Structural Transformation of Cu–Mg–Al Mixed Oxide Catalysts Derived from Hydrotalcites During Shift Reaction

Cu–Mg–Al mixed oxide catalysts were prepared from calcination of hydrotalcite-like compounds. XRD patterns showed formation of CuO and MgO phases, where aluminum oxides are well dispersed. The Cu–Mg–Al catalysts exhibited high stability during 40 h with time on stream for the shift reaction at 250 °C, which can be related to the major presence of metallic copper, confirmed by in situ XRD reaction measurements, and no change of the mean crystallite size, suggesting high resistance to sintering. DRIFTS results confirmed the major presence of metallic Cu particles at the catalyst surface but also an insignificant amount of cationic copper species.     

Permeability of porous gelcast scaffolds for bone tissue engineering

The permeability of metallic and ceramic open-cell foams prepared by the gelcasting technique was assessed by fitting of Forchheimer’s equation to experimental pressure drop curves. The ceramic composition was based on pure hydroxyapatite, while the metallic composition was based on titanium metal. Experimental Darcian (k ₁) and non-Darcian (k ₂) permeability constants displayed values in the range 0.40–3.24 × 10^?9 m² and 3.11–175.8 × 10^?6 m respectively. Tortuosity was evaluated by gas diffusion experiments and ranged from 1.67 to 3.60, with porosity between 72 and 81% and average hydraulic pore size between 325 and 473 μm. Such features were compared to data reported in the literature for cancellous bones and synthetic scaffolds for bone graft. A detailed discussion concerning the limitations of Darcy’s law for fitting laboratory data and for predicting fluid flow through scaffolds in real biomedical applications is also performed. Pore size was obtained by image analysis and was also derived from permeation-absorption-diffusion experiments. In both cases, values were within the range expected for porous scaffolds applications.     

A novel process to high-strength and controlled-shrinkage Al2O3 foams with open-cell by Al powder hollowing technology

Ceramic foams with open-cell structures have attracted extensive attention due to their unique structure and superior properties. But these materials often exhibit the weakness of high sintered shrinkage and low strength at high porosity levels. In this work, novel ceramic foams with open-cell structures have been obtained using Al powder by combining direct foaming and gelation freezing (DF–GF). The foams are assembled by hollow Al₂O₃ particles resulting from the Kirkendall effect, in which expanded particles overcome the shrinkage of sintering. The influence of sintering temperature on the microstructure and properties of foams are investigated. The Al₂O₃ foams show near-zero-shrinkage at 1773 K after undergoing the process of first expansion and then shrinkage. Compared to other conventional open-cell foam, this foam displays relatively high compressive strength of 0.35–2.19 MPa at high porosity levels of 89.45%–94.45%, attributed to hierarchical pore structure and reaction bonding between Al and O₂. This method from pore structure design provides a novel route for the preparation of controlled shrinkage and high-compressive strength alumina foam with open-cell toward potential application.     

Effect of Nitric Acid Treatment on Carbon Nanotubes (CNTs)-Cordierite Monoliths Supported Ruthenium Catalysts for Ammonia Synthesis

Nitric acid treatment of CNTs-cordierite monolith changes the amount of Mg, Si, Al and oxygen-containing functional groups, thereby influencing on the surface area and pore size distribution of composite materials. Appropriate treatment of CNTs-cordierite with nitric acid increases the surface area and the amount of micropores slightly, but improves the activities for ammonia synthesis noticeable, which might be a consequent of the variation of the amount of Mg, Si, Al and oxygen-containing functional groups. The ammonia synthesis activity of Ba–Ru/CNTs-cordierite increase by more than 30% if the support material is treated at 30 °C for 4 h with nitric acid.     

Titanium foam made with saccharose as a space holder

In this work we show a very promising method of titanium foam preparation with saccharose crystals (table sugar) as a space holder particles. The mixture of Ti and sugar particles was compacted to form a green compacts. In the next step the saccharose crystals were dissolved in the water, leaving open spaces surrounded by metallic scaffold. The sintering of the scaffold leads to foam with typical morphology and porosity. We found that 1:1 Ti/sugar ratio leads to porosities of about 72 % with pore diameter of about 0.8–1.0 mm, when we used 0.8–1.0 mm diameter sugar crystals. The foams morphology was investigated by SEM, porosity and internal structure was investigated using computed tomography and the structure was shown using XRD. The foam morphology pointed theirs potential applications in medical as well as catalyst devices.     

Enhanced Stability and Propene Yield in Propane Dehydrogenation on PtIn/Mg(Al)O Catalysts with Various In Loadings

The dehydrogenation of propane on In-promoted Pt (0.3 wt% Pt) supported on hydrotalcite Mg(Al)O with different In loadings (0.2–1.0 wt% In) was investigated at 550 °C atmospheric pressure. All the bimetallic PtIn/Mg(Al)O showed higher propane conversion and propene selectivity than the Pt/Mg(Al)O with Pt0.8In exhibited the best catalytic performances with 97.5% propylene selectivity and 27.5% yield after 5 h time-on-stream. The addition of In to the monometallic Pt catalyst could reduce the acidity strength especially the strong acid site. As revealed by the H₂-TPR and XPS results, addition of In by impregnation on Pt/Mg(Al)O also led to the formation of metallic In and PtIn alloy, which greatly enhanced the catalyst activity and reduced coke formation on the support. Nevertheless, excessive In loading (i.e., Pt1.0In) resulted in a descending trend of catalyst activity compared to the Pt0.8In, due probably to the large amount of metallic In being formed, which was disadvantageous in propane dehydrogenation.     

Effects of heating rate on the structure and properties of SiOC ceramic foams derived from silicone resin

Silicon oxycarbide ceramic foams were fabricated in a single step manufacturing process using in situ foaming of SiOC powders loaded silicone resin. The effects of heating rate on the porosity, compressive strength and microstructure of the ceramic foams were investigated. The porosity (total and open) increased firstly and then decreased with increasing heating rate. It was possible to control the total and open porosity of ceramic foams within a range of 81.9–88.2% and 62.4–72.5% respectively, by adjusting the heating rate from 0.25 °C/min to 3 °C/min while keeping the silicone resin content at 90 vol%. However, the compressive strength decreased with increasing the heating rate progressively, and the average compressive strength of the foams was in the range of 1.0–2.3 MPa. Micrographs indicated that the ceramic foams which cross-linked at a heating rate less than 1 °C/min had a well-defined open-cell and regular pore structure.     

Pore structure of reactively synthesized nanolaminate Ti3SiC2 alloyed with Al

Ti₃SiC₂ has the unique properties integrating the advantages of metals and ceramics, and good open pore structure when alloyed with Al. In this work, porous Ti₃SiC₂ compounds with different Al/Si atom ratios were prepared through the reactive synthesis of elemental powders at 1300 °C. The results indicate that the phase compositions are determined by Al element mole number, and that the pore structure can be controlled through varying Ti particle size. The MAX phase transits from Ti₃SiC₂ with Al element mole number no more than 0.6 to Ti₃AlC₂ with Al element mole number in the range of 0.8–1.2. When Al element mole number is 0.6, the porous compound has a single MAX phase of Ti₃SiC₂ with uniform microporous structure and high bending strength. Porous Ti₃SiC₂ alloyed with 0.6Al has a slow linear increase rate of 0.0083%/μm in open porosity with increasing Ti particle size, and a strict linear relationship between the maximum aperture and Ti particle size with the increase rate of 0.0342 μm/μm. The pore structure formed by the phase transition mechanism for porous MAX phase has the smallest tortuosity factor compared with that formed by the clearance mechanism and the Kirkendall effect.     

A sol–gel approach for the room temperature synthesis of Al-containing micelle-templated silica

A two-step sol–gel method has been developed for the synthesis of Al-containing micelle-templated silica (Al-MTS) with Si/Al atomic ratios in the range 8–45. The synthesis proceeds at ambient temperature, atmospheric pressure, acid medium and for only 3 h. For the calcined materials, a decrease of both the d₁₀₀ spacing and the pore size, as well as an increase in the wall thickness, are observed with ncreasing aluminum content. Thus, the average pore size changed from 2.4 nm (Si/Al=45) to 1.4 nm (Si/Al=8), i.e. the pore size reaches the micropore range without varying the chain length of the surfactant used as micellar templating agent. The ²⁷Al NMR-MAS spectra show that in the as-synthesized samples the aluminum is present as tetrahedrally coordinated species, which are partially transformed into octahedral aluminum upon calcination. Since the synthesis takes place in the absence of alkali cations, the Al-MTS materials are directly obtained in the acid form, making a subsequent ion exchange and calcination treatments superfluous.     

Extrusion of poly(ether imide) foams using pressurized CO2: Effects of imposition of supercritical conditions and nanosilica modifiers

Foams of an engineering plastic, poly(ether imide), were extruded using a single screw extruder employing pressurized CO₂ as the blowing agent. The porosity, pore size distributions, and the density of the foams were especially affected by the pressure drop, the pressure loss rate, and temperature at the die. Significant increases in porosity and pore size and corresponding decreases in density were observed when the pressure imposed on CO₂ became greater than the critical pressure values of CO₂ (i.e., the temperature was always greater than the critical temperature of the CO₂ in the extruder and the die). The viscoelastic material functions of the extruded foams depended especially on the density of the foam, with the elastic modulus increasing with density. The incorporation of nanosilica particles in the 0.08–0.6% by weight range increased only the density of the foam and did not provide any benefits in controlling of the nucleation rate and the pore size distribution, presumably due to their poor dispersibility and agglomerated state in the single screw extruder. POLYM. ENG. SCI., 54:2064–2074, 2014. © 2013 Society of Plastics Engineers     

Reactive synthesis for porous TiVAlC ceramics by TiH2,V,Al and graphite powders

Using the mixed powder of TiH₂, graphite, aluminum and vanadium as starting materials, porous TiVAlC ceramics were fabricated by the reactive synthesis technology at 1300 °C. The chemical steadiness of porous TiVAlC along with the effects of sintering temperature on the viscous permeability coefficient, strength, porosity, pore size and volume expansion rate of the porous TiVAlC were explored, and the mechanism of pore formation was also revealed. The preparation process includes five steps as follows: (i) the complete decomposition of stearic acid at 500 °C; (ii) the pyrolysis of TiH₂ at 700 °C, converting TiH₂ into hydrogen and titanium (iii) The solid-liquid chemical reaction of solid vanadium, titanium and molten aluminum at 700 °C, converting the mixture into V–Al and Ti–Al compounds; (iv) At 900–1100 °C, Surplus V and Ti interact with graphite to synthesize carbides of TiVC₂, VC, and TiC; (v) Reactive synthesized carbides (TiVC₂, VC, and TiC), Ti₂AlC, V–Al and Ti–Al compounds that yield porous TiVAlC at 1300 °C.     

Combustion synthesis and characteristics of aluminum oxynitride ceramic foams

Aluminum oxynitride (AlON) ceramic foams were prepared by combustion synthesis using Al and Al₂O₃ as starting materials under high nitrogen pressure. By introducing Al(NO₃)₃·9H₂O into reactants as active pore-forming agent, AlON ceramic foams with well-distributed pores were cost-effectively fabricated. The influences of Al(NO₃)₃·9H₂O content on the combustion process, pore content and structure were studied systematically. The experiment results showed that porous AlON ceramics containing evenly distributed and noncontiguous pores were obtained when Al(NO₃)₃·9H₂O content was 30%, in which the closed porosity of 80% was found by Archimedes’ method. Pore distribution measured by mercury intrusion porosimetry indicated that pores with several types of diameter were formed in the foams because of the different conditions of pore fusion and molten viscosity.     

Production of Lower Olefins with Highly Dispersed Ru Catalysts Supported on Al-SBA-15 in Fischer–Tropsch Synthesis

The highly dispersed Ru catalysts supported on Al-SBA-15 with different Al/Si ratios were prepared by the “two solvent technique” and tested in Fischer–Tropsch synthesis. TEM and N₂ adsorption–desorption results illustrated that Ru crystallites were uniformly embedded inside the pore channels. XRD, hydrogen temperature programmed desorption and O₂ titration results showed that the dispersion of Ru increased with the increasing Al/Si ratio. The XPS results indicated that the interaction between Ru and the supports was a function of Ru dispersion, a stronger metal-support interaction was found in the catalyst with higher Ru dispersion. The selectivity of olefins was increased by increasing Ru dispersion. When the dispersion reached 86 %, this tendency was more pronounced and the selectivity of C₂–C₄ hydrocarbons remarkably increased to 27.0 %, with very high percentage of olefins (62.7 %).     

Highly porous and hierarchical MgAl2O4 ceramics with improved pore connectivity prepared from gelled particle-stabilized foams

Ultralight ceramics with striking mechanical properties and improved pore connectivity could have wide applications in areas ranging from catalyst support to hot gas filtration. However, creating such materials has proven to be a challenging target. This work demonstrated a novel methodology to prepare porous MgAl₂O₄ ceramics by calcining gelled MgO–Al₂O₃–SiO₂ particle-stabilized foams. The striking green strength of dried foams can be achieved as a consequence of MgO hydration and subsequent formation of gelled Mg(OH)₂ and MgO–SiO₂–H₂O skeleton. The decomposition of colloidal substance at elevated temperature resulted in the formation of small pores on the cell wall, thus forming the hierarchical porous architecture and improving the pore connectivity. The highly porous MgAl₂O₄ ceramics fired at 1600°C possessed the integrated properties of ultrahigh porosity (87.0%), improved pore connectivity and satisfactory compressive strength (7.93 MPa), showing great potential to be used in multiple industrial fields.     

Optimized pore structure and high permeability of metakaolin/fly-ash-based geopolymer foams from Al– and H2O2–sodium oleate foaming systems

Geopolymer foams have been widely studied as adsorbents owing to their high specific surface areas, high heavy metal immobilization efficiencies, low cost, environmental friendliness, and resource-recycling benefits. In this study, geopolymer foams with different pore structures were prepared from Al– and H₂O₂–sodium oleate foaming systems, and their chemical properties, pore structures, and permeabilities were characterized. The effects of the foaming agent type and surfactant content on the crystal structure and chemical bonding of the materials were analyzed by X-ray diffraction analysis and Fourier-transform infrared spectroscopy, and the pore morphology and structural characteristics were characterized by morphological observations, three-dimensional (3D) reconstruction, and compression tests. Numerical simulations were also carried out to study the structural characteristics of the 3D-reconstructed pores. Furthermore, variations in the permeability coefficient and flow characteristics were tested and analyzed by experiments and simulations. The pores in the geopolymer from the H₂O₂–sodium oleate foaming system tended to be more connected, whereas those in the Al–sodium oleate geopolymers were more complete and closed. The highly connected pore structure facilitates the even diffusion of the solution and effectively increases the amount of adsorption sites. These properties are significant for adjusting the adsorption capacity of geopolymer foams as adsorption monoliths.     

(Mg1–xCax)2Al4Si5O18陶瓷的相演变与微波介电性能

采用固相烧结反应法制备(Mg1–x,Cax)2Al4Si5O18陶瓷。XRD测试结果表明:0≤x<0.2时,陶瓷以(Mg,Ca)2Al4Si5O18堇青石单一相固溶体形式存在;0.2≤x<0.8时,陶瓷以Mg2Al4Si5O18/CaAl2Si2O8两相复合形式存在;0.8≤x<1.0时,陶瓷以单一相(Ca,Mg)Al2Si2O8固溶体形式存在。SEM结果显示:Ca2+掺杂可以有效地降低堇青石陶瓷的气孔率与微裂纹,并能有效地控制Mg2Al4Si5O18/CaAl2Si2O8复相陶瓷的颗粒分布与晶粒尺寸。微波介电性能测试结果表明:0≤x≤0.4时,(Mg1–xCax)2Al4Si5O18陶瓷介电常数εr为7.0左右;0.6≤x≤1.0时,εr从7.0增加到8.6,然后又降低到6.9。随着x增加,品质因数Qf值从24100GHz降低到4400GHz。但是,在x=0.6时,由于[Si,AlO4]四面体中Al/Si原子排列的有序化,(Mg0.4,Ca0.6)2Al4Si5O18陶瓷Qf值(Qf=5500GHz)比两侧x值成分点Qf值有较大提高。(Mg1–x,Cax)2Al4Si5O18陶瓷谐振频率温度系数在整个x值范围内保持在–20×10–6～–35×10–6℃–1。     

Effect of interfacial-active elements addition on the incorporation of micron-sized SiC particles in molten pure aluminum

Ceramic particles generally have poor wettability by liquid metal, leading to a major drawback in fabrication of cast metal matrix composites (MMCs). In this work,  the effect of 1 wt. % of Ca, Mg, Si, Ti, Zn and Zr interfacial-active alloying elements was studied on the incorporation of micron-sized SiC particles into the molten pure aluminum using the vortex casting method at 680 °C. The results indicated that Ti, Zr, Zn and Si were not positively effective in improving particulate incorporation, while Ca and especially Mg were very efficient at increasing the incorporation of ceramic particles into the molten Al. Also, it was revealed that Al₃Ti, and Al₃Zr intermetallic phases were formed for samples containing Ti and Zr, making hybrid MMCs with a higher amount of hardness. Finally, it was found that a reaction layer between Al and SiC particles was formed at the Al/SiC interface for all of the samples, expect for the ones containing Si and Ti, indicating that for most of the samples at 680 °C an exothermic reaction took place between the Al and SiC particles.     

Fabrication and characterization of alumina and zirconia-toughened alumina porous structures

In the present study, alumina (Al₂O₃) and zirconia-toughened alumina (ZTA) porous structures (foams) were manufactured using the space holder technique. Al₂O₃ and ZTA foams with varying porosities from 20% to 69% were fabricated by adding different sizes (10, 20, and 40 μm) and different volume % of polystyrene beads (space holders) to Al₂O₃ and ZTA powders. All the fabricated foams were investigated under static conditions to assess the compressive behavior. It is observed that the compressive strength of these foams strongly depends on porosity, pore size, pore size distribution and pore wall thickness. Among all fabricated foams, Al₂O₃ foams with 20 vol% beads of 10 μm size showed a higher compressive strength of 700 MPa with low porosity (21%) and a higher pore wall thickness (2.8 μm). It is also observed that the pore wall thickness decreased with the increase in beads size and the volume % of the beads, resulting in a low compressive strength value of 8 MPa with a lower pore wall thickness of 1.75 μm at 80 vol% of 40 μm beads. All the foams, irrespective of pore size, showed a typical ceramic failure phenomenon up to 70 vol% of beads; after that, the failure behavior changed to complete open-cell fracture.     

Microstructural and mechanical behavior of multi-walled carbon nanotubes reinforced Al–Mg–Si alloy composites in aging treatment

Microstructural and mechanical behavior of heat treatable Al–Mg–Si (6XXX series) alloy composites reinforced with multi-wall carbon nanotubes (MWCNTs) fabricated by powder metallurgy process were investigated by SEM-EDS, XRD, tensile test and Vicker’s hardness test. As-extruded P/M 6063 alloy composites with CNT reinforcements indicated a small increment of mechanical strength compared to the monolithic 6063 alloy with no CNT before T6 heat treatment. When T6 heat treatment was applied to the specimens, the 6063 composite with CNTs showed a noticeable decrease of yield stress (YS) improvement, compared to the monolithic Al alloy. It means that Mg₂Si precipitates hardening effect by the artificial aging treatment was insufficient for the composite containing CNTs. This was mainly because Mg alloying elements were diffused around CNTs and consumed to form Al₂MgC₂ compounds, and resulted in the incomplete matrix strengthening behavior by Mg₂Si precipitation after the aging treatment.     

Physicochemical and catalytic properties of iron‐promoted Raney‐nickel catalysts obtained by mechanical alloying

Raney‐type catalysts were prepared by means of a two‐step procedure: (i) mechanical alloying of the metals and (ii) alkaline aluminum leaching. Mechanical alloying is a novel alternative related to the synthesis of skeletal Ni catalysts. Catalysts characterization was performed by atomic absorption, X‐ray diffraction, electron microscopy, and Mössbauer spectroscopies. Textural studies were also carried out. Binary Al–Ni and ternary Al–Ni–Fe alloys were produced by mechanical alloying from pure metallic powders; in particular, the intermetallic α‐(AlNi) phase was formed with a fine microstructure as a non‐equilibrium phase; then, aluminum was selectively removed. After aluminum leaching the α‐(AlNi) phase was transformed into the more stable nickel fcc structure. The effect of iron addition to the Ni–Al catalysts depends on iron concentration and reduction temperature; both parameters determine catalysts composition and activity. This work reports physicochemical properties and benzene hydrogenation activity of these materials, compared with conventional catalysts obtained by melting and leaching.