Pressureless sintering of dense hydroxyapatite–zirconia composites

Hydroxyapatite (HA)–TZP (2.5 mol% Y₂O₃) containing 2, 5, 7.5 and 10 wt% TZP were prepared using calcium nitrate, diammonium hydrogen orthophosphate, zirconium oxychloride and yttrium nitrate. The composite powder was prepared by a reverse strike precipitation method at a pH of 10.5. The precipitates after aging and washing were calcined at 850°C to yield fine crystallites of HA and TZP. TEM study of the calcined powder revealed that while HA particles had both spherical and cuboidal morphology (∼50–100 nm) the TZP particles were only of spherical nature (∼50 nm). X-ray analysis showed that the calcined powder of all the four composition had only HA and t-ZrO₂. Uniaxially compacted samples were sintered in air in the temperature range 1,150–1,250°C. High sintered density (>95% of theoretical) was obtained for composites containing 2 and 5 wt% TZP, while it was 92% for 7.5 wt% and 90% for 10 wt% TZP compositions. X-ray analysis of sintered samples shows that with 2 wt% TZP, the retained phases were only HA and t-ZrO₂. However, for 5, 7.5 and 10 wt% TZP addition both TCP and CaZrO₃ were also observed along with HA and t-ZrO₂. Bending strength was measured by three point bending as well by diametral compression test. While in three point bending, the highest strength was 72 MPa, it was 35.5 MPa for diametral compression. The strength shows a decreasing trend at higher ZrO₂ content. SEM pictures show near uniform distribution of ZrO₂ in HA matrix. The reduction in sintered density at higher ZrO₂ content could be related to difference in the sintering behaviour of HA and ZrO₂.     

Characterisation of the bioactive behaviour of sol–gel hydroxyapatite–CaO and hydroxyapatite–CaO–bioactive glass composites

The fabrication and characterization of sol–gel derived hydroxyapatite–calcium oxide (HAp–CaO) material is investigated focusing on the effect of the addition of a bioactive glass on the material bioactive behaviour through the fabrication of a novel HAp–CaO (70 wt.%)–bioactive glass (30 wt.%) composite material. The bioactive behaviour of the materials was assessed by immersion studies in Simulated Body Fluid (SBF) and the alterations of the materials surfaces after soaking periods in SBF were characterized by Scanning Electron Microscopy (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). A brittle and weakly crystalline carbonate hydroxyapatite (HCAp) layer was found to develop on the surface of all samples, few hours after immersion in SBF, confirming the high bioactivity of the material. Alterations of the morphology of the developed HCAp layer, which led to a more compact structure, were observed on the surface of composite samples after 7 days of immersion in SBF. The presence of the CaO phase seems to accelerate the formation of HCAp, while the bioactive glass affects both the morphology and cohesion of the developed layer.     

The effect of partially stabilized zirconia on the mechanical properties of the hydroxyapatite–polyethylene composites

The effect of partially stabilized zirconia (PSZ) on the mechanical properties of the hydroxyapatite-high density polyethylene composites was studied by investigating the effect of hydroxyapatite and the simultaneous effect of hydroxyapatite and PSZ volume fractions on fracture strength, modulus of elasticity, and absorbed energy in the composite samples. The results showed a decrease in fracture strength, and absorbed energy with an increase in the volume fraction of hydroxyapatite content in the hydroxyapatite-polyethylene samples. Partial replacement of hydroxyapatite with PSZ particles was beneficial in the improvement of both the fracture strength and failure energy values in the composite samples. A transition from ductile to brittle behavior was observed as the volume fraction of ceramic filler particles increased in the samples.     

Preparation and characterization of hydroxyapatite/polycaprolactone–chitosan composites

Hydroxyapatite (HA)/polycaprolactone (PCL)–chitosan (CS) composites were prepared by melt-blending. For the composites, the amount of HA was varied from 0% to 30% by weight. The morphology, structure and component of the composites were evaluated using environmental scanning electron microscope, X-ray diffraction and Fourier transform infrared spectroscope. The tensile properties were evaluated by tensile test. The bioactivity and degradation property were investigated after immersing in simulated body fluid (SBF) and physiological saline, respectively. The results show that the addition of HA to PCL–CS matrix tends to suppress the crystallization of PCL but improves the hydrophilicity. Adding HA to the composites decreases the tensile strength and elongation at break but increases the tensile modulus. After immersing in SBF for 14 days, the surface of HA/PCL–CS composites are covered by a coating of carbonated hydroxyapatite with low crystallinity, indicating the excellent bioactivity of the composites. Soaking in the physiological saline for 28 days, the molecular weight of PCL decreases while the mass loss of the composites and pH of physiological saline increase to 5.86% and 9.54, respectively, implying a good degradation property of the composites.     

Hydrogen peroxide versus water synthesis of bioglass–nanocrystalline hydroxyapatite composites

This article reports a comparison of the structural and textural properties of bioglass–hydroxyapatite (HA) composites obtained in the SiO₂–CaO–P₂O₅ system by sol–gel method, with different amounts of hydrogen peroxide (3% H₂O₂) or water (H₂O). X-ray diffraction, Raman, and FT-IR spectroscopy reveal the presence of nanocrystalline HA. Scanning electron microscopy images illustrate that the HA phase is mainly distributed on the glass surface. The results point out that the sintering at 550 °C of a sol–gel derived SiO₂–CaO–P₂O₅ bioglass leads to a single crystalline phase of HA, and validate a new processing method for obtaining bioglass–HA composites. Structural analyses of the investigated composites indicate the existence of a silicate network built up from Q₃ and Q₂ units. The replacement of water with hydrogen peroxide has as consequence the increase of depolymerization degree of silica network. Textural properties were investigated with N₂-adsorption technique. The composites prepared with hydrogen peroxide exhibit a more uniform and narrow mesoporous distribution that recommends them for drug uptake and release applications. It was found that the specific surface area and pore volume are clearly influenced by the H₂O₂(H₂O):TEOS molar ratio.     

The fabrication of nanocomposites via calcium phosphate formation on gelatin–chitosan network and the gelatin influence on the properties of biphasic composites

A novel biodegradable polymer–ceramic nanocomposite which consisted of gelatin (Gel), chitosan (CS), and calcium phosphate (CaP) nanoparticles was prepared based on in situ preparation method. The fabricated biocomposites were characterized by FTIR, X-ray diffraction (XRD), transmission electron microscopy (TEM) as well as scanning electron microscope with X-ray elemental analysis (SEM-EDX). The characterization results confirmed that the crystalline calcium phosphate nanoparticles were mineralized in polymeric matrix and the interaction between Ca2+ in calcium phosphate and functional groups in polymers molecular chains was formed. XRD result showed that in addition to hydroxyapatite (HA), Brushite (BR) and tricalcium phosphate (β-TCP) particles also were formed due to lack of complete penetration of the basic solution into the polymeric matrix. However, SEM image indicated that the polymeric matrix has the controlling role in the particle size of calcium phosphate. The size of particles in three component composites was about 100 nm while in two component composites proved to be more in μm size. TEM observation supported SEM results and showed that the three component composites have calcium phosphate nanoparticles. The elastic modulus and compressive strength of the composites were also improved by the employment of gelatin and chitosan together, which can make them more beneficial for surgical applications.     

Solid solution formation at the sintering of hydroxyapatite–fluorapatite ceramics

Fluorhydroxyapatite ceramics are increasingly studied for the use as biomaterials due to their good integration ability in the bone tissue and higher resorption resistance compared to the common hydroxyapatite (HA) ceramics. This study is aimed at the X-ray diffraction investigation of the interaction between HA and fluorapatite (FA) particulates in the sintering temperature range up to 1300 °C. The lattice parameters were calculated in dependence of both the FA content in the powder mixtures and the sintering temperature. From those data, the solid solution formation is concluded, at least in the temperature range from 1200 to 1300 °C. Energy-dispersive X-ray microanalysis confirmed the fluorine distribution to be almost uniform in the sintered at 1300 °C ceramics.     

Toughening characterization in alumina platelet–hydroxyapatite matrix composites

Fracture toughness of Al₂O₃ platelet-reinforced hydroxyapatite (HAP) ceramics was investigated using the Vickers' indentation technique. The geometrical anisotropy of alumina platelets induces an anisotropic toughening. The efficiency of reinforcing mechanisms remains maximum for a crack propagating with an angular deviation inferior to 30° around the direction perpendicular to alumina disc faces. This is assumed to result from a crack deflection mechanism which induces a favorable contribution of mode II failure. A small effect of hydroxyapatite grain size becomes noticeable in the direction parallel to alumina disc faces. The toughening depends on the size and volume content of alumina platelets. Large size platelets provoke a spontaneous microcracking of the HAP matrix which is detrimental to the mechanical reliability, whereas small platelets lead to a strong toughening. The results relate to the intensity of thermoelastic residual stresses within the matrix around alumina inclusions. © 1999 Kluwer Academic Publishers     

Factors influencing the compressive strength of an injectable calcium sulfate–hydroxyapatite cement

A biphasic injectable bone substitute, suitable for filling bone defects, that sets in the body, based on calcium sulfate and hydroxyapatite (HA), is presented. For applications in bone defects the compressive strength is important to assure support of the defect site during loading when the patient is weight bearing. To control the strength, the influence of four different factors; the liquid-to-powder (L/P) ratio, the HA particle morphology, the HA content and the amount of accelerator, were investigated. -Calcium sulfate hemihydrate (CSH) and four different HA powders (three sintered and one spray-dried) were used. All differed in size and morphology. CSH and each HA powder were mixed together with distilled water to form the bone substitute. An accelerator, in form of calcium sulfate dihydrate, was added to the powder phase to obtain an adequate setting time. Cylindrical specimens were compression tested. A lower L/P-ratio gave stronger cement, but was more difficult to inject. The shape and the morphology of the HA particles influenced the strength, and reducing the amount of HA increased the strength. The amount of accelerator (calcium sulfate dihydrate) had no influence.     

The effective thermoelectric properties of core–shell composites

Thermoelectric materials are capable of converting heat directly into electricity and vice versa, and they have been explored for both waste heat recovery and thermal management. In this work, we analyze axially symmetric thermoelectric problems, motivated by energy harvesting using waste heat from an automobile exhaust pipe. Thermoelectric field distributions in both homogeneous shell and core–shell composites are solved, and the effective thermoelectric properties of the core–shell composites are analyzed. Numerical results show that higher thermoelectric conversion efficiency can be achieved in core–shell composites, and the mechanism responsible for the enhanced conversion efficiency is also identified. The analysis thus points to a new direction in developing high-performance thermoelectric materials.     

Effect of pyrophosphate ions on the conversion of calcium–lithium–borate glass to hydroxyapatite in aqueous phosphate solution

The conversion of glass to a hydroxyapatite (HA) material in an aqueous phosphate solution is used as an indication of the bioactive potential of the glass, as well as a low temperature route for preparing biologically useful materials. In this work, the effect of varying concentrations of pyrophosphate ions in the phosphate solution on the conversion of a calcium–lithium–borate glass to HA was investigated. Particles of the glass (150–355 μm) were immersed for up to 28 days in 0.25 M K₂HPO₄ solution containing 0–0.1 M K₄P₂O₇. The kinetics of degradation of the glass particles and their conversion to HA were monitored by measuring the weight loss of the particles and the ionic concentration of the solution. The structure and composition of the conversion products were analyzed using X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectroscopy. For K₄P₂O₇ concentrations of up to 0.01 M, the glass particles converted to HA, but the time for complete conversion increased from 2 days (no K₄P₂O₇) to 10 days (0.01 M K₄P₂O₇). When the K₄P₂O₇ concentration was increased to 0.1 M, the product consisted of an amorphous calcium phosphate material, which eventually crystallized to a pyrophosphate product (predominantly K₂CaP₂O₇ and Ca₂P₂O₇). The consequences of the results for the formation of HA materials and devices by the glass conversion route are discussed.     

Strength properties of ice–soil composites created by method of cryotropic gel formation

This paper considers the strengthening of both frozen soils and the soils after thawing in ice–soil composites generated by the method of cryotropic gel formation. The properties of the frozen soils (ice–soil composites) have to be improved for example in order to create reliable materials with low filtration factor for building weirs and other hydrotechnical constructions, which operate under a wide range of temperatures, including positive temperatures. The application of this method is very promising in terms of the creation of almost impermeable curtains for hydrotechnical structures in cold regions. It is very important that such elements are safe in both frozen and thawing conditions. Aqueous solutions of polyvinyl alcohol (PVA) are used for the formation of cryogels. This paper shows that the ice–soil composites, obtained by using the method of cryotropic gel formation, are sufficiently strong and watertight during thawing. Experimental data are provided.     

Effect of filler type on the mechanical properties of self-reinforced polylactide–calcium phosphate composites

Bioabsorbable polymers are of interest as internal fracture fixation devices. Self-reinforcement has been developed to improve the mechanical properties of the material and the addition of calcium phosphate fillers improves the bioactivity. Composite plates, produced by compression molding preimpregnated sheets of polylactide fibers coated in a polylactide matrix have been degraded in simulated body fluid for up to 12 weeks. Some samples also contained hydroxyapatite or tricalcium phosphate filler particles. Degradation was measured by monitoring the water uptake and mass decrease of the samples, as well as carrying out four point bend tests to assess the mechanical properties of the material. By 12 weeks, it was found that the unfilled samples absorbed more water and showed greater mass loss than the samples containing calcium phosphate fillers. Also, the flexural modulus and yield stress decreased significantly at week 12 for the unfilled samples. Adding hydroxyapatite (HA) or tricalcium phosphate (TCP) to the composite increased the flexural modulus and yield strength to values within the range of those reported for cortical bone and these values were maintained over the 12-week period.     

Effect of vinyl acetate content on the sintering behavior of hydroxyapatite–ethylene vinyl acetate copolymer composites

Ethylene vinyl acetate copolymer (EVA) alone could be used as a binder material for the fabrication of hydroxyapatite (HAP) into intricate shapes for various bone substitute applications. It was observed that as the vinyl acetate content in the polymer was increased from 12 to 28 wt % an increase in the sintered density of the HAP was observed. Retention of the shapes of HAP in the molded form was also observed.     

The role of oxidation in time-dependent response of ceramic–matrix composites

In this work, a model is constructed to account for the effect of oxidation of the fiber, fiber interface coating and surrounding matrix on the stress distribution and strain accumulation in ceramic–matrix composites. The model includes the role of the fabric architecture, the effect of porosity and the distribution of cracks in its formulation and utilizes oxidation rate constants and phenomenological models for the progress of oxidation as reported in literature.Dwell fatigue experiments were carried out for silicon carbide/silicon carbide nitride (SiC/SiNC) and Melt infiltrated silicon carbide/silicon carbide (MI SiC/SiC) composites to evaluate their time-dependent strain accumulation. Strain accumulation due to oxidation calculated by the model was compared to time-dependent strain obtained from experiment and showed that the rate of strain accumulation due to oxidation was low before the fibers were exposed to the environment but drastically increased after that. Such high rate of strain accumulation can be one of the main causes for failure of the composite.Model results showed that strain accumulation in both composites due to oxidation was dependent on the stress level with the SiC/SiNC accumulating more strain at similar stress levels. This can be explained by the higher modulus of the MI SiC/SiC that limits deformation, reducing crack density and accordingly decreasing the chance of oxygen to infiltrate the specimen and oxidize the fibers. Strain accumulation due to oxidation was also dependent on the fabric architecture and stress distribution within the unit cell. Additionally, comparing the effect of the value of the linear and parabolic oxidation rate constants reported by different researchers showed that not only is their absolute value important, but also their ratio to one another.     

Characterization of hydroxyapatite– and bioglass–316L fibre composites prepared by spark plasma sintering

To improve the mechanical properties of pure hydroxyapatite (HA) ceramics and pure 45S5 bioglasses, HA–316L fibre composites and bioglass 45S5–316L fibre composites were produced by spark plasma sintering (SPS) at 950 and 850 °C, respectively. While the HA phase in the HA–316L fibre composites did not decompose after the SPS process, microcracks were found around the 316L fibres in the composites. Consequently, the HA–316L fibre composites could not effectively improve the mechanical properties of the pure HA ceramics. In contrast, the bioglass 45S5–316L fibre composites showed no microcracks around the 316L fibres and thus exhibited bending strengths of up to 115 MPa.     

In situ formation of sintered cordierite–mullite nano–micro composites by utilizing of waste silica fume

This study aims at in situ formation of sintered cordierite–mullite nano–macro composites having high technological properties using waste silica fume, calcined ball clay, calcined alumina, and magnesia as starting materials. The starting materials were mixed in different ratios to obtain different cordierite–mullite composite batches in which the cordierite contents ranged from 50 to 100 wt.%. The batches were uni-axially pressed at 100 MPa and sintered at 1350, 1400 and 1450 °C to select the optimum temperature required for cordierite–mullite nano–macro composites formation. The formed phases were identified by X-ray diffraction (XRD) pattern. The sintering parameters in terms of bulk density (BD) and apparent porosity (AP) were determined. The microstructure of composites has been investigated by scanning electron microscope (SEM). Cold crushing strength (CCS) of the sintered batches was evaluated. The result revealed that the cordierite–mullite nano–macro composites were in-situ formed at 1400 °C. The batch containing 70 wt.% cordierite showed good physical and mechanical properties.     

Preparation of hydroxyapatite/α-tricalcium phosphate composites by colloidal process

Synthesised hydroxyapatite (HAp)/α-tricalcium phosphate (α-TCP) composites were prepared by colloidal process. Mixed slurries composed of 30 vol% HAp/α-TCP powder and 70 vol% aqueous solution containing a small amount of polymer dispersant were produced. HAp/α-TCP powder was blended with 0, 0.2, 0.4 or 1.0 mass fraction of α-TCP. Rheological behaviour of the HAp/α-TCP mixed slurries depended on the quantity of polymer dispersant and pH. Addition of suitable quantity of polymer dispersant at higher pH resulted in an increase in the stability of colloidal state due to electrostatic stabilisation. The optimum quantity of dispersant for the colloidal process was found to be proportional to the mass fraction of α-TCP. In the dehydration process of the slurries, pressure filtration technique was effective in preventing the gravity segregation of α-TCP. Thus, HAp/α-TCP composites with homogeneous microstructure and high relative density could be prepared by optimising the colloidal process.     

Pulse electric current sintering of hydroxyapatite/β-tricalcium phosphate composites

Hydroxyapatite (HA)/β-tricalcium phosphate (β-TCP) composites attract attentions as bone implant materials. As one of the fabrication method of HA/β-TCP is mixing of HA and β-TCP powder in advance of sintering. This method enables to control the ratio of content of β-TCP easier. However, it is difficult to obtain dense composites. In this study, we focused on pulse electric current sintering (PECS) to obtain dense HA/β-TCP composites. The sinterability is evaluated with relative density and grain size measurements. Composition of sintered body was also characterized by X-ray diffraction. In comparison with pressureless sintering, PECS increased relative density of the composites without grain growth. In HA/β-TCP sintered by PECS, the phase transformation from β-TCP to α-TCP was promoted. This is due to higher thermal energy by spark discharge during PECS. On the other hand, sintering additives (MgO) inhibited phase transformation. It was suggested that sinterability of HA/β-TCP composites was improved by PECS.     

A study of the formation of amorphous calcium phosphate and hydroxyapatite on melt quenched Bioglass® using surface sensitive shallow angle X-ray diffraction

Melt quenched silicate glasses containing calcium, phosphorous and alkali metals have the ability to promote bone regeneration and to fuse to living bone. These glasses, including 45S5 Bioglass^® [(CaO)_26.9(Na₂O)_24.4(SiO₂)_46.1(P₂O₅)_2.6], are routinely used as clinical implants. Consequently there have been numerous studies on the structure of these glasses using conventional diffraction techniques. These studies have provided important information on the atomic structure of Bioglass^® but are of course intrinsically limited in the sense that they probe the bulk material and cannot be as sensitive to thin layers of near-surface dissolution/growth. The present study therefore uses surface sensitive shallow angle X-ray diffraction to study the formation of amorphous calcium phosphate and hydroxyapatite on Bioglass^® samples, pre-reacted in simulated body fluid (SBF). Unreacted Bioglass^® is dominated by a broad amorphous feature around 2.2 Å^?1 which is characteristic of sodium calcium silicate glass. After reacting Bioglass^® in SBF a second broad amorphous feature evolves ~1.6 Å^?1 which is attributed to amorphous calcium phosphate. This feature is evident for samples after only 4 h reacting in SBF and by 8 h the amorphous feature becomes comparable in magnitude to the background signal of the bulk Bioglass^®. Bragg peaks characteristic of hydroxyapatite form after 1–3 days of reacting in SBF.