α-Al2O3 Matrix Research Articles

Alumina-embedded HZSM-5 with enhanced behavior for the catalytic cracking of biomass pyrolysis bio-oil: Insights into the role of mesoporous matrix in the deactivation by coke

The behavior of HZSM-5 zeolite embedded in γ-Al2O3/α-Al2O3 matrix in the catalytic cracking of biomass pyrolysis bio-oil was studied with special focus on the role of the matrix in the deactivation by coke. The runs were conducted at 450 ºC, space-time of 0.35 gzeolite.h.gfeed−1 and 0.5 h, 1 h, and 2 h on stream. The analysis of reaction products was conducted by in-line GC and GC/MS, and deactivated catalyst samples were extensively characterized by N2 physisorption, NH3-TPD, TPD-TPO, TPD-GC/MS and LDI FT-ICR MS. The alumina-embedded catalyst (Cat−Z30) exhibited greater stability, keeping the bio-oil conversion above 88 % for 0.5 h on stream, in comparison to the bulk HZSM-5 that deactivated rapidly entailing a bio-oil conversion plunge (from 95 % to 40 %). Consequently, the yield of gasoline-range hydrocarbons (> 70 % MAH monoaromatics) was boosted from 18 % to 26 % for the same reaction time. The aqueous liquid byproduct concurrently produced has a suitable composition for H2 production by further steam reforming. The enhanced behavior was caused by the weak-acid mesoporous structure of the matrix playing a dual function: i) diffusive-shell (inwards) role by promoting fragmentation/deoxygenation of bulky molecules (e.g., saccharides and guaiacols) into reactive oxygenates, and deposition of thermally-induced coke away from the zeolite micropores; ii) spread-growth (outwards) role by enhancing the diffusion of hydrocarbons outwards from zeolite crystals, and by promoting the development of heavy polycyclic structures that delay the blockage of active sites in micropores. The results are useful to progress towards the integral valorization of raw bio-oil, the development of active and stable catalysts, and the proposal of regeneration strategies necessary for the scaling up.

Growth and scintillation properties of Ce 3+:LuAG–Al2O3 chemically deposited eutectics

Directionally solidified eutectics (DSEs) in Al2O3-based systems have been studied as high-temperature structural ceramics, also recently attracting attention as a phosphor screen for high-resolution radiation imaging. However, it is difficult to obtain uniform and thinly processed DSE in large areas. Here, we report a chemically deposited eutectic (CDE) of Lu3Al5O12 (LuAG)–Al2O3 using a laser-assisted chemical vapor deposition method. The Ce3+-doped LuAG lamella were epitaxially grew with an α-Al2O3 matrix on an r-cut sapphire substrate. The Ce3+:LuAG–α-Al2O3 CDE film exhibited green emissions due to the 5d–4f transitions of the Ce3+ center under UV light and α-/X-ray irradiations. The α-ray dose in the Ce3+:LuAG phase was estimated with Monte Carlo simulations to study a scintillation light yield of the CDE film. The X-ray radiograph of the semiconductor storage device was successfully obtained using a 15-μm-thick Ce3+:LuAG–α-Al2O3 CDE film as an X-ray phosphor screen.

Nanostructured PEO-coatings on silumin as environmental catalysts

The process of the formation of mixed oxide coatings on a high-silicon aluminum alloy in the cobalt-containing pyrophosphate electrolyte by the plasma-electrolytic oxidizing (PEO) method has been studied. It has been shown that AK12M2MgN chemical composition heterogeneity causes the consumption of part of the anode current to homogenize the treated surface, which is reflected in minimizing the content of doping components at the initial processing stage. It has been established that the growth of mixed oxides Al2O3·CoOx relative mass is a function of time with the maximum at 55 min. The chemical, phase composition and surface morphology of the formed oxide layer depend on the oxidizing time. The catalytic component content in the surface oxides varies from 0.2 to 23.3 at.% with an increase in processing time from 10 to 60 min. Maximum cobalt incorporation into the oxide layer occurs at PEO of 35–50 min, while the silicon content in the surface layers does not exceed 2 at.%, which is favorable for the catalytic properties. The cobalt oxide, preliminary Co3O4, incorporation in alumina α-Al2O3 matrix is visualized by the blue-violet color spheroid surface structures at the sites of micro-arc discharges. The mixed alumina and cobalt oxides layer are characterized by a developed micro globular surface consisting of conglomerate spheroids with an average size of 1–2 µm. There are some amorphous phases in the structure of mixed oxides due to non-equilibrium PEO conditions. The set of detected factors is a prerequisite for the high catalytic properties of oxide coatings. A promising field of Al2O3·CoOx systems application is an intra-cylinder catalysis in the internal combustion engines.

Functionally Graded Tunable Microwave Absorber with Graphene-Augmented Alumina Nanofibers.

Graphene is currently attracting attention for radiation absorption particularly at gigahertz and terahertz frequencies. In this work, composites formed by graphene-augmented γ-Al2O3 nanofibers embedded into the α-Al2O3 matrix are tested for X-band absorption efficiency. Composites with 15 and 25 wt % of graphene fillers with shielding effectiveness (SE) of 38 and 45 dB, respectively, show a high reflection coefficient, while around the electrical percolation threshold (∼1 wt %), an SE of 10 dB was achieved. Furthermore, based on the dielectric data obtained for varying fractions of graphene-/γ-Al2O3-added fillers, a functionally graded multilayer is constructed to maximize the device efficiency. The fabricated multilayer offers the highest absorption efficiency of 99.99% at ∼9.6 GHz and a full X-band absorption of >90% employing five lossy layers of 1-3-5-15 and 25 wt % of graphene/γ-Al2O3 fillers. The results prove a remarkable potential of the fillers and various multilayer designs for broad-band and frequency-specific microwave absorbers.

In-situ formation of ceramic layer on Mo-based composites via laser powder bed fusion

Poor oxidation resistance is a longstanding disadvantage of Mo-based materials for ultrahigh-temperature applications. In this study, we developed a facile strategy for depositing an in-situ ceramic layer on the surface of Mo-based composites via laser powder bed fusion (L-PBF) using Mo-based alloy powders covered with uniform Al2O3 nanoparticles and bridged by functionalized carbon nanotubes. The surface layer consisted of an α-Al2O3 matrix with a dispersed TiC phase and had a controllable thickness. The formation mechanism of this layer was investigated systematically through single-track observations and finite-element simulation. Moreover, the increased nanohardness can be attributed to the uniformly dispersed, intimately contacted ceramic nanoparticles in the matrix. The results indicated the multifunctionality of L-PBF-processed metallic parts, introducing the possibility of fabricating advanced ultrahigh-temperature materials.

PECULIARITIES OF COBALT CONTAINING OXIDE COATINGS FORMATION ON SILUMIN

The process of mixed oxide coatings formation on a high-silicon aluminum alloy in a cobalt-containing pyrophosphate electrolyte by the plasma-electrolytic oxidation (PEO) method is studied. It was shown that AL25 chemical composition heterogeneity causes the consumption of a part of the anode current to homogenize the treated surface, which is reflected in minimizing the content of doping components at the initial processing stage. It was established that the growth of mixed oxides Al2O3·CoxOy relative mass is a function of time with a maximum at 55 minutes. The chemical, phase composition and surface morphology of the formed oxide layer depend on the oxidation time. The catalytic component content in the surface oxides varies from 0.2 to 23.3 at.% with an increase in processing time of 10 to 60 minutes. Maximum cobalt incorporation into the oxide layer occurs at PEO of 35–50 minutes, while the silicon content in the surface layers does not exceed 2 at.%, which is favorable for the catalytic properties. The cobalt oxide, preferably Co3O4, incorporation in the alumina α-Al2O3 matrix is visualized by the blue-violet color steroidal surface structures in the sites of micro-arc discharges. The alumina cobalt oxide mixture layers are characterized by a developed micro-globular surface which consists of spheroid conglomerate with an average size of 1–2 microns. There are some amorphous phases in the structure of mixed oxides due to non-equilibrium PEO conditions. The set of detected factors is a prerequisite for the high catalytic properties of oxide coatings. A promising field of Al2O3·CoxOy systems application is intra-cylinder catalysis in internal combustion engines.

Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties

A novel Mo-based composite with simultaneously improved mechanical and oxidation properties was fabricated by laser powder bed fusion (L-PBF), using uniform Al2O3-nanoparticle-decorated MoTiAl powders bridged by functionalized carbon nanotubes. A tight ceramic coating ∼2.57 μm in thickness consisting of an α-Al2O3 matrix with dispersed TiC particles was formed on the surface of the Al2O3-CNT/MoTiAl composite, which has been proved to effectively increase resistance to oxidation at 1173 K. Meanwhile, the nanoparticles were homogenously dispersed and tightly contacted with the matrix, giving rise to an enhanced Vickers hardness. This work shed light on designing and producing high-performance Mo-based composites for application to ultrahigh-temperature materials.

Effects of CNTs content on microstructure and mechanical properties of Cnt/α-Al2O3 and Cnt-Csf/α-Al2O3 composites

Abstract In this work, CNTs and short carbon fiber reinforced α-Al2O3 matrix composites (i.e., Cnt/α-Al2O3 and Cnt-Csf/α-Al2O3) were prepared by sol-gel dispersing method followed by hot pressing process. Effects of CNTs content on the mechanical properties of Cnt/α-Al2O3 and Cnt-Csf/α-Al2O3 composites were investigated and the inter- and transgranular fracture mechanisms in Cnt-Csf/α-Al2O3 composites were analyzed. The hardness and relative density of Cnt/α-Al2O3 and Cnt-Csf/α-Al2O3 composites slightly decrease as CNTs content increases. The flexural strength and fracture toughness monotonically increase with the increase in Cnt content for both Cnt/α-Al2O3 and Cnt-Csf/α-Al2O3 composites. Addition of Csf and CNTs into α-Al2O3 matrix results in a bimodal grain size microstructure of Cnt-Csf/α-Al2O3 composites, which accounts for the unique fractograph and the enhanced toughness of Cnt-Cf/α-Al2O3 composites. The flexural strength and fracture toughness of Csf5-nt06 composite are 465 MPa and 7.08 MPa/m2, 37% and 13% higher than that of Cnt06, and 42% and 34% higher than that of Csf5, respectively.

Al2O3/Y-TZP ceramic composite with unidirectional functional gradient

Ceramic composites are developed with the aim of improving the properties of single phase materials, adapting it to the demands of their applications. In this work, composite ceramics with an unidirectional functional gradient based on Al2O3 reinforced with 3, 5, 10 and 15 wt-% of Y-TZP (Yttria stabilized tetragonal zirconia polycrystals) nano-sized particles were prepared, using multilayer deposition and uniaxial compaction. The composites were characterized by their relative density, microstructure, phase composition, hardness and fracture toughness. The results showed the presence of α-Al2O3 and t-ZrO2 as crystalline phases and relative density near to 98%. The hardness varied according to the indented region varying between 1750 and 1540 (HV), for layers containing 3 and 15 wt-% of Y-TZP, respectively. Fracture toughness of the composite layers is also sensitive to the increasing ZrO2 content, presenting average values of 3.2 to 4.8 MPa.m1/2, indicating the influence of toughening mechanisms such as the t → m ZrO2 phase transformation and compressive residual stresses between the α-Al2O3 matrix and the reinforcing t-ZrO2 particles, generated by their thermal mismatch.

Growth and luminescence properties of Eu-doped HfO2/α-Al2O3 eutectic scintillator

HfO2/α-Al2O3 eutectics were grown by the micro-pulling down (μ-PD) method at the composition of 63.2 mol.% α-Al2O3, 35.512 mol.% HfO2, 0.92 mol.% Eu2O3 and 0.368 mol.% Y2O3. Rod phases with around 3–4 μm diameter were observed on the transverse cross-section. The phase-separated scintillator fibers (PSSFs) structure with fibers of around 3–4 μm diameter and 10–20 μm length was observed. The electron back scattering patterns indicated crystal orientation of [101] for monoclinic HfO2 rod phase and [0001] for α-Al2O3 matrix in the transverse cross-section. Eu3+ 4f-4f emission was observed at 550–750 nm.

Machinability evaluation and desirability function optimization of turning parameters for Cr2O3 doped zirconia toughened alumina (Cr-ZTA) cutting insert in high speed machining of steel

In present study, mechanical properties, microstructure and machining parameter optimization of Cr2O3 doped zirconia toughened alumina (ZTA) ceramic insert have been investigated for application in high speed turning of AISI 4340 steel with achieving maximum tool life. The yttria stabilized zirconia (YSZ) in α-Al2O3 matrix with varying percentage of co-doped chromia (Cr2O3) is prepared to study the phase transformation behaviour. The samples are uniaxially pressed in the form of cutting inserts and subsequently sintered at 1600°C to evaluate the mechanical properties. Hardness and fracture toughness reaches the highest value i.e. 17.40GPa and 7.20MPam1/2 respectively at 0.6% Cr2O3 doped ZTA due to more metastable tetragonal ZrO2 phase present in the alumina matrix. After 50min of machining, the flank wear and surface roughness are found well below the tool rejection criteria. The cutting force also does not affect detrimentally on the job–tool interface. Turning experiments have been adopted as per central composite design (CCD) of response surface methodology (RSM) with varying 3 levels of cutting speed (140m/min, 280m/min, 420m/min), feed rate (0.12mm/rev, 0.18mm/rev, 0.24mm/rev) and depth of cut (0.50mm, 1.00mm, 1.50mm). The effect of each input parameter on output responses is investigated using analysis of variance (ANOVA) and modelled using regression analysis. The influence of cutting speed, feed rate and depth of cut is observed maximum for determination of flank wear, cutting force and surface roughness respectively. Cutting speed of 420m/min with feed rate of 0.12mm/rev and depth of cut of 0.5mm has been shown as optimized condition with 83.32% desirability for minimum tool failure and maximum tool life.

Orientation relationship between α-Fe precipitate and α-Al2O3 matrix in iron-implanted sapphire

Fe ions were implanted into α-Al2O3 single crystals (sapphire) at room temperature and annealed in a reducing atmosphere. The orientation relationships (ORs) between α-Fe particles and sapphire matrix were investigated using transmission electron microscopy (TEM). All the α-Fe particles have the orientation relationship (OR) of (111)α-Fe||(0001)sapphire and [11¯0]α-Fe||[112¯0]sapphire with sapphire. This OR is predicted precisely by the coincidence of reciprocal lattice points (CRLP) method. The other OR of (110)α-Fe||(0001)sapphire and [111]α-Fe||[51¯4¯0]sapphire reported before is confirmed by the same method to be one of the secondary preferred orientation relationships in the α-Fe/sapphire system.

Perfect High‐Temperature Plasticity Realized in Multiwalled Carbon Nanotube‐Concentrated α‐Al2O3 Hybrid

We investigate the high‐temperature compressive deformation behavior of a novel, fully dense and structurally uniform, 20 vol% multiwalled carbon nanotube (MWCNT)–α‐Al2O3 matrix hybrid, which has a strong room‐temperature interfacial shear resistance (ISR) and a unique MWCNT‐concentrated grain‐boundary (GB) structure. We realized a perfect plastic deformation at 1400°C and a rather high initial strain rate of 10−4 s−1 by a low ~30 MPa flow stress, which is contrary to the strain hardening response of fine‐grain monolithic Al2O3. This unique performance in CNT–ceramic system in compression is explained as follows: the concentrated network of individual MWCNTs perfectly withstands the high‐temperature and shear/compressive forces, and strongly preserves the nanostructure of Al2O3 matrix by preventing the dynamic grain growth, even during a large ~44% deformation. Furthermore, the presence of large amount of radially soft/elastic, highly energy‐absorbing MWCNTs in the GB and specially multiple junction areas, and a potentially weak 1400°C‐ISR, could greatly facilitate the GB sliding process (despite the hybrid's strong room‐temperature ISR), as evidenced by the formation of some submicrometer‐scale MWCNT aggregates in GB area, the equiaxed grains and dislocation‐free nanostructure of the deformed hybrid. The results presented here could be attractive for the ceramic forming industry and could be regarded as a reference for oxide systems in which, the GB areas are occupied with soft/elastic, highly energy‐absorbing nanostructures.

Ilmenite and Nickel as Catalysts for Upgrading of Raw Gas Derived from Biomass Gasification

Two metal oxides, naturally occurring ilmenite (iron titanium oxide) and manufactured nickel oxide supported on an α-Al2O3 matrix (NiO/AL2O3), were compared as catalysts for secondary biomass gas upgrading. The experiments were conducted in a chemical-looping reforming (CLR) reactor, which combines biomass gas upgrading with continuous regeneration of coke deposits. The CLR system was fed with a tar-rich producer gas from the Chalmers 2–4-MW biomass gasifier, and the possibilities to reduce the tar fraction and to increase the yield of hydrogen were evaluated for temperatures between 700 and 880 °C. A system-wide molar balance was established to enable calculations of tar removal efficiency on a mass basis; these results were further compared with those for the more widely used tar-to-reformed gas ratio, yielding tar concentrations in units of grams of tars per normal cubic meter of product gas (gtar/Nm3gas). Both materials exhibited activity with respect to tar decomposition and increased the yield of hydrogen. In addition, both tar removal and hydrogen production were increased with increasing temperature. All the phenolic compounds and a large proportion of the one-ring branched tars were decomposed at 800 °C by the two catalysts, despite the fact that the tar load in the raw gas was as high as 30 gtar/Nm3gas. Results from the mole balance showed that it is important to specify on what basis the tar removal efficiency is calculated. The tar removal efficiency was calculated as 95% for the Ni/Al2O3 catalyst at 880 °C and as 60% for the ilmenite catalyst at 850 °C on tar-to-reformed gas basis. When the produced permanent gases were removed from the reformed gas, the same calculations yielded tar removal efficiency of 86% and 42%, respectively. The testing of serial samples of the effluent stream from the regeneration reactor for carbon oxides showed that coke was removed from the catalyst, and no deactivation by coke deposits was detected during the 8 h of operation of the CLR reactor.

A study on in situ growth of TaC whiskers in Al2O3 matrix powder for ceramic cutting tools

In situ growth of tantalum carbide (TaC) whiskers was synthesized in an α-Al2O3 matrix powder via a carbothermal reduction technique within a temperature range of 1350–1500°C in an argon atmosphere. The starting materials consisted of Ta2O5, C, Ni and NaCl powders. Different mixing methods and various reaction temperatures were employed. Most of the prepared whiskers were 0.2–0.5μm in diameter and 5–15μm in length. The reaction temperature of 1400–1450°C was suitable for the growth of TaC whiskers and a wet mixing method was beneficial to increase the whisker yield. Some of the whiskers exhibited the needle shape while others exhibited the screw shape. The growth mechanism of the whiskers was a complex mechanism involving a helical screw dislocation mechanism and a vapor–liquid–solid process. No obvious influences of the Al2O3 matrix powder on the growth of TaC whiskers were found and the major impurities in the obtained powder were TaC particles, nickel and unreacted carbon.

High-temperature cyclic oxidation behavior of Al2O3–YAG composite coating prepared by EPD and microwave sintering

Al2O3–YAG (Y3Al5O12) composite coatings were successfully fabricated on NiCoCrAlY alloy substrates by electrophoretic deposition (EPD) in Y3+ doped composite sol–gel suspensions and pressure filtration microwave sintering (PFMS). The composite coating was dense, crack-free and had good adherence with substrate even after cycling oxidation at 1000°C for 200h. Filed emission SEM showed that nano-size YAG particles were embedded in α-Al2O3 matrix. The high-temperature cyclic oxidation tests revealed that the oxidation resistance and spallation resistance were greatly improved. The oxidation resistance of alloy is mainly determined by α-Al2O3 sealing effect and reactive elements effects (REE) on the thermally grown oxide (TGO) scale. Moreover, the enhanced fracture toughness and strength of this composite coating can be attributed to the nano/submicro composite structure and YAG particles toughening effects.

Preparation and high-temperature properties of Au nano-particles doped α-Al2O3 composite coating on TiAl-based alloy

Au nano-particles doped α-Al2O3 composite coatings were successfully prepared on TiAl-based alloy by electrodeposition, using the Al2O3 sols with minor addition of HAuCl4 solution. The even distribution of Au nano-particles (<2.0wt.%) in the α-Al2O3 matrix has been observed. Isothermal oxidation tests of the samples coated with the as-prepared novel coatings at 900°C in static air for 200h shown that the oxygen inward diffusion can be effectively suppressed to a low level. The results of high-temperature cyclic oxidation test at 900°C in air revealed that the oxidation and spallation resistance of TiAl-based alloy were improved significantly under thermal cycling. In the as-prepared coatings, cracks were shielded by means of crack bridging and the fracture resistance of the formed scales can be improved by toughening effects of the composite structure. Surface scratching tests after the cyclic oxidation exhibited that the adhesion of the formed composite scale on TiAl-based alloy was remarkably improved by the Au nano-particles doped α-Al2O3 composite coating.

Tool ceramics based on Al2O3–ZrO2–TiCN composite

Oxide composite Al2O3–ZrO2(Y2O3) powders containing 50 and 70 vol.% Al2O3, are deposited from salt solutions. The surface area of the deposited powders exceeds 75 m2/g. It is established that the α-Al2O3 and t-ZrO2(Y2O3) matrix has an effect on stepwise crystallization of α-Al2O3 and the powder production history on the sinterability of oxide composites. Ceramics based on Al2O3–ZrO2(Y2O3) composite powder containing 20 vol.% titanium carbonitride are hot pressed to be used as a cutting tool. The microstructure and mechanical properties of the ceramics are analyzed. The best mechanical properties are shown by ceramic samples sintered at 1550°C at 35 MPa: bending strength 950 MPa, fracture toughness 10.1 MPa ⋅ m1/2, and hardness 18.0 GPa. The performance and lifetime of cutting tools based on these ceramics are tested according to GOST 26613–85.

Microstructures and Properties of Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;/ZrO&lt;sub&gt;2&lt;/sub&gt; Composite Ceramics from In Situ Growth in the Melts Produced by Combustion Synthesis

By introducing ZrO2 (3Y) micron powder into the thermit, Al2O3/18%, 21%, 24%, 27% and 30%ZrO2 composite ceramics were obtained from in-situ growth in the melts produced by combustion synthesis. As the volume fraction of ZrO2 is below 27%, the ceramics is comprised of lath-shaped and rod-shaped α-Al2O3 matrix, within the rod-shaped grain the nano-micron fibers are oriented in essentially the same direction. As volume fraction of ZrO2 is over 27%, the ceramics is mainly composed of lamina-shaped grains with alternating α-Al2O3 and t-ZrO2 layers. The flexural strength and the fracture toughness measured simultaneously the maximum values with increasing volume fraction of ZrO2 to 24%, indicating that flexural strength and fracture toughness are simultaneously under the control of rod-shaped grains in which ZrO2 nano-micron monocrystal fibers are embedded.

Preparation and Properties of Alumina Baria Nano Composites.

Barium doped alumina was prepared by calcination of gels obtained from metal chloride solutions by homogeneous precipitation with urea. The gels were either air-dried, oven-dried, microwave-dried or autoclave treated. The materials were analysed by SAXS, XRD, DTA/TG and TEM. The phase composition of the calcined products depended on the gel drying procedure. Barium aluminates formed directly at low temperatures in the microwave treated gel because of the uniform distribution of barium in the dried gel. After calcination at 1200 °C the aluminates were uniformly distributed as nanoscale particles within an α-Al2O3 matrix. In contrast, autoclave treated samples initially consisted of boehmite crystals coated with barium carbonate which reacted on calcination to form a surface layer of BaAl2O4. The autoclaved material withstood 1 h at 1400 °C in air without transformation to α-Al2O3. This increased thermal stability of transition alumina (θ-Al2O3) is attributed to the presence of the BaAl2O4 surface layer. The final transformation to α-Al2O3 was associated with the transformation of a surface layer of BaAl2O4 to nanoscale particles of BaAl9.2O14.8.