The correlation between grain size, optical birefringence, and transparency is discussed for tetragonal zirconia (ZrO2) ceramics using the Mie, Rayleigh, and Rayleigh–Gans–Debye scattering models. ...Our results demonstrate that at the degree of mean birefringence in the range (0.03–0.04) expected for tetragonal ZrO2, only the Mie theory provides reasonable results. At small particle size (<50 nm) the more straightforward Rayleigh approximation correlates with the Mie model. A real in‐line transmission of ∼50% at visible light and 1 mm thickness is expected at a mean grain size <40 nm and ∼70% at a mean grain size <20 nm. At an infrared (IR) wavelength of 5 μm there should not be any scattering caused by birefringence for grain sizes <200 nm. Our simulations were validated with experimental data for tetragonal ZrO2 (3 mol% Y2O3) ceramics made from a powder with an initial particle size of ∼10 nm by sintering in air and using hot‐isostatic pressing. The maximum in‐line transmission of about 77% was observed at IR wavelengths of 3–5 μm.
Tetragonal zirconia polycrystals (TZP) represent a favorite material for monolithic ceramic dental restorations. However, all approaches employed so far to improve the translucency of dental zirconia ...ceramics are accompanied by a significant decline in strength. In this investigation, we developed dental 3Y‐TZP ceramics that can provide excellent strength combined with enhanced translucency. The machinable tetragonal zirconia discs and blocks were prepared from fine mesostructured zirconia particles stabilized with 3 mol% of yttria using the gelcasting method. Zirconia ceramics with an average biaxial strength of 1184 MPa and translucency of 41.1% for a 1 mm thick sample were obtained. Due to its unique microstructure, this tetragonal ceramic provided a favorable combination of high translucency comparable to the high‐translucent, tetragonal/cubic 4Y‐TZP and very high strength achievable only in the pure tetragonal 3Y‐TZP. The applicability and resistance to low‐temperature degradation of the new dental ceramics was demonstrated.
In this investigation, we developed dental 3Y‐TZP ceramics that can provide excellent strength combined with enhanced translucency. The CAD/CAM tetragonal zirconia discs and blocks were prepared from fine mesostructured zirconia particles stabilized with 3 mol% of yttria using the gelcasting method. Zirconia ceramics with an average biaxial strength of 1184 MPa and translucency of 41.1% for a 1 mm thick sample were obtained. Newly developed 3Y‐TZP ceramic provided a favorable combination of high translucency comparable to the high‐translucent, tetragonal/cubic 4Y‐TZP and very high strength achievable only in the pure tetragonal 3Y‐TZP. The applicability and resistance to low‐temperature degradation (LTD) of the new dental ceramics was demonstrated
Fracture toughness of nanocrystalline zirconia ceramic stabilized with 1.5
mol.% Y
2O
3 was measured and compared with fracture toughness of submicrometer-grained zirconia ceramics stabilized with 3
...mol.% Y
2O
3. It was found that nanocrystalline ceramics demonstrated higher fracture toughness than submicrometer-grained zirconia ceramics with both optimized and common grain sizes. The high fracture toughness of nanocrystalline zirconia was confirmed by two different testing methods: indentation method (15.5
MPa
m
0.5) and chevron notched beam method (7.9
MPa
m
0.5).
Four nanometer‐sized zirconia powders stabilized by 3 mol% Y2O3 were used for the preparation of dense bulk ceramics. Ceramic green bodies were prepared by cold isostatic pressing at pressures of ...300–1000 MPa. The size of the pores in ceramic green bodies and their evolution during sintering were correlated with the characteristics of individual nanopowders and with the sintering behavior of powder compacts. Only homogeneous green bodies with pores of <10 nm could be sintered into dense bodies (>99% t.d.) at a sufficiently low temperature to keep the grain sizes in the range <100 nm. Powders with uniform particles 10 nm in size yielded green bodies of required microstructure. These nanoparticle compacts were sintered without pressure to give bodies (diameter 20 mm, thickness 4 mm) with a relative density higher than 99% and a grain size of about 85 nm (as determined by the linear intercept method).
A novel approach for producing predetermined, complex 3d ceramic architectures by robotic deposition where UV radiation is used for solidification is presented. Homogeneous, highly loaded, solvent ...free colloidal inks with controlled viscoelastic properties are achieved by proper selection of monomers and surfactants. Room temperature deposition of complex 3d fiber networks having filaments in the 100μm range is demonstrated for Al2O3 and hydroxyapatite model particles for structural and biomedical applications. Solidification of the structures by UV radiation allows additional shaping of the structures by post-printing processes such as cutting, folding and bonding. 2d and 3d architectures with high aspect ratios retain their shape and transform to macroscopic ceramics after thermal debinding and sintering procedures. Sintered alumina fiber networks functionalized with a 3–5μm layer of TiO2 nanoparticles exhibit photocatalytic activity for the decomposition of formaldehyde as a similar weight of loose powder, indicating possible applications in catalytic reactors prototypes.
The current limitations of calcium phosphate cements (CPCs) used in the field of bone regeneration consist of their brittleness, low injectability, disintegration in body fluids and low ...biodegradability. Moreover, no method is currently available to measure the setting time of CPCs in correlation with the evolution of the setting reaction. The study proposes that it is possible to improve and tune the properties of CPCs via the addition of a thermosensitive, biodegradable, thixotropic copolymer based on poly(lactic acid), poly(glycolic acid) and poly(ethylene glycol) (PLGA⁻PEG⁻PLGA) which undergoes gelation under physiological conditions. The setting times of alpha-tricalcium phosphate (α-TCP) mixed with aqueous solutions of PLGA⁻PEG⁻PLGA determined by means of time-sweep curves revealed a lag phase during the dissolution of the α-TCP particles. The magnitude of the storage modulus at lag phase depends on the liquid to powder ratio, the copolymer concentration and temperature. A sharp increase in the storage modulus was observed at the time of the precipitation of calcium deficient hydroxyapatite (CDHA) crystals, representing the loss of paste workability. The PLGA⁻PEG⁻PLGA copolymer demonstrates the desired pseudoplastic rheological behaviour with a small decrease in shear stress and the rapid recovery of the viscous state once the shear is removed, thus preventing CPC phase separation and providing good cohesion. Preliminary cytocompatibility tests performed on human mesenchymal stem cells proved the suitability of the novel copolymer/α-TCP for the purposes of mini-invasive surgery.
Dense multi-cationic Sm-Co-O, Sm-Ni-O, Al-Co-O, Al-Ni-O, and Al-Ni-Co-O oxide aerogels were prepared by epoxide-driven sol-gel synthesis. Catalysts for dry reformation of methane, Sm
O
/Co, Sm
O
/Ni, ...Al
O
/Co, Al
O
/Ni, Al
O
/Co, and Ni were prepared by reduction of aerogels with hydrogen and their catalytic activities and C-deposition during dry reformation of methane were tested. Catalytic tests showed high methane conversion (93-98%) and C-deposition (0.01-4.35 mg C/g
h). The highest content of C-deposits after catalytic tests was determined for Al
O
/Co and Al
O
/Ni catalysts, which was related to the formation of Al alloys with Co and Ni. A uniform distribution of Co
and Ni
nanoparticles (in the form of a CoNi alloy) was found only for the Al
O
/Co and Ni catalysts, which showed the highest activity as well as low C deposition.
An approach to the preparation of customized hydroxyapatite scaffolds for bone regeneration based on subtractive manufacturing has been developed. The developed method is intended as a simple ...alternative to rapid prototyping based on additive manufacturing methods. The method for the preparation of customized scaffolds consists in computer numerical controlled (CNC) milling of porous hydroxyapatite foam. The machinable foam for customized scaffolds was prepared by direct foaming of a colloidal hydroxyapatite suspension and the foam structure was consolidated by the gelcasting method. The optimal foam after sintering reached a high porosity of 83.3% and the cellular-like structure contained spherical pores with an average diameter of 613µm, which were connected through windows with an average diameter of 161µm. The compressive strength of the sintered foam reached an average value of 2MPa. CNC milling was tested at different stages of the production cycle of the porous foam and after the reinforcing of the foam by impregnation. The best results were obtained by milling the foam presintered at 1100°C. The milling process was demonstrated and scaffold examples were successfully manufactured.
Zirconia nanoparticles were densified by pressureless sintering and spark plasma sintering (SPS). The evolution of relative density and pore size distribution in powder compacts during these ...sintering processes were compared. It was found that pore size increase was suppressed during SPS and that this facilitated the densification. The combined sintering process (low-temperature pre-sintering by SPS followed by pressureless sintering) was performed to confirm the advantage of the low-temperature SPS technique (referred to as SPS warm pressing).
A method for manufacturing bi-layered zirconia rods of core-shell geometry
with a porous core and a dense shell has been developed. Core-shell rods were
successfully prepared by thermoplastic ...co-extrusion of assembled feed rods
composed of core and shell zirconia feedstocks. Rheological analysis and
adjustment of the feedstock viscosities enabled co-extrusion of regular
core-shell rods of uniform shell thickness. Tapioca starch was used as a
pore-forming agent in the core feedstock. Binder removal and high temperature
treatment had to be modified in order to safely remove the starch particles.
Sintering analysis revealed constrained sintering of porous cores in the
core-shell rods due to the rigid dense shell, which resulted in an increased
porosity in the core. Defect-free core-shell rods with a core porosity of up
to 40% and different thicknesses of the dense shell were prepared.
nema
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