrinkling in the powders and indicated that various preferential directions played a role in the growth of hydroxyapatite crystals. The results showed that in general, graphene sheets increased the mechanical properties of HA. In the samples synthesized with injected gases, this increase was more significant. Interface analysis results indicate that reduced graphene oxide (rGO)/HA interface is likely coherent. These nanocomposites were biocompatible and showed some hydrophobicity compared to pure HA. [ABSTRACT FROM AUTHOR]

In this study, the effect of the argon, nitrogen, and hydrogen gases on the final properties of the reduced graphene oxide- hydroxyapatite nanocomposites synthesized by gas injected hydrothermal method was investigated. Four samples were synthesized, which in the first sample the pressure was controlled by volume change at a constant concentration. In subsequent samples, the pressure inside the autoclave was adjusted by the injecting gases. The initial pressure of the injected gases was 10 bar and the final pressure considered was 25 bar. The synthesized powders were consolidated at 950 °C and 2 MPa by spark plasma sintering method. The final samples were subjected to Vickers indentation analysis. The findings of this study indicate that the injection of argon, hydrogen, and nitrogen gases improved the mechanical properties of the nanocomposites. Injection of gases increased the crystallinity and particle size of hydroxyapatite, and this increase was greater for nitrogen gas than for others. Injection of these gases increased the rate of graphene oxide reduction and in this case the effect of nitrogen gas was greater than the others. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate enhanced mechanical properties of three-dimensional graphene/hydroxyapatite (3DG/HA) nanocomposites by employing the Taguchi method. The hydrothermal process time, the total pressure, the hydrothermal temperature, and the weight percent of graphene to the hydroxyapatite were considered as control parameters. The results showed that all parameters were effective in controlling the process. According to the findings, the hydrothermal temperature had the greatest effect on the results. The predicted strength fracture toughness was 2.08 ± 0.03 MPa.m0.5, which is within the 90% confidence interval. Microscopic analysis confirmed the presence of graphene sheets with folding and wrinkling in the powders and indicated that various preferential directions played a role in the growth of hydroxyapatite crystals. Image 1 • Hydrogen gas has led to reduction of graphene oxide. • The presence of graphene sheets improved mechanical properties of hydroxyapatite. • The results showed that all parameters are considered as an effective parameter in the process. • According to the findings, the hydrothermal temperature had the greatest effect on the results. • The predicted fracture toughness was 2.08 ± 0.03 MPa.m0.5. [ABSTRACT FROM AUTHOR]

In this study, we demonstrate the fabrication of three-dimensional reduced graphene oxide/hydroxyapatite (HA)/Gelatin scaffolds by employing two steps, including a hydrothermal autoclave with hydrogen gas injection to synthesize three-dimensional graphene (3DG)/HA powders and a hydrogel 3D-printing method to fabricate the scaffolds. Evaluations were performed separately for synthesized powders and fabricated scaffolds, which included field emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, inductively coupled plasma, Raman spectroscopy, and instrumented compression experiments. The powders characterization results showed that the nanostructured hybrid powders were successfully synthesized. The graphene sheets were stacked in 3D and the HA nanorods were attached on the surface of the graphene. Interface analysis showed that the two phases (HA/Graphene) are connected coherently and various preferential directions played a role in the growth of hydroxyapatite crystals. Characterization of the scaffolds showed that the addition of HA and graphene/HA powders resulted in smaller pores and higher dimensional accuracy of the fabricated scaffolds. Mechanical evaluation showed that the addition of HA (0.3 gr) increased the mechanical properties of the scaffolds. Also, increasing the small amount of graphene (0.0045 gr) made the mechanical properties much higher than that of pure HA. Image 1 • Interface analysis showed that the two phases (HA||Graphene) are connected coherently. • The presence of HA and graphene sheets increased mechanical properties of scaffolds. • Addition of powders resulted in smaller pores and higher dimensional accuracy of the scaffolds. • Increasing the small amount of graphene made the mechanical properties much higher than that of pure HA. [ABSTRACT FROM AUTHOR]

In this investigation, reduced graphene oxide/hydroxyapatite (rGO/HA) hybrid powders have been synthesized using hydrogen gas injection into hydrothermal autoclave. The powders were then consolidated with spark plasma sintering. The results showed that this method of synthesis caused the obtained powders to have high crystallinity. Microscopic analysis confirmed the presence of rGO sheets with folding and wrinkling in the nanocomposite and indicated that various crystalline planes such as (002) and (300) played a role in the growth of hydroxyapatite crystals. The results of interface analysis (HA||rGO) showed that the HA is coherently connected by its (300)-planes with the surface of the rGO sheets and this coherency is accomplished in the rGO cross-section with the (002) planes of HA. The hardness and the Young's modulus of the composite samples were 5.9–6.6 GPa and 132–146 GPa respectively. Unlabelled Image • Various crystalline planes such as (002) and (300) played a role in the growth of hydroxyapatite crystals. • The HA is coherently connected by its (300)-planes with the surface of the rGO sheets. • Interface coherency is accomplished in the rGO cross-section with the (002) planes of HA. • The hardness and the Young's modulus of the composite samples were 5.9–6.6 GPa and 132–146 GPa respectively. [ABSTRACT FROM AUTHOR]

Abstract Three dimensional graphene-hydroxyapatite (3DG-HA) powders were synthesized at various hydrothermal pressures and the effect of pressure change on the properties of the powder was investigated by the change in the volume of the solution. Graphene oxide, calcium nitrate tetrahydrate, and diammonium hydrogen phosphate solutions were used as a precursor to synthesis by hydrothermal method. The initial solution was prepared by mixing the precursors, and then the four samples P1, P2, P3, and P4 were identical with the same concentration, and fill percent of 16.67, 33.33, 66.67, and 83.33 in the hydrothermal conditions of 180 ᵒC, 8 h, pH = 11, and pressures at 13, 16, 21, and 19 bar for the above samples, respectively. The synthesized powders were characterized with Fourier-Transform Infrared spectroscopy (FT-IR), Field Emission Scanning Electron Microscope (FESEM), X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), Energy Dispersive Spectroscopy (EDS), and Raman spectroscopy. X-ray analysis results showed that increased pressure increased crystallinity and decreased crystallite size. FTIR and Raman spectroscopy analysis showed that the highest reduction rate for graphene oxide occurred at a fill percentage of 33.3. In general, when the fill percent ranged from 33% to about 75%, the efficiency of the hydrothermal process was appropriate for the synthesis of the 3dG-HA powders. [ABSTRACT FROM AUTHOR]