钛掺杂对羟基磷灰石烧结稳定性和晶粒尺寸的影响

采用化学沉淀法通过钛掺杂合成了不同钛掺杂量(0.2%~2.4%(质量分数))的羟基磷灰石纳米粉,研究了钛掺杂量对其烧结稳定性和晶粒尺寸的影响规律。结果表明,不同钛掺杂量对羟基磷灰石的烧结稳定性和晶粒尺寸有非常显著的影响。随着钛掺杂量的增加,羟基磷灰石的晶格结构发生变化,a、c轴尺寸及其晶胞体积都在增加,并且和钛掺杂量成正比。在1 000~1 200℃烧结,不同钛掺杂量羟基磷灰石均未发生分解,表明钛掺杂抑制了羟基磷灰石的高温分解,提高了它的烧结稳定性。同时,钛掺杂有效限制了烧结过程中羟基磷灰石晶粒的长大,0.8%(质量分数)钛掺杂羟基磷灰石晶粒尺寸较纯羟基磷灰石晶粒尺寸明显细化。     

Effect of polyvinyl alcohol additive on the pore structure and morphology of the freeze-cast hydroxyapatite ceramics

Porous hydroxyapatite (HAP) ceramics with different morphologies were fabricated by the freeze casting method. The morphologies of HAP ceramics were modified by adjusting the concentration of polyvinyl alcohol (PVA) additive in the HAP slurries. HAP ceramics without PVA additive were composed of non-interconnected macroscopic lamellar pores and porous ceramic walls. With PVA additive, the HAP ceramics were made up of small lamellar pores or three-dimensional reticulate pores and porous ceramic walls. PVA additive had no effect on the phase composition of HAP ceramics. The open porosity and pore connectivity were improved because of the addition of PVA.     

Compressive strength and processing of camphene-based freeze cast calcium phosphate scaffolds with aligned pores

Porous calcium phosphate (CaP) scaffolds with aligned pores were fabricated by unidirectionally freezing a CaP/camphene slurry at 32 °C for various times (1, 2, 3 days). During this process, camphene dendrites grew preferentially from the bottom to the top of the cast body. The frozen samples were then freeze-dried to remove the solid camphene and sintered at 1200 °C for 3 h to densify the CaP walls. All of the fabricated samples showed a highly aligned pore structure with a porosity of 62-65 vol.%, regardless of the freezing time. As the freezing time was increased from 1 to 3 days, the pore size increased from 122 to 166 μm due to the continual overgrowth of camphene dendrites, while the compressive strength decreased from 9.3 ± 1.6 to 6.2 ± 1.3 MPa due to the increase in pore size. However, it should be noted that the compressive strength of the sample tested parallel to the freezing direction was much higher than that of the sample tested normal to the direction of freezing, indicating the utility of the aligned pores.     

Effect of silica and hydroxyapatite mineralization on the mechanical properties and the biocompatibility of nanocomposite collagen scaffolds

A recently established materials concept of biomimetic composites based on silica, collagen, and calcium phosphates was adapted for the preparation of porous scaffolds suitable for tissue engineering applications. Mineralization was achieved by directed nucleation of silica on the templating organic phase during a sol-gel process with or without addition of hydroxyapatite. Both mineral phases (25 wt %, individually or combined in equal shares) influenced the scaffold's morphology at the nanoscale. Enhancement of apparent density and compressive strength was similar for silica or hydroxyapatite mineralization; however the stiffening effect of hydroxyapatite was much higher. All scaffold modifications provided proper conditions for adhesion, proliferation, and osteogenic differentiation of human bone marrow stromal cells. The open porosity allowed cells to migrate throughout the scaffolds while maintaining their viability, both confirmed by MTT staining and confocal laser scanning microscopy. Initial cell distributions were graduated due to collagen mineralization, but balanced out over the cultivation time of 28 days. RT-PCR analyses revealed higher gene expression of ALP but lower expression of BSP II and osteocalcin because of collagen mineralization. The results demonstrate that both silica and hydroxyapatite offer comparable possibilities to tailor mechanical properties of collagen-based scaffolds without being detrimental to in vitro biocompatibility.     

Effect of hybridization of liquid rubber and nanosilica particles on the morphology, mechanical properties, and fracture toughness of epoxy composites

A liquid carboxyl-terminated butadiene–acrylonitrile copolymer (CTBN) and SiO₂ particles in nanosize were used to modify epoxy, and binary CTBN/epoxy composites and ternary CTBN/SiO₂/epoxy composites were prepared using piperidine as curing agent. The morphologies of the composites were observed by scanning electron microscope (SEM) and transmission electron microscope (TEM), and it is indicated that the size of CTBN particles increases with CTBN content in the binary composites, however, the CTBN particle size decreases with the content of nanosilica in the ternary composites. The effects of CTBN and nanosilica particles on the mechanical and fracture toughness of the composites were also investigated, it is shown that the tensile mechanical properties of the binary CTBN-modified epoxy composites can be further improved by addition of nanosilica particles, moreover, obvious improvement in fracture toughness of epoxy can be achieved by hybridization of liquid CTBN rubber and nanosilica particles. The morphologies of the fractured surface of the composites in compact tension tests were explored attentively by field emission SEM (FE-SEM), it is found that different zones (pre-crack, stable crack propagation, and fast crack zones) on the fractured surface can be obviously discriminated, and the toughening mechanism is mainly related to the stable crack propagation zone. The cavitation of the rubber particles and subsequent void growth by matrix shear deformation are the main toughening mechanisms in both binary and ternary composites.     

Mullite-zirconia composites: Effect of citric acid addition on slip and cast properties

Dispersion of mixed suspensions of zircon and α -alumina using citric acid as dispersant followed by pressure filtration produced after sintering (1600^∘C—2 h) dense mullite-zirconia composites. The effect of citric acid addition and pH on the rheological behavior of the mixed suspensions (57 vol%) was examined. Most of flow curves showed a shear thinning behavior at low shear rates reaching the viscosity a plateau at high shear rates. Newtonian viscosity at high shear rates values decreased to a minimum and then increased with increasing the amount of dispersant added. Compacts prepared from well stabilized suspensions achieved a maximum relative density of 72% (theoretical) which is slightly higher than that obtained from suspensions dispersed with a polyelectrolyte. Characteristics of sintered compacts such as density and crystalline phase composition by XRD were determined.     

Fabrication of titanium scaffolds with porosity and pore size gradients by sequential freeze casting

This paper reports a novel method for producing porous Ti scaffolds with a gradient in porosity and pore size using the freeze casting method, in which TiH₂/camphene slurries with various TiH₂ contents (40, 25, and 10 vol.%) were cast sequentially into a mold, followed by freeze drying and heat-treatment in a vacuum at 1300 °C for 3 h. This simple sequential freeze casting method produced good bonding between the layers with different porosities of 35, 53, and 75 vol.% obtained using the TiH₂ contents of 40, 25 and 10 vol.%, respectively. In addition, the pore size could be increased significantly by increasing the freezing time. The pore sizes obtained in the regions produced using 40, 25, and 10 vol.% TiH₂ after freezing for 7 days were 96, 166, and 270 μm, respectively.     

Effect of ZrO2 addition on the mechanical properties of porous TiO2 bone scaffolds

This study aimed at the investigation of the effect of zirconium dioxide (ZrO₂) addition on the mechanical properties of titanium dioxide (TiO₂) bone scaffolds. The highly biocompatible TiO₂ has been identified as a promising material for bone scaffolds, whereas the more bioinert ZrO₂ is known for its excellent mechanical properties. Ultra-porous TiO₂ scaffolds (> 89% porosity) were produced using polymer sponge replication with 0–40 wt.% of the TiO₂ raw material substituted with ZrO₂. Microstructure, chemical composition, and pore architectural features of the prepared ceramic foams were characterised and related to their mechanical strength. Addition of 1 wt.% of ZrO₂ led to 16% increase in the mean compressive strength without significant changes in the pore architectural parameters of TiO₂ scaffolds. Further ZrO₂ additions resulted in reduction of compressive strength in comparison to containing no ZrO₂. The appearance of zirconium titanate (ZrTiO₄) phase was found to hinder the densification of the ceramic material during sintering resulting in poor intergranular connections and thus significantly reducing the compressive strength of the highly porous ceramic foam scaffolds.     

Influence of hydrogen on the phase composition and physicomechanical properties of structural materials

The results of investigations of the influence of hydrogen on the processes of phase formation in structural materials are analyzed. On the basis of the study of self-diffusion, interdiffusion , and atomic ordering in hydrogenated nickel and alloys of the Fe–Ni, Ni–Mo, and Ni–Mo–Re systems, it is shown that hydrogen accelerates these processes by increasing the degree of atomic ordering and decreasing the critical temperatures of formation and decomposition of intermetallic phases. By analyzing these alloys and VKh-4A steel taken as an example, we demonstrate the possibilities of application the obtained results to the improvement of the physicomechanical properties of these materials.     

The effect of nanosilica addition on flowability,strength and transport properties of ultra high performance concrete

The experimental study herein presented was conducted aiming to evaluate the influence of nanosilica (nS) addition on properties of ultra-high performance concrete (UHPC). Thermo gravimetric analysis results indicated that nS consumes much more Ca(OH)₂ as compared to silica fume, specifically at the early ages. Mercury intrusion porosimetry measurements proved that the addition of nS particles leads to reduction of capillary pores. Scanning electron microscope observation revealed that the inclusion of nS can also efficiently improve the interfacial transition zone between the aggregates and the binding paste. The addition of nS also resulted in an enhancement in compressive strength as well as in transport properties of UHPC. The optimum amount of cement replacement by nS in cement paste to achieve the best performance was 3 wt.%. However, the improper dispersion of nS was found as a deterrent factor to introduce higher percentage of nS into the cement paste.     

The effect of alginate addition on the structure and morphology of hydroxyapatite/gelatin nanocomposites

A series of hydroxyapatite/gelatin/alginate nanocomposites with different amount of alginate were synthesized by a co-precipitation method. With the increase of alginate amount, a cross-linked alginate/gelatin polymer network formed, which induced a gradual red shift of organic absorption peaks in FT-IR analysis. TEM images indicated that the development of HAP nanocrystals in an aqueous gelatin/alginate mixture was highly influenced by the alginate content. On increasing alginate content, the dimensions of the crystals increased and their morphology changed from needle-like to long fiber-like, and at high alginate content, the crystals tended to aggregate in separate clusters. The results of the electron diffraction strongly indicated alginate promoted the preferential alignment in c direction of HAP nanocrystals. SEM results showed that high amount of alginate led to regular shape and large size of HAP crystals after the composites were calcined for 4 h at 600 °C.     

Effect of carbon nanotube and aluminum oxide addition on plasma-sprayed hydroxyapatite coating's mechanical properties and biocompatibility

This study reports on the synthesis of novel bioceramic composite coating of hydroxyapatite (HA) reinforced with carbon nanotubes (CNTs) and aluminum oxide (Al₂O₃) using plasma spray technique. Fracture toughness of HA–20 wt.% Al₂O₃ improved by 158% as compared to HA coating whereas HA–18.4 wt.% Al₂O₃–1.6 wt.% CNT showed an improvement of 300%. Carbon nanotubes provided reinforcement via rebar mechanism. Human fiber osteoblast cell-growth studies showed that biocompatibility of the coating remained unaltered, as Al₂O₃ retained its bio-inertness and CNT, its bioactivity, within the composite coatings. Composite coating showed lower attachment, but higher proliferation rate, for the osteoblast cells, which has been attributed to the surface roughness. An optimized relation between coating composition, its biocompatibility and mechanical properties was established to predict the most suited coating material for orthopedic implants. HA–Al₂O₃–CNT composite coating displayed most improved mechanical properties while retaining its biocompatibility.     

Effect of irradiation on physicomechanical properties of polymer materials

Investigations of the effect of irradiation on physicomechanical properties of polymers are reviewed, and it is shown that studies of physicomechanical properties of irradiated specimens must be regarded as only the first stage of researches directed toward elucidating this problem. Complete understanding of the effect of irradiation on the performance of materials of this type can be obtained only by studies of the effects of simultaneous action of irradiation and mechanical loads, especially studies of the long-time strength carried out with a view to determining the range of application of polymers under these conditions. It is shown also that in studies of this kind it is essential to take into account the effect of working media.     

Effect of mechanical activation on the morphology and structure of hydroxyapatite

We have studied the effect of grinding in planetary mills on the phase composition, morphology, and water content of hydroxyapatite powder. The results indicate that milling for even relatively short times, which reduces the average particle size by a factor of 2, causes the monetite present in the unmilled powder to disappear and reduces the crystallite size of the hydroxyapatite. The fraction of nanoparticles in the powder is then 98% and remains constant during further milling. Milling for longer times leads to hydroxyapatite amorphization. For an average size of large particles R ≥ 1 μm, the surface area of the particles per unit volume, E (cm⁻¹), is determined only by R (E ∼ 1/R).     

Effect of stress-induced phase transformation on the properties of polycrystalline zirconia containing metastable tetragonal phase

Polycrystalline zirconia containing a high content of metastable tetragonal phase shows high strength ( 700 MPa), high fracture toughness (K _c = 6 to 9 MN m^–3/2) and small grain size (<0.3jm). The strength and grain size remain nearly constant over a wide range of tetragonal phase content (100 to 30%). At a low concentration of tetragonal phase <30%, there is a rapid decrease in strength accompanied by a rapid increase in grain size. These results are explained by means of a stress-induced phase transformation in the metastable tetragonal phase.     

Effect of solid content on pore structure and mechanical properties of porous silicon nitride ceramics produced by freeze casting

Porous Si₃N₄ ceramics were prepared by freeze casting using liquid N₂ as refrigerant. The pore structure, porosity, α → β-Si₃N₄ transformation and mechanical properties of the obtained materials were strongly affected by the solid contents of the slurries. Increasing the solid content would reduce the porosity, decrease the pore size and change the pore structure from the aligned channels with dendrites to the round pores with decreased pore size. The formation of this round pores impeded the α → β-Si₃N₄ phase transformation, but was beneficial to the mechanical properties of the obtained porous Si₃N₄ due to its unique pore structure.