Zinc-substituted hydroxyapatite produced from calcium precursor derived from eggshells

The availability of natural calcium derived from biowaste materials is an inexpensive alternative to commercial calcium reagents used to produce calcium phosphate bioceramics. This approach is more ecofriendly, cost effective and sustainable owing to its abundant availability. In this study, zinc-substituted hydroxyapatite (ZnHA-Es) was synthesised using calcium precursor derived from eggshells through precipitation method. Zn concentration was varied from 1 mol% to 5 mol% during the synthesis, and the derived powders were calcined at 700 °C in air atmosphere. XRD analysis revealed that ZnHA-Es powders were highly crystalline and comprised biphasic mixtures of HA as the major phase and β-tricalcium phosphate as the minor phase. FTIR examination indicated that all ZnHA-Es samples had a similar structure to pure HA as evidenced by the presence of frequency bands corresponding to phosphate, carbonate and hydroxyl functional groups. The disappearance of vibrational bands of carbonate groups became more apparent with increased Zn concentration. Microstructure analysis revealed that the derived nanostructured particles were spherical (approximately 40–100 nm in diameter) regardless of Zn substitution.     

In-situ grown hydroxyapatite whiskers reinforced porous HA bioceramic

To improve the mechanical properties of a porous bioceramic without reducing its porosity, a new kind of porous hydroxyapatite (HA) bioceramic with in-situ grown HA whiskers was fabricated using a simple sintering method. CaSO₄·2H₂O was used as a pore-forming medium and also as a catalyst for the growth of in-situ HA whiskers. The bioceramic was analyzed by XRD, SEM and mechanical tests. In-situ grown HA whiskers were stratified on the cliffs of pores in the bioceramic. The compressive strength is as high as 21.7 MPa with the porosity of about 26%. The results show that porous HA bioceramic can be improved in both compressive strength and porosity by the addition of CaSO₄·2H₂O. This novel HA bioceramic has a higher compressive strength without reducing its porosity in a certain weight ratio of CaSO₄·2H₂O, which depends on its two-step fracture pattern. This novel structure provides a new and promising reinforced pattern for porous materials.     

Sr,Mg co-doping of calcium hydroxyapatite: Hydrothermal synthesis,processing, characterization and possible application as dentin substitutes

The aim of this study was to investigate the influence of binary Sr, Mg ion-doping compared to the single-ion doping effects on the phase composition and morphology of hydrothermally obtained calcium hydroxyapatite (HAP) powders and on the phase composition, microstructure, and mechanical properties of the sintered materials. Additionally, the focus of this study was to investigate the possible utilization of the binary doped Sr,Mg-HAP compacts as dentin substitutes in restorative dentistry by evaluating their bonding ability with commercially available restorative materials using the shear bond strength test (SBS). The hydrothermally obtained mono-doped Mg5 and Sr5 showed the monophasic apatite structure, while in all co-doped samples β-TCP phase was formed, resulting in biphasic calcium phosphate (BCP) powders. It was confirmed that co-doping with Sr and Mg ions led to the thermal stabilization of the β-TCP phase by suppressing the phase transition into α-TCP when sintered at 1200 °C. Moreover, the co-presence of Mg ions significantly improved the hardness of Sr-doped HAP from 3.74 to 5.02 GPa. Sr,Mg-HAP dental inserts were found to achieve sufficient bonding (13.53 MPa) through the micromechanical interlocking with Z250 dental composite and Single Bond Universal dental adhesive applied with a total etch approach. The SBS values obtained for the SrMg-HAP insert were similar to the literature data on bonding ability with human dentin, indicating that binary doped Sr,Mg-HAP ceramics present a promising material for application in restorative dentistry as dentin substitutes.     

Mechanical and optical properties evaluation of rapid sintered dental zirconia

The purpose of this study was to compare the effects of conventional sintering (CS) and rapid sintering (RS) on the mechanical and optical properties of different generations of dental zirconia ceramics. Five commercial zirconia ceramics were used for the CS and RS processes. The microstructure, phase composition, biaxial strength, 3-point bending strength, and Vickers microhardness of the zirconia after different sintering processes were studied. Weibull analysis was also used to check the structural reliability of the zirconia. The translucency parameter (TP) and the color difference (ΔE) were determined from the L*, a*, and b* values. The surface morphology of each specimen was dense, with no obvious pores or microcracks. The grain size of the RS group was smaller than that of the CS group. Between the two sintering processes, the same characteristic peaks were detected in the X-ray diffraction (XRD) patterns. There was no statistically significant difference (p > 0.05) in the biaxial strength, 3-point bending strength, or Vickers microhardness in terms of the sintering process. After RS, the Weibull modulus of the materials ranged from 9.3 to 19.3. It seems that the RS process affected the TP and ΔE values to some degree and depended on the zirconia generation. The RS process had no impact on the mechanical properties and produced smaller grain sizes than the CS process. The effect of the RS process on the optical properties depended on the type of material used. These findings should assist the production of dental zirconia restorations through the RS process, which saves time and energy.     

Processing of natural resourced hydroxyapatite from eggshell waste by wet precipitation method

Abstract

Abstract

In the present paper, synthesis of hydroxyapatite (HAp) powder from eggshell waste is produced through wet precipitation process at 50–55°C. The thoroughly washed and dried eggshell powder is treated with dilute nitric acid followed by diammonium hydrogen phosphate under controlled reaction conditions such as pH, temperature, stirring time, etc. to obtain white HAp precipitate. The formation of the HAp phase and its thermal stability were identified through thermogravimetric/differential thermal analysis, Fourier transform infrared spectroscopy and X‐ray diffraction studies at different calcined temperatures. The dried powder at 90–100°C was wet ball milled for several hours and compacted to cylindrical shapes at a lower pressure of 850?kg?cm^?2. The green compacts were sintered at different temperatures for 2?h in dry atmosphere. Average grain sizes of sintered samples are mostly in submicrometre range. The morphologies of the green as well as sintered compacts were observed by field emission scanning electron microscopy.     

Enhanced mechanical property of Ca5(PO4)2SiO4 bioceramic by a biocompatible sintering aid of zinc oxide

In the present work, a biocompatible additive ZnO was used to enhance the sintering of calcium phosphate silicate (CPS)¹ bioceramic and improve its mechanical property. Sol-gel prepared CPS powders were mechanically mixed with various amount of ZnO and then sintered at high temperatures to prepare Zn-CPS bioceramics. The effects of ZnO content and sintering temperature on microstructure and bending strength of Zn-CPS bioceramics were investigated and the related mechanism was also discussed. The results showed that ZnO could significantly enhance the sintering of CPS and improve its bending strength accordingly. The Zn-CPS bioceramic with 1?wt% ZnO sintered at 1300?°C exhibited a highest bending strength of 80.8?MPa. The enhanced densification of Zn-CPS bioceramics might be attributed to the liquid phase appeared during the sintering process and the formation of Ca₂ZnSi₂O₇² (HT) nano-grains at grain boundaries. It is expected that Zn-CPS bioceramics could have both good mechanical property and excellent bioactivity.     

Microstructure and mechanical properties of B4C matrix composites prepared via hot-pressing sintering with Pr6O11 as additive

High-performance B₄C-PrB₆ composites were prepared via hot-pressing sintering with matrix phase B₄C and with 2–5 wt% Pr₆O₁₁ as additive. The effects of different sintering processes and Pr₆O₁₁ content on the microstructure and mechanical properties of the composites were studied in detail. It is found that increasing sintering temperature and pressure will contribute to the densification of B₄C-PrB₆ composites. Coarse grains are formed in B₄C without additives at high temperature conditions, resulting in the decrease of the densification. Pr₆O₁₁ can effectively hinder the formation of coarse grains and finally promote the densification of the composites. The main toughening mechanisms of composites was crack deflection. The composites with 4 wt% Pr₆O₁₁ prepared at 2050 °C and 25 MPa had the best comprehensive mechanical properties. The relative density, hardness, flexural strength and fracture toughness reached to 98.9%, 37.6 GPa, 339 MPa and 4.4 MP am^1/2, respectively.     

Effect of fluoride additive on the mechanical properties of hydroxyapatite/alumina composites

The effect of fluoride additives on the mechanical properties of hydroxyapatite/alumina composites was investigated. When MgF₂ (5 vol%) was added to hydroxyapatite/alumina composites, the decomposition of hydroxyapatite was suppressed due to the substitution of F⁻ for OH⁻ in the crystal structure. Comparing two additives, such as MgF₂ and CaF₂, MgF₂ showed much more effective for the suppression of phase decomposition in the hydroxyapatite/alumina composites due to the enhanced substitution of F⁻ for OH⁻. In the case of MgF₂ addition, a relatively high-mechanical properties (flexural strength: ∼170 MPa; Vickers hardness: ∼7 GPa) was obtained compared to MgF₂-free composites.     

Effect of sintering additives on microstructures and mechanical properties of short-carbon-fiber-reinforced SiC composites prepared by precursor pyrolysis–hot pressing

Short-carbon-fiber-reinforced SiC composites were prepared by precursor pyrolysis–hot pressing with MgO–Al₂O₃–Y₂O₃ as sintering additives. The effects of the amount of sintering additives on microstructure and mechanical properties of the composites were investigated. The results showed that the composites could be densified at a relatively low temperature of 1800 °C via the liquid-phase sintering mechanism and the composite density and mechanical properties improved with the amount of additives. The amorphous interphase in the composites with more additive content, not only avoided the direct contact of the fibers with matrix, but also improved the fiber–matrix bonding. It proved that the fiber–matrix interphase characteristics played a key role in controlling mechanical properties of the composites.     

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.     

Synthesis,characterization, bactericidal activity,and mechanical properties of hydroxyapatite nano powders impregnated with silver and zinc oxide nanoparticles (Ag-ZnO-Hap)

Hydroxyapatite (Hap) doped or embedded with silver has shown improved bactericidal properties, and its mechanical properties were greatly improved by doping or impregnating Hap with metals such as Magnesium or Zinc, or by impregnating Hap with metal oxides such as MgO, or ZnO. This work describes the preparation of Ag-ZnO-Hap nanocomposites with 4 different Ag-ZnO–Ca mole ratios. XRD, FTIR, SEM, and TEM analysis of all prepared materials identified Hap as the only crystalline phase present in all samples exhibiting a uniform rod-like morphology with particles in the 20–40 nm size range. Microwave Plasma-Atomic Emission Spectroscopy confirmed the presence of zinc and silver in all embedded Hap samples. The antibacterial activity was tested against two different strains; Escherichia coli (E. coli (MV10Nal), and Gram-negative Aggregatibacter actinomycetemcomitans (A.a). The mechanical testing consisted of evaluating breaking force, work of fracture, and brittleness/ductility of Hap and Ag/ZnO/Hap composites. Our study clearly shows that reinforcing Hap with silver and zinc oxide yields superior bactericidal and mechanical properties.     

Microporous activated carbon prepared from coconut shells using chemical activation with zinc chloride

Microporous activated carbon samples were prepared from coconut shells (low-cost lignocellulose waste), using chemical activation with zinc chloride followed by physical activation. Textural characterization was performed using nitrogen adsorption at 77 K. The sample that presented the best characterization results was then evaluated for methane adsorption at pressures between 0.1 MPa and 7 MPa and temperatures in the range 283–333 K. At 298 K and 40 bar, a capacity of ca. 122 mg of methane/g of carbon (80 v/v) was observed, just short of the target established in Brazil for ANG in remote sites transportation (100 v/v). These results suggest that activated carbons prepared from coconut shells, using chemical activation followed by physical activation, may be further developed as potential adsorbents for natural gas storage applications.     

Sintering and mechanical properties of MgO-doped nanocrystalline hydroxyapatite

Hydroxyapatite (HA) has been extensively studied for its exceptional ability in promoting osseointegration as in bone graft substitute and biomimetic coating of prosthetic implants. However poor mechanical properties of HA, in particular its low fracture toughness, has made its widespread adaption in a number of biomedical applications challenging. Here we employ an optimized wet precipitation method to synthesize nanocrystalline HA with significantly improved mechanical properties. In addition doping by MgO is found to effectively suppress grain growth and enhance fracture toughness by nearly 50% while good densification and phase stability in all samples regardless of concentration of dopant are fully maintained. Microstructural analysis further suggests that the exceptionally superior mechanical properties can be explained by migration of MgO to grain boundaries where they transform the more common transgranular fracture into an intergranular mode. Our biodegradation tests also confirm that MgO-doped HA is indeed a suitable candidate for load bearing implants.     

Effect of TiO2 on sinterability and physical properties of pressureless sintered Ti3AlC2 ceramics

TiO₂ was selected as effective sintering aid for pressureless sintering of Ti₃AlC₂ ceramics in this study. The addition of only 5?wt% TiO₂ largely promotes the densification and nearly dense Ti₃AlC₂ ceramic was obtained by pressureless sintering at 1500?°C. Significant strengthening and toughening effects were observed with the addition of TiO₂. High Vickers hardness, flexural strength and fracture toughness of 3.22?GPa, 298?MPa and 6.2?MPa?m^?1/2, respectively, were achieved in specimen pressureless sintered with 10?wt% TiO₂. Additionally, the addition of 5?wt% TiO₂ had no deleterious effect on the excellent oxidation resistance of Ti₃AlC₂ ceramic under 1200?°C water vapor atmosphere, while addition of 10?wt% TiO₂ accelerates the oxidation rate by two orders of degree.     

Improvement of wetability,sinterability, mechanical and electrical properties of Al2O3-Ni nanocomposites prepared by mechanical alloying

The wetability improvement and particle size reduction of alumina/Ni composites through mechanical alloying were addressed. Their effect on the sinterability (at high temperature), mechanical and electrical properties were studied. Al₂O₃ matrix nanocomposites reinforced with different volume fractions of Ni up to 10 vol% were prepared by mechanical alloying. The milled powders were cold pressed and sintered at different firing temperatures up to 1600 °C. The morphology of powders and the microstructure of sintered bodies were investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), respectively. Furthermore, relative density, apparent porosity, mechanical properties and electrical resistivity of the sintered composites were investigated. The results revealed that Al₂O₃ matrix was successfully coated with Ni thin film through mechanical alloying; the thickness of coat was increased with increasing the Ni content. Moreover, the increasing of both Ni content and sintering temperature up 1600 °C, led to a remarkable increase in the relative density and facture toughness of the sintered specimen. On the other hand, microhardness and elastic modulus were decreased with increasing of Ni content, while they increased significantly with the increase of sintering temperature. The electrical resistivity was decreased with increasing Ni content and sintering temperature.     

Effect of annealing process on IR transmission and mechanical properties of spark plasma sintered Yttria

In this study, fully dense Yttria ceramics were successfully fabricated by spark plasma sintering (SPS) at temperatures of 1300 and 1350 $℃$. The effects of post-annealing on IR transmission were investigated by Fourier transform infrared spectroscopy (FTIR) at various temperatures ranging from 1050 to 1250 $℃$. It was found that the optimum annealing temperature depends strongly on the sintering temperature. Annealed samples showed white opaqueness mainly due to the increase and coalescence of pores after annealing and showed an absorption band around 6.6 $\mu m$ which limits usage of yttria in IR applications. Sintering at 1350 $℃$ and annealing at 1250 $℃$ led to the maximum IR transmittance above 80% at wavelength of 5 $\mu m$ for a 3.5-mm-thick sample. The hardness and the fracture toughness of the samples were analyzed in detail and hardness of 9.2 GPa and fracture toughness of 1.65 MPa m^1/2 were obtained for the above sample.     

Structural and electrical properties of 0.57PSN-0.43PT ceramics prepared by mechanochemical synthesis and sintered at low temperature

In this investigation we show that the dielectric, ferroelectric and piezoelectric properties of stoichiometric 0.57Pb(Sc_1/2Nb_1/2)O₃-0.43PbTiO₃ (0.57PSN-0.43PT) ceramics prepared by mechanochemical synthesis are comparable or even better than the properties of 0.57PSN-0.43PT ceramics with Nb doping, which was proposed to enhance the electrical properties. Here, the stoichiometric ceramic was sintered to 97% of theoretical density at a temperature of 1000 °C, which is 200-300 °C lower than previously reported. The dielectric constant ?, remnant polarization P_r, piezoelectric coefficient d₃₃, coupling coefficients k_p and k_t and mechanical quality factor Q_m of the ceramics prepared by mechanochemical synthesis were 2200, 43 μC/cm², 570 pC/N, 0.71, 0.56 and 38, respectively.The effects of the poling field on the structural and electrical properties of the 0.57PSN-0.43PT ceramics were investigated. The results show that the ratio of the monoclinic to the tetragonal phases is influenced by the application of the poling electric field. The non-poled ceramics contain 71% of the monoclinic phase and 29% of the tetragonal phase. The highest d₃₃, k_p and k_t were measured for ceramics poled at an electric field of 3 kV/mm. For these poled ceramics a phase determination of 86% monoclinic phase and 14% tetragonal phase was obtained from Rietveld refinements.     

Properties of concrete pavements prepared with ferrochromium slag as concrete aggregate

This paper presents the results of investigation related to both the properties of the ferrochromium slag and the standard physical and mechanical properties of Portland cement concrete pavements (PCCP) made with this slag as aggregate, according to the relevant Croatian standards. Slag is formed as a liquid at 1700 °C in the manufacture of the high-carbon ferrochromium metal and, by slow cooling in the air, the slag crystallizes to give a stable CaO–MgO–Al₂O₃–silicate product with mechanical properties similar to basalt. With a proper selection of slag as an artificial aggregate, concrete pavements with compressive strengths, wear resistance and specific weight higher than in those from natural (limestone) aggregate in commercial Portland cement, type CEM II/B-S 42.5 (EN 197), can be made. The 28-day compressive strength of the concretes made with original unfractioned slag and with standard limestone as aggregates (w/c=0.64 and 350 kg/m³) reached the values of 57.00 MPa and 36.70 MPa, respectively. Volume stability, high volume mass, good abrasion resistance to wear and crushability make this reinforced slag concrete suitable for wearing courses of concrete pavements for traffic load classes 1 and 2 where carbonate stone material (limestone) mainly does not meet the Standard Technical Requirements for cement concrete slab pavements according to the relevant Croatian standard.     

Replacement of hydroxyapatite from chicken eggshell waste for ceramic properties improvement

The chicken eggshell waste from food processing was synthesized as the hydroxyapatite for fluxing agent replacement in ceramic manufacturing. The main fluxing agents in Thailand ceramic manufacturing are natural potash feldspar (k-feldspar) and animal bone ash. To overcome the problems of inconstant properties and the lack of k-feldspar, the hydroxyapatite from chicken eggshell waste was selected as fluxing agent for the enhancement of the ceramic product. In this work, the hydroxyapatite with 0, 5, 10, and 15 wt% was replaced with the k-feldspar in the ceramic samples. The results revealed the physical and mechanical properties of the ceramic samples with various hydroxyapatite contents were investigated after heat treatment in the temperature range of 1000–1200 °C. The ceramic samples added with hydroxyapatite have higher linear shrinkage and bulk density as compared with the ceramic sample without hydroxyapatite. The apparent porosity and water absorption decreased to near zero after the heat treatment at a temperature of 1200 °C. Moreover, the results showed that the physical properties affected the mechanical properties improvement after the hydroxyapatite addition and heat treatment process.     

Nano-scale mechanical behaviors and material removal mechanisms of zirconia ceramics sintered at various temperatures

The present work aims to address the effects of the sintering temperature (800–1500 °C) on the mechanical properties and material removal mechanisms of 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) based on a series of nanoindentation and nanoscratch experiments. The impacts of the sintering temperature on the mechanical properties, including plasticity, deformation behavior, indentation energies, and cracking resistance were rigorously studied. The acquired results indicate that 1100 °C signifies the transition threshold for the ceramic microstructure, leading to the entirely different mechanical properties and indentation responses for materials sintered at temperatures lower or higher than the threshold value. Moreover, 1100 °C is also a transition threshold of the material removal mechanism, before which the material removal mechanism is dominated by the plastic regime, and beyond which, the material removal mechanism tends to show ductile-brittle characteristics.