Transparent hydroxyapatite ceramics with nanograin structure prepared by high pressure spark plasma sintering at the minimized sintering temperature

A hydrothermally processed bulky powder composed of loosely aggregated nano-sized rods was consolidated by spark plasma sintering. The use of a high pressure cell allows the application of pressure up to 500 MPa. It was found that applying of high pressure is beneficial for widening up the kinetic window for attaining dense HAp nanoceramics. The high transparency of HAp nanoceramics obtained in this study is ascribed to the high density and homogeneous nano-grained structure achieved besides the unique intrinsic optical properties of the HAp crystal itself, i.e. its low refractive index and very small birefringence. Achieving full densification at the minimized sintering temperature allows for the first time the preparation of transparent HAp nanoceramics with stoichiometric composition, i.e. avoiding the loss of structural water that commonly takes place during the conventional ways of sintering.     

Effective diffusion distance for isothermal sintering of hydroxyapatite

Abstract

The microstructural evolution of hydroxyapatite (HAp) was quantified for isothermal sintering at 1100°C. The aggregated state of the powder particles is thought to be responsible for the relatively high value of the average pore separation throughout isothermal sintering. The measured grain size exponent of 6·7 is not compatible with the values expected for volume diffusion or grain boundary diffusion under the assumptions of the Combined Stage Sintering Model and hence the Master Sintering Curve. The measured exponent for a previously defined flux weighted effective diffusion distance gave a more reasonable value of 3·4. The theoretical exponent in the Combined Stage Sintering Model was then corrected to accommodate a non-linear relationship between the effective diffusion length and pore size. The results demonstrate how the effective lengthscale in the Combined Stage Sintering Model can be corrected to accommodate aggregation in the microstructure.     

Gelcasting and sintering of hydroxyapatite materials: Effect of particle size and Ca/P ratio on microstructural,mechanical and biological properties

The sintering behaviour and microstructural evolution of two batches of a commercial calcium-deficient hydroxyapatite powder were investigated. First, the sintered density as a function of the starting particle size distribution was studied, and the minimum particle size to get the desired target density was determined. Then, as the two batches were characterized by a slight difference in Ca/P ratio, the role of such ratio on phase and microstructural evolutions during sintering, as well as on mechanical and biological properties was investigated.It was observed that the powder with lower Ca/P ratio underwent significant hydroxyapatite (HA) to β-tricalcium phosphate (β-TCP) decomposition, with a simultaneous formation of tetracalcium phosphate (TTCP). The microstructure of sintered gelcast samples evolved during isothermal sintering at 1300 °C, moving from a starting homogeneous and narrow grain size distribution to a bimodal distribution after 3 h sintering. In fact, over time, large grains decomposed into smaller ones, finally providing a microstructure composed of coarse grains surrounded by plenty of ultra-fine grains. On the contrary, the powder with the higher Ca/P ratio provided a limited HA to β-TCP transformation, and normal grain growth by increasing the sintering time. Such differences lead to different mechanical properties for gelcast samples produced by the two powder batches, as the material with the lower Ca/P ratio affected by lower mechanical strength. Finally, sintered samples from both powders showed in-vitro bioactivity, with a larger surface coverage observed for the lower Ca/P ratio material. The morphology of the apatite layer seemed to be affected by the material composition, too, showing flake-like and needle-like morphologies depending on the Ca/P ratio of the starting powder.     

Effect of the temperature and sintering time on the thermal,structural, morphological,and vibrational properties of hydroxyapatite derived from pig bone

This paper focuses in the study of the effect of the temperature and sintering time on structural, morphological, thermal, and vibrational properties of hydroxyapatite obtained from pig bone (BHA). A three-step process was used to get BHA: hydrothermal cleaning, calcination, and cooling. Samples were calcined and cooled inside the furnace under atmosphere air. The samples were calcined at 600 and 1000 °C and sintered at 1, 7, 20, 50 h and studied accordingly. XRD and Raman studies showed an improvement in the crystalline quality as a function of the sintering time, while the samples calcined at 1000 °C did not exhibit structural changes as a function of the sintering time. The presence of magnesium oxide (MgO) was confirmed by XRD and micro EDS analysis as a result of the temperature treatment because this compound was not found in the samples calcined at 600 °C. The crystalline quality of these samples was studied using XRD and Raman spectroscopy.     

Enhancing the physical and mechanical properties of pellet-shaped hydroxyapatite by controlling micron- and nano-sized powder ratios

Hydroxyapatite (HA) was fabricated in microns as its basic size. The particle size distribution was controlled by mixing micron- and nano-sized HA to obtain the optimum amount of mixture to improve its properties. HA powder with a size of 2.5 μm was mixed with that with a size of 200 nm, with a variety of concentrations of up to 20 wt%. A green body was fabricated using the uniaxial pressing method at a pressure of 200 MPa. The sintering process was conducted at a temperature of 1200 °C, heating rate of 3 °C/min, and holding time of 2 h in air. The physical characteristics of the HA sintered body were determined using X-ray diffraction, scanning electron microscopy, linear shrinkage, and density testing. The mechanical properties of the HA sintered body were tested using compressive strength testing. The test results indicated that the mechanical properties of the HA sintered body increased with the addition of nano-sized HA. The mechanism of the increasing strength occurred because nano-sized HA particles filled the gaps between the micron-sized particles. In this study, the highest mechanical properties were obtained by adding 20 wt% nano-sized HA. The compressive strength in the sample without added nano-sized HA was 132.2 MPa and increased significantly to 208.6 MPa with the addition of nano-sized HA of 20 wt%. No change in the phase in HA was observed within a sintering temperature of 1200 °C.     

Hydroxyapatite nanocomposites: Synthesis,sintering and mechanical properties

Two different hydroxyapatite based composites reinforced by oxide ceramic (20 wt%) nano crystals were synthesized by high-energy ball milling and sintered by pressure less technique. Alumina and titania nanoparticles as secondary phases improved densification and mechanical behavior of apatite and postponed its decomposition to the tricalcium phosphate (TCP) phases at elevated temperatures. Increasing the relative density of apatite using nano reinforcements leads to enhance the bending strength by more than 40% and 27% (as compared to the pure HA) and increase the hardness from 2.52 to 5.12 (Al₂O₃ composite) and 4.21 (TiO₂ addition) GPa, respectively. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction spectroscopy were employed to study morphologies, fracture surfaces and phase compositions, respectively. The morphological study and micro structural analysis confirm the X-ray diffraction and relative density diagrams.     

Effect of sintering temperature on structural and dielectric properties of Bi and Li co-substituted barium titanate ceramic

Bi and Li co-substituted barium titanate, Ba_0.98(Bi,Li)_0.02TiO₃ ceramic samples were sintered at different temperatures using conventional solid state sintering technique. X-Ray Diffraction patterns confirm tetragonal phase in all the sintered samples. Microstructure analysis using Scanning Electron Microscopy (SEM) reveals increasing grain sizes with an increase in sintering temperature. Dielectric spectroscopy performed in the range of 40 Hz to 2 MHz at room temperature shows that the dielectric constant increases with increasing sintering temperature, reaching a maximum of value of 1200 at 40 Hz where the dielectric loss observed was less than 0.02 for samples sintered at 1300 °C. Temperature dependant impedance spectroscopy data in the range of 30–300 °C was used to measure AC conductivity. The activation energy of grains was deduced through Arrhenius plots. Loss tangent at different frequencies for 1300 °C sintered samples was less than 0.1 over the entire temperature range. The high dielectric constant with a low dielectric loss at elevated temperatures make Ba_0.98(Bi,Li)_0.02TiO₃ samples suitable for Multi-Layer Ceramic Capacitors (MLCC)s used in high-temperature applications.     

Effect of sintering temperature on electrical properties of SiC/ZrB2 ceramics

SiC/20?wt% ZrB₂ composite ceramics were fabricated via pressureless solid phase sintering in argon atmosphere at different temperature. The effect of sintering temperature on microstructure, electrical properties and mechanical properties of SiC/ZrB₂ ceramics was investigated. Electrical resistivity exhibits twice significant decreases with increasing sintering temperature. The first decrease from 1900?°C to 2000?°C is attributed to the obvious decrease of continuous pore channels in as-sintered materials. The second decrease from 2100?°C to 2200?°C results from the improvement of carbon crystallization and the disappearance of amorphous layers enveloping ZrB₂ grains. Additionally, the increase of sintered density with increasing temperature caused greatly advance of flexural strength, elastic modulus and Vickers hardness. But excessive temperature is detrimental to flexural strength because of SiC grain growth.     

Improvement of microstructural properties of 3Y-TZP materials by conventional and non-conventional sintering techniques

3 mol% Y₂O₃-stabilized zirconia nanopowders were fabricated using various sintering techniques; conventional sintering (CS) and non-conventional sintering such as microwave (MW) and pulsed electric current-assisted-sintering (PECS) at 1300 °C and 1400 °C. A considerable difference in the densification behaviour between conventional and non-conventional sintered specimens was observed. The MW materials attain a bulk density 99.4% theoretical density (t.d.) at 1300 °C, while the CS materials attain only 92.5% t.d. and PECS 98.7% t.d. Detailed microstructural evaluation indicated that a low temperature densification leading to finer grain sizes (135 nm) could be achieved by PECS followed by MW with an average sintered grain size of 188 nm and CS 225 nm. It is believed that the high heating rate and effective particle packing are responsible for the improvements in these properties.     

Effect of sintering temperature on the morphology,crystallinity and mechanical properties of carbonated hydroxyapatite (CHA)

Effect of sintering temperature on the physical and mechanical properties of synthesized B-type carbonated hydroxyapatite (CHA) over a range of temperature in CO₂ atmosphere has been investigated. The B-type CHA in nano size was synthesized at room temperature by using a direct pouring wet chemical precipitation method. The synthesized CHA powders were subsequently consolidated by sintering treatment from 800 to 1100 °C. The sintered CHA samples were evaluated using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrometry, X-ray fluorescence (XRF), carbon-hydrogen-nitrogen-sulfur-oxygen (CHNS/O) elemental analyzer, Field emission scanning electron microscopy (FESEM), and Vicker's indentation technique. The results obtained from XRD and FESEM indicated that the synthesized B-type CHA powders were nanometer in size. The crystallinity and crystallite size of the sintered CHA samples were increased due to increasing sintering temperature. The heat treatment between 800 °C and 1000 °C has resulted in coarsening and increased hardness of the sintered CHA samples. However, these properties began to deteriorate when sintering beyond 1100 °C due the formation of calcium oxide.     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Improvement of the hydroxyapatite mechanical properties by direct microwave sintering in single mode cavity

The main purpose of this study consists in investigating the direct microwave sintering of hydroxyapatite (HA) in a single mode cavity. Firstly, stoichiometric HA powders were synthesized by a coprecipitation method from diammonium phosphate and calcium nitrate solutions and shaped by slip-casting. Then, using the one-step microwave process, dense pellets with fine microstructures were successfully obtained in short sintering timespan. A parametric study permitted to determine the influence of powder grain size, sintering temperature and dwell time on the sintered samples microstructures. The Young's modulus (E) and hardness (H) were measured by nanoindentation and the values discussed according to the microstructure. Finally, the resulting mechanical properties determined on the microwave sintered samples (E = 148.5 GPa, H = 9.6 GPa, σ_compression = 531.3 MPa and K_IC = 1.12 MPa m^1/2) are significantly higher than those usually reported in the literature, whatever the sintering process, and could allow the use of HA for structural applications.     

Study of microstructural,mechanical, and biomedical properties of zirconia/hydroxyapatite ceramic composites

In this study, hydroxyapatite was obtained by the sol-gel method, and zirconia/hydroxyapatite composites (YSZ/HAp) were produced with weight proportions of 95/5, 90/10, 85/15, and 80/20, respectively. The samples were characterized by X-ray diffraction (XRD), Archimedes' principle, Fourier-transform infrared spectroscopy (FT-IR), Vickers microhardness, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). The calvarial critical-sized defect experimental model in rats was used to evaluate the biological interaction between YSZ/HAp scaffolds and bone tissue by Micro CT analysis. The XRD patterns of composites showed the major intensity of the zirconia phase and lower intensity of the hydroxyapatite phase, but the FT-IR analysis confirmed the presence of hydroxyapatite. Dense composite materials were verified by way of the Archimedes’ principle, where the YSZ/HAp 85/15 sample had lower apparent porosity (0.60%) and water absorption (0.10%). Vickers microhardness showed that composite material hardness decreased with the increase of hydroxyapatite, varying from 1367.43 to 711.37 HV. SEM images were possible to quantify the crack sizes in the indentations and to identify the elements presents by EDS, while FESEM was applied to analyze the morphology of the powders and microstructure of the composites. Among the composite studied, YSZ/HAp 85/15 and YSZ/HAp 80/20 samples were the compositions that demonstrated the best mechanical behavior with a fracture toughness of 9.2 and 9.3 MPa m^1/2, respectively. The YSZ/HAp scaffold showed an interaction with bone tissue. The percent bone volume (BV/TV, p < 0.001) and bone mineral density (BMD, p < 0.01) were significantly increased in Zirconia/hydroxyapatite scaffold.     

Effects of particle size distribution and sintering temperature on properties of alumina mold material prepared by stereolithography

Alumina mold materials prepared by stereolithography usually have considerable sintering shrinkage, and their properties related to casting have been rarely studied. In this study, alumina molds materials were prepared by stereolithography, and the effects of particle size distribution and sintering temperature on the properties of the materials were investigated. Results show that the viscosity of the slurries decreases as the fraction of fine powder increases, and the particle size distribution affects the curing behaviors slightly. Sintering shrinkage increases as the fraction of fine powder or the sintering temperature increases. Although lower sintering shrinkage can be achieved by sintering at 1350 °C or 1450 °C, the mold materials sintered at lower temperatures would continue to shrink under the service temperature of 1550 °C, and thus 1550 °C is determined as the optimal sintering temperature. As the fraction of fine powder increases, the creep resistance first increases and then decreases, and specimens prepared with 0.1 fraction of fine powder exhibit the best creep resistance with the droop distance of 4.44 ± 0.45 mm. Specimens prepared with 0.1 fraction of fine powder and sintered at 1550 °C exhibit linear shrinkage of 6.36% along the X/Y direction and 11.39% along the Z direction, and have a flexural strength of 78.15 ± 3.50 MPa and porosity of 30.12 ± 0.08%. The resulting material possesses relatively low sintering shrinkage, proper mechanical strength, porosity and high-temperature properties that meet the requirements for casting purposes.     

Digital light processing of high-strength hydroxyapatite ceramics: Role of particle size and printing parameters on microstructural defects and mechanical properties

Dense hydroxyapatite (HA) bars were fabricated using digital light processing. The roles of HA median particle size (MPS), curing depth-to-layer thickness ratio (CD/LT), and debinding process on the printing/debinding flaws and flexural strength of the sintered parts were investigated. Commercial HA was milled for different times to provide powders with an MPS ranging from 0.3 to 2.7 µm. Thermal debinding led to delamination and vertical cracks, which decreased with increasing MPS; the minimum value required to fabricate specimens with appreciable flexural strength was 0.9 µm. At a given MPS (2.7 µm), the CD/LT varied between 1.4 and 3.3, indicating a progressive disappearance of the above major flaws. Finally, the positive effect of water debinding prior to thermal debinding on reducing crack formation was demonstrated. After optimisation, the bars achieved a flexural strength greater than 100 MPa, which is the highest value among dense HA fabricated using lithography-based techniques.     

Spark plasma sintering to restrict sintering reactions and enhance properties of hydroxyapatite–mullite biocomposites

Herein, we demonstrate how spark plasma sintering (SPS) can be useful in restricting the sintering reactions and faster densification in Hydroxyapatite–Mullite system, which otherwise shows extensive sintering reactions during conventional pressureless sintering, as reported in a recent study [Nath et al. J. Am. Ceram. Soc. 93 (2010) 1639–1649]. The microstructure of SPSed Hydroxyapatite (HAp)-20 wt% mullite composites was characterized by submicron sized HAp and equiaxed mullite grains. Another important result has been the achievement of higher hardness of 7 GPa, which is much higher than pressureless sintered composites. The cell culture study including cellular viability using MTT analysis establishes good cytocompatibility of SPSed composites.     

Improvements in microstructural,mechanical, and biocompatibility properties of nano-sized hydroxyapatites doped with yttrium and fluoride

Hydroxyapatite was doped with Y³⁺ (2.5, 5 and 7.5 mol%) and F⁻ (2.5 mol%) ions (2.5YFHA, 5YFHA, 7.5YFHA, respectively) to compare its structural and mechanical properties and cellular response with pure-hydroxyapatite. No second phases were observed by X-ray diffraction spectra of 2.5YFHA. Doped hydroxyapatites had F⁻ bonds in addition to OH⁻ bonds. Hydroxyapatites sintered at 900 and 1100 °C were in nano-size. 7.5YFHA sintered at 1300 °C had the highest microhardness value. 2.5YFHA sintered at 1100 °C had the highest fracture toughness value. MTT viability assays showed high cell attachments on 2.5YFHA. Cell proliferation on 2.5YFHA and 5YFHA sintered at 1100 and 1300 °C was comparable with the control after 5-day culture. The highest ALP production and calcium deposition were observed on all hydroxyapatites sintered at 1100 °C. 2.5YFHA sintered at 1100 °C can be an alternative for hydroxyapatite in orthopedic applications.     

Influence of sintering temperature on conductivity and mechanical behavior of the solid electrolyte LATP

To warrant long-term reliability for application of electrolytes in solid state batteries also mechanical properties have to be considered. Current work concentrates on Li_1+xAl_xTi_2-x(PO₄)₃ (LATP), which based on its conductivity is a very promising material. Effect of sintering temperature (950, 1000, 1050, 1100 °C) on mechanical properties and conductivity was tested. Impedance tests were carried out and as main focus of the work the mechanical behavior of LATP samples was determined. The impedance tests results revealed that LATP sintered at 1100 °C had the highest ion conductivity. The LATP sintered at 1100 °C revealed also the highest elastic modulus and hardness, which appeared to be related mainly to a smaller lattice parameter with additional effects of lower porosity especially when tested at higher loads. The results indicate that enhancement of both mechanical behavior and conductivity requires lowering secondary phase content and densifying the microstructure of the material.     

Superplastic deformation of hydroxyapatite ceramics with B2O3 or Na2O addition fabricated by pulse current pressure sintering

High-density and fine-grained transparent hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂: HAp) ceramics with B₂O₃ and Na₂O addition were fabricated using pressureless sintering and pulse-current pressure sintering between 1000 and 1100 °C; the superplastic deformation of these HAp specimens was evaluated. The relative density of pure HAp compacts pulse-current pressure sintered at 1000 °C for 10 min under a pressure of 50 MPa attained 99.9% and exhibited translucency. The tensile elongation of the pure HAp specimen, which was measured at 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 364%. The relative density of HAp compacts with 3.0 mol.% B₂O₃ addition pulse-current pressure sintered under the same conditions as those of pure HAp compacts was 98.9%, whereas the grain size was as low as 0.24 μm. The elongation of HAp specimens, measured at a test temperature of 1000 °C under a strain rate of 1.48 × 10⁻⁴ s⁻¹, was as high as 578%.     

Micro-hydrothermal reactions mediated grain growth during spark plasma sintering of a carbonate-containing hydroxyapatite nanopowder

A carbonate-containing hydroxyapatite nanopowder was consolidated by spark plasma sintering at the temperatures ranging from 650 to 1100 °C. It was found that the water released by dehydroxylation was trapped inside the nanopores in the densified HAp bodies over 900 °C. Based on the analysis by the X-ray diffraction, Fourier-transform infrared spectrometry and scanning electron microscope, the water-nanopore system was evaluated and its effect on the grain growth was also investigated. It was revealed that the water existed inside the closed nanopores most probably resulted in the formation of local micro-hydrothermal environments inside bulk HAp ceramics during SPS. Therefore, the grain growth was enhanced by the local micro-hydrothermal reactions activated above 900 °C. In addition, abnormal grain growth was also observed when a higher temperature or higher heating rate was employed, which may be attributed to the local highly active hydrothermal reactions.