Porous titanium (Ti) scaffolds by freezing TiH2/camphene slurries

Porous titanium (Ti) scaffolds with interconnected pores were fabricated by freezing titanium hydride (TiH₂)/camphene slurries at 33 °C for 24 h, followed by freeze-drying and subsequent heat-treatment at 1300 °C for 2 h in vacuum. All of the fabricated samples revealed highly porous structures having large pores up to 100 μm in size surrounded by Ti metal walls without any secondary phases. When the initial TiH₂ content was increased from 15 to 25 vol.%, the porosity was decreased from 63 to 49%, while the compressive strength was significantly improved from 81 to 253 MPa.     

Highly porous hydroxyapatite bioceramics with interconnected pore channels using camphene-based freeze casting

We fabricated highly porous hydroxyapatite (HA) bioceramics using the camphene-based freeze casting method. In this method, HA/camphene slurries with various HA contents (10, 15, and 20 vol.%) were prepared by ball-milling at 60 °C and then cast into a mold at room-temperature. This method allowed the fabricated sample to have completely interconnected pore channels by removing the frozen camphene network via sublimation and dense HA walls by sintering the highly packed HA powder networks at 1250 °C for 3 h. As the initial HA content was increased from 10 to 20 vol.%, the porosity decreased from 75 to 56%, while the compressive strength remarkably increased from 0.94 to 17 MPa.     

Fabrication of highly porous titanium (Ti) scaffolds with two interlaced periodic pores

This paper reports a novel type of porous titanium (Ti) scaffolds with two interlaced periodic pores that were produced by coating the surfaces of a dual-channeled hydroxyapatite (HA) scaffold, as a supporting framework, with a titanium hydride (TiH₂) slurry followed by heat-treatment at 1200 °C for 3 h in a vacuum to convert TiH₂ to Ti metal. This method allowed the porous Ti scaffolds to mimic the original pore structure of the dual-channeled HA scaffold in a tightly controlled manner. It was observed that the Ti layer was strongly adhered to the HA layer, owing to the diffusion of P ions into the Ti layer. The fabricated sample showed a high compressive strength of 6.0 ± 0.77 MPa and a porosity of 78 vol.% due to its unique pore structure, as well as perfect interconnections between the pores.     

Elaboration and mechanical characterization of multi-phase alumina-based ultra-fine composites

Al₂O₃-10 vol.% YAG and Al₂O₃-10 vol.% ZrO₂ bi-phase composites as well as Al₂O₃-5 vol.% YAG-5 vol.% ZrO₂ tri-phase composite were developed by controlled surface modification of an alumina powder with inorganic precursors of the second phases. Green bodies were produced by dry pressing and slip casting and then sintered at 1500 °C. In particular, slip casting led to fully dense, defect-free, and highly homogenous samples, made of a fine dispersion of the second phases into the micronic alumina matrix, as observed by SEM. The mechanical characterization proved the predominant role of the final density on the Vickers hardness, while the elastic modulus was affected by the volume fraction of the constituent phases, in fairly good agreement with the rule of mixture prediction. The fracture toughness values of the bi- and tri-phase materials were similar, and their crack paths revealed the importance of the thermal residual stresses at the matrix-reinforcement interfaces, promoting inter-granular propagations.     

Superplasticity and machinability in a four-phase ceramic

Fine grained four-phase ceramic materials were fabricated to have a combination of high temperature superplasticity and room temperature machinability. The composite ceramics were made of 25 vol.% 3Y-TZP/8YSZ–25 vol.% Al₂O₃–25 vol.% MgAl₂O₄–25 vol.% LaPO₄, using LaPO₄ powders fabricated in-house. X-ray diffraction and scanning electron microscopy revealed that the grain size for the best mixed samples was in the range of 600 nm, tetragonal ZrO₂ transformed into monoclinic, and reactions in the sintered samples produced a new phase, magnetoplumbite (LaMgAl₁₁O₁₉) with lath-like grains. The formation of magnetoplumbite was facilitated by the presence of yttria and by a liquid phase generated at elevated temperatures. These four-phase ceramics had a maximum hardness of 12 GPa and fracture toughness was no more than 3 MPa√m. Deformation rates at 1400 °C under 40 MPa stress were in the superplastic range of 10^?3 s^?1 for most compositions. These four-phase ceramics were machinable as demonstrated using conventional tungsten drill bits.     

Fabrication of porous titanium scaffolds with high compressive strength using camphene-based freeze casting

We herein report the fabrication of highly porous titanium (Ti) scaffolds with unusually high compressive strength by freezing a titanium hydride (TiH₂)/camphene slurries at 42 °C. As the freezing time was increased from 1 to 7 days, the pore size obtained was increased significantly from 143 to 271 μm due to the continual overgrowth of camphene dendrites. However, interestingly, the formation of the micro-pores inside the Ti walls was suppressed at longer freezing time. This resulted in a significant increase in compressive strength up to 110 ± 17 MPa with a porosity of 64%. It is believed that this unusually high compressive strength with large interconnected pores makes this material suitable for applications as load-bearing parts.     

Influence of the reactive N2 gas flow on the properties of rf-sputtered ZnO thin films

Nitrogen (N)-doped ZnO thin films were RF sputtered with different N₂ volume (ranging from 10% to 100%) on sapphire (001) substrates. The influence of N₂ vol.% on the properties of ZnO films was analyzed by various characterization techniques. The X-ray diffraction studies showed that the films grow along the preferential (002) crystallographic plane and the crystallinity varied with varying N₂ vol.%. The films sputtered with 25 vol.% N₂ showed better crystallinity. The transmittance was decreased with increasing N₂ volume until 25% and was almost constant above 25%. A maximum optical band gap (2.08 eV) obtained for 10 vol.% N₂ decreased with increasing N₂ volume to reach a minimum of 1.53 eV at 100%. The compositional analysis confirmed the incorporation of N into ZnO films, and its concentration increased with increasing N₂ volume to reach a maximum of ∼ 3.7 × 10²¹ atom/cm³ at 75% but then decreased slightly to 3.42 × 10²¹ atoms/cm³. The sign of Hall coefficient confirmed that the films sputtered with ≤ 25 vol.% N₂ possess p-type conductivity which changes to n-type for > 25 vol.% N₂.     

High-porosity NiTi superelastic alloys fabricated by low-pressure sintering using titanium hydride as pore-forming agent

Porous NiTi shape memory alloys (SMAs) were successfully fabricated by low-pressure sintering (LPS), and the pore features have been controlled by adjusting the processing parameters. The porous NiTi SMAs with high porosity (45%) and large pore size (200–350 μm) can be prepared by LPS using TiH_1.5 as pore-forming agent. These alloys exhibit isotropic pore structure with three-dimensional interconnected pores. The porous NiTi SMA produced by LPS exhibits superelasticity and mechanical properties superior to that by conventional sintering.     

Effect of mechanical milling on Ni–TiH2 powder alloy filler metal for brazing TiAl intermetallic alloy: The microstructure and joint's properties

A TiH₂–50 wt.% Ni powder alloy was mechanically milled in an argon gas atmosphere using milling times up to 480 min. A TiAl intermetallic alloy was joined by vacuum furnace brazing using the TiH₂–50 wt.% Ni powder alloy as the filler metal. The effect of mechanical milling on the microstructure and shear strength of the brazed joints was investigated. The results showed that the grains of TiH₂–50 wt.% Ni powder alloy were refined and the fusion temperature decreased after milling. A sound brazing seam was obtained when the sample was brazed at 1140 °C for 15 min using filler metal powder milled for 120 min. The interfacial zones of the specimens brazed with the milled filler powder were thinner and the shear strength of the joint was increased compared to specimens brazed with non-milled filler powder. A sample brazed at 1180 °C for 15 min using TiH₂–50 wt.% Ni powder alloy milled for 120 min exhibited the highest shear strength at both room and elevated temperatures.     

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.     

Dynamic freeze casting for the production of porous titanium (Ti) scaffolds

This paper proposes dynamic freeze casting as a new manufacturing technique for producing porous Ti scaffolds with a uniform porous structure and good ductility. In this method, Ti/camphene slurries with various initial Ti contents (15, 20, and 25 vol.%) were frozen at 44 °C for 12 h in rotation, which allowed for the extensive growth of camphene crystals and the uniform construction of walls made of Ti particles. All the fabricated samples showed spherical-like pores surrounded by dense Ti walls that were uniformly formed after sintering at 1300 °C for 2 h in a vacuum. The porosity decreased from 71 to 52 vol.% with an increase in Ti content from 15 to 25 vol.%, whereas the pore size decreased from 362 to 95 μm. On the other hand, the compressive strength and stiffness increased considerably from 57 ± 4 to 183 ± 6 MPa and from 1.3 ± 0.5 to 5.0 ± 0.8 GPa, respectively, due to the decrease in the porosity of the samples.     

Growth mechanism of TiN reaction layers produced on AlN via active metal bonding

Interfacial reactions related to the TiN layer growth process between nanocrystalline epitaxial layers of AlN deposited on c-plane sapphire and a Ti-containing metal brazing or sintering layer using Ag–Cu–TiH₂, Ag–TiH₂ and Cu–TiH₂ pastes have been investigated. The brazed/sintered samples were heated in vacuum at 850 °C for 30 min. The TiN layer produced at the metal/AlN interfaces consists of TiN particles?<?50 nm in size and grain boundary phases including Al-containing Ag and Al-containing Cu. The Al concentration within the TiN layer decreases as the distance increases from the AlN epitaxial layer. These experimental observations all suggest that when AlN is used as a starting material in the active metal bonding method, interfacial reaction processes take place with the generation of a local Al-based eutectic liquid phase and elemental transport through this eutectic liquid phase.

     

Freeze casting of porous Ni-YSZ cermets

Dual-phase porous Ni-YSZ cermets were fabricated via the freeze casting of a ceramic/camphene slurry. After removing the frozen camphene via sublimation at room temperature, the green samples were sintered for 3 h in air at various temperatures, ranging from 1100 to 1350 °C, and then reduced in an Ar-5% H₂ atmosphere at 700 °C for 3 h. The fabricated Ni-YSZ cermets showed 3-D pore channels formed by the replication of the entangled camphene dendrite network and small pores in the Ni-YSZ walls produced by partial sintering of the NiO-YSZ composite. Furthermore, the fabricated samples were found to possess reasonable electrical conductivities, thus rendering them suitable for use as the basic components of planar solid oxide fuel cells (SOFCs).     

Effect of TiH2 particle size distribution on aluminum foaming using the powder metallurgy method

TiH₂ decomposes over a range of temperatures strongly influenced by diverse factors including particle size. In the present research, a systematic study of the dehydrogenation behavior of TiH₂ powder of different particle size distribution was undertaken with the aid of thermogravimetric analysis. The effect of this parameter on aluminum foaming was evaluated. It was found that when TiH₂ exceeds a critical particle size (around 50 μm), dehydrogenation occurs as a single desorption event with onset temperatures around 500 °C. The reduction of particle size, besides reducing the onset of hydrogen release, decreases the dehydrogenation rate. As a result, the first dehydrogenation event gets sharper and tends to overlap with the second with increasing particle size. The use of selected powders on foaming showed that the final foam expansion and porosity features, such as pore size, pore density, and homogeneity are largely influenced by the particle size distribution of the foaming powder. TiH₂ of the largest particle size was the most suitable for foaming pure aluminum.     

The morphology of limestone-based pellets prepared with kaolin-based binders

Modifications and pelletization of limestone were investigated in order to improve the utilization of CaO-based materials for different catalytic reactions and environmental applications. Attempts to purify the limestone by ion-exchange with CaCl₂ solution did not result in significant removal of impurities. On the other hand, acetification with 10 vol.% acetic acid enhanced pore surface area and pore volume of the sorbent by 42% and 3-fold, respectively. The acetification was found to widen small pores, and thus create a beneficial pore size distribution with more pores in the range of 25–100 nm. In order to utilize such powdered materials in fluidized beds, pelletization is the next step. Unfortunately, pelletization results suggested that natural kaolin is an unsuitable binder for preparing CaO-based pellets due to its negative impact on pellet morphology. By contrast, Al(OH)₃ binder obtained from kaolin leaching had a strong positive effect on the porous texture of the pellets, demonstrated by pore surface area and volume of 22.48 m² g⁻¹ and 0.051 cm³ g⁻¹ for 1 mm pellets with CaO/binder ratio of 5.5, compared to 10.92 m² g⁻¹ and 0.039 cm³ g⁻¹ for natural materials. The enhancement in pellet morphology is mainly attributed to transformation of Al(OH)₃ to the highly porous Al₂O₃ at high temperatures. Pellets synthesized from limestone modified with 10 vol.% acetic acid with Al(OH)₃ binder (ratio of 5.5) exhibited high pore surface area and volume, represented by 1.3-fold and 44% increase over those for natural limestone. It was concluded that the combination of acetified limestone with Al(OH)₃ binder is a promising approach for synthesis of CaO-based pellets with enhanced morphology.     

Combination of SHS and SPS Techniques for fabrication of fully dense ZrB2-ZrC-SiC composites

A novel method for the fabrication of fully dense ZrB₂-ZrC-SiC Ultra High Temperature Ceramic (UHTC) materials is proposed. It consists of first synthesizing ZrB₂-40 vol.% ZrC-12 vol.% SiC powders by Self-propagating High-temperature Synthesis (SHS) and subsequently consolidating them by Spark Plasma Sintering (SPS). Specifically, when starting from Zr, B₄C, Si, and graphite, the SHS technique leads to the complete conversion of reactants to the desired product. In addition, the use of the SPS apparatus allows for the full consolidation of the SHS powders. This result is achieved under the optimal conditions of 10 min total time and with a maximum temperature of 1800 °C. The proposed method is particularly rapid and convenient as compared to other techniques available for the preparation of analogous materials and for the consolidation of commercial ZrB₂, ZrC, and SiC, using the same SPS conditions.     

Spontaneous patterning and nanoparticle encapsulation in carboxymethylcellulose/alginate/dextran hydrogels and sponges

Novel polysaccharide sponges containing a network of capillaries and pore structures have been prepared by freeze drying of Ca²⁺ ion cross-linked sodium carboxymethylcellulose/sodium alginate hydrogels with or without addition of dextran. The iontropic gels consisted of capillaries, 5 to 40 µm in width, which comprised small pores, 1–25 µm in size. Gold, Fe₃O₄ or TiO₂ nanoparticles were encapsulated in the patterned gels and the mechanical strength of the resulting sponges investigated.     

Effects of aluminium powder content and cold rolling on foaming behaviour of xAlp/Al5Si4Cu4Mg/0.8TiH2 composites

Abstract

Aluminium foams were produced by applying powder metallurgy technology. The process began by making aluminium powder and mixing it with alloy powder (Al5Si4Cu4Mg) and foaming agent (TiH₂). The mix was compacted to the form of a billet by cold pressing and then it was hot extruded to a dense foamable strip, which was cold rolled to give 40% thickness reduction. The resulting precursor composites of both the extruded strip and the extruded plus rolled strip were then freely foamed without a mould at a constant temperature of 700°C for different foaming times. The effects of aluminium powder content and cold rolling on the foaming characteristics of the foamable composite strip were studied. It is noted that aluminium powder fibre in the extruded composite strip acts as a barrier to pore initiation and evolution due to the higher melting point of pure aluminium fibre than that of the alloy matrix. Cold rolling promotes foaming of the composite strip due to the TiH₂ cracking and debonding between TiH₂ particles and metal matrix. The morphological and microstructural evolution of composite foams was also investigated. The foaming mechanism can be described by the following sequence: cracklike pore nucleation between elongated powder fibres; ellipsoidal, spherical, and polygonal pore growth; and the collapse of pores as a result of coalescence.     

Surface patterning nanoparticle-based arrays

In this study, focused ion beam lithography is used to pattern different size and shape island arrays on silicon wafers. Cavity arrays of inverse shapes are then made on silicone mold surfaces by polymerization. After that, Al₂O₃ nanoparticle-based island arrays are created by a surface feature transfer and freeze casting process using an Al₂O₃ colloidal suspension. The effects of silicone mold surface wettability and freezing rate on the Al₂O₃ nanoparticle pattern quality are investigated. The results show that coating the silicone mold surface with a 10 nm thick Au–Pt layer makes the Al₂O₃ nanoparticle suspension more wetting on the mold surface and also likely reduces the dry Al₂O₃ nanoparticle adhesion to the mold surface. Freezing rate should be lower than 1 °C/min to avoid cracks or loose Al₂O₃ nanoparticle packing in the freeze cast features. When these factors are properly controlled, the reported patterning process allows reproduction of micron-size feature arrays from Al₂O₃ nanoparticle suspensions. The studied approach should be applicable to most nanoparticle-based materials and open numerous opportunities for direct-device fabrication.     

Fabrication of metallic composite foam using ceramic porous spheres “Light Expanded Clay Aggregate” via casting process

Composite metal foam was produced as an advanced porous material, using gravity casting technique. Light Expanded Clay Aggregate “LECA” was used as space holder for the produced composite metal foam. The used LECA density was 0.33–0.43 g/cm³ and the volume fraction of its porosity was from 73 to 88 vol.% and aluminum A355.0 was selected as matrix in order to produce the composite foam. Structural characterization, relative density, hardness and compressive test were studied. The composite metal foam was well investigated and found to have homogeneous structure, relatively equal pore, distributable pore and isotropy properties. The study resulted in that relative density, yield strength and energy absorption capacity were 0.44, 35.9 MPa and 18 MJ/m³, respectively.