Fast preparation of ZTA-TiC-FeCrNi cermets by high-gravity combustion synthesis

ZTA-TiC-FeCrNi cermets are prepared by a fast and furnace-free way called high-gravity combustion synthesis. The synthesized cermet samples show the maximum relative density of 97.6% and a hierarchical microstructure with grain sizes from submicron to >50 µm. The content of TiC has a strong influence on the microstructure and mechanical properties of the cermet samples. A higher TiC content results in refined microstructure, improved hardness, and reduced coefficient of friction. With increasing TiC content, the strength and toughness of the samples first increase and then drops, and reach the maximum of 469±26 MPa and 11.3±0.2 MPa m^1/2 at 20% TiC. Compared with commercial polycrystalline Al₂O₃ ceramics, the ZTA-TiC-FeCrNi cermets exhibit better wear resistance, and the volume loss is lower by one magnitude than Al₂O₃ under the same condition.     

非离子模板剂制备有序介孔氧化铝及表征

以硝酸铝为原料,采用非离子型表面活性剂聚乙二醇为模板剂,磷酸氢二铵为辅助剂,通过沉淀方法合成有序介孔氧化铝。探讨了模板剂的选取、反应终点pH值、反应温度、后处理工艺和辅助剂等条件对合成的有序介孔氧化铝结构影响,采用BET、XRD、TEM等测试技术对样品进行了结构表征,研究结果表明合成的有序介孔氧化铝比表面积大且孔分布窄,形成的蠕虫状孔道具有一定的有序性。     

Mesoporous beta zeolite obtained by desilication

Zeolite beta crystals (Si/Al = 35) synthesized in fluoride medium were treated in aqueous 0.2 M NaOH solution for mesopore formation by selective extraction of framework silicon. A 16 parallel-batch reactor was used to study the influence of the treatment time and temperature on the physico-chemical properties of the zeolites, which were characterized by ICP-OES, XRD, N₂ adsorption at 77 K, SEM, TEM, DRIFTS, and in situ ATR-IR. Alkaline treatment of H-beta within the optimal window of Si/Al ratios identified for other zeolite families leads to extensive silicon extraction at mild treatment conditions. This originates substantial mesoporosity and presumably improved transport, but negatively impacts on the microporous and acidic properties of the resulting sample. Consequently, the alkaline-treated beta zeolites show lower catalytic activity in the acid-catalyzed liquid-phase benzene alkylation than the purely microporous parent material. The relatively low stability of framework aluminum in BEA, compared to MFI and MOR, is detrimental for the controlled mesopore formation by Si dissolution, since aluminum cannot optimally exert its pore-directing role.     

多孔氧化铝模板的制备及形成过程研究

在直流恒电压条件下,以草酸为电解液制备了多孔氧化铝模板。采用扫描电子显微镜对多孔氧化铝模板的形貌进行表征。氧化铝模板的厚度测量表明,当氧化时间为6h时,氧化膜厚度达到最大值35.0mm。根据氧化过程的j-t曲线研究了多孔氧化铝模板的形成过程。     

Mesoporous alumina (I): Comparison of synthesis schemes using anionic, cationic, and non-ionic surfactants

A series of mesoporous alumina was prepared using a selected group of anionic, non-ionic, and cationic surfactants with Al-tri-sec-butoxide or Al³⁺ in organic solvent with stoichiometric amount of water, and in aqueous solution with different hydrolyzing agents, or with complexing agent at room temperature to 100 °C. A comparison of the physicochemical properties of the materials prepared was made with respect to surface area, thermal stability, and porosity. Anionic surfactants in aqueous solution produced thermally unstable materials but in organic solvent resulted in a moderately stable alumina phase. Both cationic and non-ionic surfactants produced thermally stable mesoporous alumina after removal of the surfactant upon calcination at 500 °C when the materials were synthesized in organic solvents. Overall, mesoporous alumina prepared with cationic cetyltrimethylammonium bromide in sec-butanol in the optimized substrate composition of Al:surfactant:H₂O:solvent = 1.0:0.3:2.0:15 exhibited the best textural property among the various synthesis systems investigated.     

Preparation of alumina ceramic template by the molten salt method

ABSTRACT

Single-crystal alpha alumina (α-Al₂O₃) platelets were synthesised with aluminium sulphate in the study. Moreover, the effects of the heating temperature, holding time, and different power mixtures of γ-Al₂O₃ and sodium sulphate (Na₂SO₄) on the morphology of as-prepared α-Al₂O₃ platelets were explored. The crystalline phase evolution and morphology of the samples were characterised by X-ray diffraction and scanning electron microscopy. According to the further analysis results, most of α-Al₂O₃ platelets were hexagonal particles. After the power mixture of γ-Al₂O₃ and Na₂SO₄ (1:2) was heated at 1200°C for 1?h, the platelet size became relatively uniform with an average diameter of 3–5?μm and a thickness of 50–100?nm. This method is an easily controllable, simple, and green method to produce high-quality α-Al₂O₃ platelets.     

Solution combustion synthesis (SCS) of chrome alumina as a high temperature pink pigment

In this article, one step synthesis of the chrome alumina pink pigments is investigated. Results indicated that adjusting ignition parameters such as the fuel type (glycine, citric acid, and urea) and the oxidizer to fuel ratio, is the essential factor to obtain the straight corundum structure of the pigment. According to this, chrome alumina pink pigment was just synthesized by the urea fuel in 1.8 oxidizer to fuel (O/F) ratio. The effect of ignition parameters on the morphology was also studied. The microstructure was changed from ultra‐fine irregular agglomerates to sponge‐like flakes by the change in the fuel type from citric acid to urea. Colorimetric characteristics confirmed the relationship between the formation of the corundum structure and one step synthesis of the pink pigment. The pink color of synthesized pigments was comparable with commercial purplish pink pigments of tiles.     

Electrochemical synthesis of nickel hexacyanoferrate nanoarrays with dots, rods and nanotubes morphology using a porous alumina template

Transition metal hexacyanoferrate (MeHCF) have attracted extensive attention because of their outstanding properties including, electrocatalysis, molecular magnetism, biosensing and ion-exchange. This paper describes an approach for fabrication of ordered nanoarrays of Ni hexacyanoferrate (NiHCF) structures with different morphologies such as dots, rods and tubes in order to advance their properties and applications. The method is based on the conversion of Ni into NiHCF nanostructures by electrochemical oxidation in the presence of hexacyanoferrate ions, using nanoporous anodic alumina oxide (AAO) as a template. The structure and morphology of formed Ni and NiHCF nanoarrays were confirmed by scanning electron microscopy (SEM), showing agreement with the pore structures of the AAO template. The electrocatalytic activity of NiHCF nanorod array electrodes showed high catalytic properties for the detection of hydrogen peroxide and the potential to be used as a platform for direct biosensing applications. The ion-exchange ability of fabricated NiHCF nanostructures (nanorods and nanotubes) toward alkali cations such as Na⁺ has been successfully confirmed.     

Colloidal behaviour of mullite powders produced by combustion synthesis

The combustion synthesis method is very attractive to produce homogeneous powders of pure or mixed oxidic ceramics without further thermal treatment. Previous work reports the synthesis of monophasic mullite powders by a suspension combustion process using two combustions aids, ammonium nitrate and hydrogen peroxide, and an excess of colloidal silica suspension.

The combustion synthesis leads to coarse, agglomerated powders that must be crushed. The effect of attrition milling on both particle size and morphology was studied. Aqueous suspensions of the milled powders were prepared using two dispersants: citric acid and a polyacrylic acid-based polyelectrolyte. The stability of the powders was studied by measuring the zeta potential. The effect of the dispersants on the rheological behaviour of concentrated suspensions was also studied. The sintered densities and the microstructures of specimens obtained by slip casting were studied, demonstrating that mullite powders obtained by a single step combustion process can be processed through conventional shaping routes.     

Synthesis of mesoporous alumina by using a cost-effective template

A salt of stearic acid, i.e., magnesium stearate [(C₁₇H₃₅COO)₂Mg], can be used as a chemical template for the formation of mesoporous alumina, and is a less expensive reagent than stearic acid. Mesoporous alumina prepared using this cost-effective surfactant shows similar pore properties with respect to pore size (3.5 nm) and surface area (above 300 m²Vg) to that prepared using stearic acid. In addition, textural porosity, arising from non-crystalline intraaggregate voids and spaces, was effectively removed by the addition of magnesium nitrate. The entire transformation from aluminum hydroxide to active alumina was performed at 550 °C, and the crystallinity of the product was confirmed by powder XRD analysis.²⁷A1 MAS NMR result shows the phase of mesoporous alumina is the γ-alumina form.     

Inward template synthesis of intact hollow spheres

A facile and general one-step approach is presented to synthesize hollow spheres with varied composition by an aerosol-assisted solvent evaporation process. The monomer of ethyl-2-cyanoacrylatemide contained in the aerosol droplets can form an outer shell by a fast polymerization around the droplets. Materials inside the droplets further grow inwardly against onto the interior surface of the first shell forming another shell forming composite hollow spheres. The hollow spheres are derived by dissolution of the outer shell, therefore the intact shell can be well preserved. Many approaches can be exploited forming the second shell for example sol-gel process of oligomers and phase separation from polymer solutions. Microstructure of the hollow spheres can be tuned from smooth to porous. The methodology is general.     

Nano-sized plate-like alumina synthesis via solution combustion

Nano-sized plate-like alumina is an important inorganic material in catalytic, ceramic and electronic applications due to its unique two-dimensional structure and properties, while its facile synthesis is still under investigation. In this work, nano-sized alumina plate was synthesized by solution combustion method with glycine and polyvinyl alcohol as composite fuels for the first time. The calcination temperature during solution combustion process was optimized as confirmed by XRD, FT-IR, TG-DSC, Raman, ²⁷Al MAS NMR, SEM, TEM, and N₂ physisorbtion. Obtained alumina displayed special nano-sized plate morphology and porous structure. It was further adopted as support for cobalt acetate and used as catalyst for selective oxidation of cyclohexane. This work develops a facile approach to synthesize two-dimensional plate-like alumina for catalytic application.     

模板反应在大环化合物合成中的应用

本文综述了模板反应在大环化合物合成中的应用,及含不同配位原子的大环化合物的合成方法。     

Facile synthesis of ordered nanocrystalline alumina thin films with tunable mesopore structures

Mesoporous alumina layers have attracted attention for their potential use in ultrafiltration of salts, as a heterogeneous catalyst support, an adsorbent in environmental cleanup, and in petroleum refinement. The ability to control the fast hydrolysis rate of the inorganic precursors using simple and inexpensive routes is important for that potential to be realized. Herein, we introduce a novel and facile route to synthesize mesoporous alumina thin films from the combination of inexpensive and commercially available copolymer with aluminum chloride or nitrate (salts) in an EtOH–surfactant–NH₃ · H₂O–salts (EsNs) system through the evaporation-induced self-assembly (EISA) method. Mesoporous alumina layers obtained utilizing the EsNs system have ordered and tunable pore structures. The ability to easily control the mesophases of the alumina layers within a short time provides distinct advantages over previously reported synthesis procedures. Most importantly, we demonstrate that the binding of surfactant and NH₃ · H₂O for the formation of hydrogen bond between them in the EsNs system controls the fast hydrolysis rate of the inorganic species. This allows for the synthesis of nanocrystalline alumina layers via the aluminum oxo-clusters’ assembly with the surfactant. Such simple route may be applied in the synthesis of other non-silica mesostructured oxides.     

Gradient microstructure of titanium nitride fabricated by combustion synthesis with liquid nitrogen

Titanium nitride has been fabricated by combustion synthesis from a titanium powder compact and liquid nitrogen in a closed vessel. This method drastically increases the pressure around the sample, resulting in combustion propagation. The products obtained in this system are non-homogeneous in morphology and composition along the propagation direction. The nitridation and densification of the products have improved with the increase of the maximum pressure or pressure gradient. With a pressure gradient of 9.5 MPa s⁻¹, the first combustion part of the product is almost unreacted titanium, but the main product at the last combustion part is titanium nitride (TiN_0.75) with a dense body. The Vickers micro-hardness of the product (at 0.98 N force) along the propagation direction is gradually increased from 9.7 to 17.3 GPa.     

A novel foam combustion approach for the synthesis of nano-crystalline cobalt oxide powder

This article focuses on the nano-crystalline tricobalt tetraoxide (Co₃O₄) synthesis using the combustion method. The fabrication process involves the combustion a mixture of cobalt nitrate as oxidized and expanded polystyrene (EPS) as a novel fuel. The effects of fuel/oxidizer ratio on the phases formed, Co₃O₄ as a major phase and CoO as an impurity, and the Co₃O₄ crystallite size have been addressed. Based on the thermal analyses data (TGA and DTA), which follow the phase changes during the heat treatment of the parent fuel/oxidizer precursor, the different parent precursors have been calcined at 500?°C. Characterization of the formed solids has been performed using various tools (XRD, FT-IR, SEM, TEM, and XPS). It was concluded that the EPS content, in the combustion precursor mixtures, is a key parameter that controls the phase features of the prepared nanomaterials. CoO impurity was detected for the solids with EPS?>?4?wt%. The crystallite size, morphology, and the surface elemental concentration are influenced by changing the EPS content. The activity of the different powders towards hydrogen peroxide decomposition has been evaluated at the 35–50?°C temperature range.     

Nanostructural evolution in mesoporous networks using in situ High-Speed Temperature Scanner

Mesoporous networks with a special distribution of functional centers are an important class of high performance materials for catalysts, bioscaffolds, energy conversion and storage systems. Methods of fabrication allowing to control over special distribution of the functional sites of required phases and designed morphology are still of a great challenge. Here, we report a wet-combustion method for functionalization of the mesoporous network of alumina nanofibers with a single fiber diameter of 7?±?2?nm by Ni-based nanoparticles. The second-scale duration of the combustion process developing high flame temperature (>1000?°C) allows fabrication of mesoporous functional networks with tailored size, morphology and homogeneity of the dopants in a single-stage. The mechanism of the combustion process are studied by in situ high-speed temperature scanner, which continuously records the temperature history and applied power during the process. It readily allows quenching of the samples at a specified point, demonstrates the critical effect of a fuel on the decomposition temperature, combustion enthalpy, functional group, and pH. As a result of the wet-combustion synthesis, aligned double core-shell NiO-NiAl₂O₄-Al₂O₃ fibers or NiO nanoparticles deposited onto alumina nanofibers can be formed.     

Combustion synthesis of perovskite-type catalysts for natural gas combustion

Combustion synthesis has been applied to LaMnO₃ production with a view to boosting its activity towards natural gas combustion by enhancing its specific surface area. With a highly exothermic and self-sustaining reaction, this oxide can be quickly prepared from an aqueous solution of metal nitrates (oxidisers) and urea (fuel).

The favourable conditions for LaMnO₃ formation were sought: only fuel-rich mixtures are effective, but carbonaceous deposits are formed when too much urea is used. In the field of operating conditions in which the combustion synthesis reaction takes place, the specific surface areas were not dramatically higher than those obtained with traditional methods; moreover, even short thermal treatments have been found to rapidly deactivate the catalysts by rapid sintering. With a view to tackling these problems, NH₄NO₃ was chosen as an additive for its low costs, highly exothermic decomposition and because it generates gaseous products only, without altering the proportion of the other elements in the catalysts. With ammonium nitrate, specific area was enhanced from 4 m²/g up to about 20 m²/g. A short thermal treatment at 900 °C partially deactivates also the NH₄NO₃-derived catalysts. It was found that NH₄NO₃-boosted mixtures produce materials whose activity, after a similar thermal treatment, behave practically as the perovskites obtained by the “citrates” method.

Combustion synthesis is though rather cheap—in terms of reactants employed—and quick, given that the process requires few minutes at low temperature without successive calcination. However, the main drawback of this method is that hazardous or polluting compounds are emitted during the synthesis (mainly NH₃ or NO_x).

The MgO introduction, which should act both as a structural promoter and as a sulphur poisoning limiting agent, has proved to be harmful: since MgO does not physically interpose between perovskite grains, it does not offer resistance to deactivation induced by high temperatures.     

Controlling the morphological,structural, and optical properties of one-dimensional PCDTBT nanotubes by template wetting

In this study, the synthesis of poly [N-9′-heptadecanyl-2, 7-carbazole-alt-5, 5-(4′, 7′-di-2-thienyl-2′, 1′, 3′-benzothiadiazole)] (PCDTBT) nanotubes via a templating method is reported. PCDTBT nanotubes were successfully grown by immersing the porous alumina template into 15 mg/ml of solution concentration for 2- and 24-h periods and annealed at 50°C. Changes in morphological and optical properties between nanotubes of different infiltration times (2 and 24 h) as well as its thin films are observed. The longer infiltration time of 24 h produced nanotubes with enhanced morphological, structural, and optical properties. Nanotubes that are formed between 2 and 24 h of infiltration show enhancement in absorption, photoluminescence, and shift in Raman peak if compared to their thin films.     

Characterization of mesoporous alumina prepared by surface alumination of SBA-15

Mesoporous aluminas of controlled pore sizes were synthesized by multiple grafting of Al isopropoxide in organic solvents on mesoporous silica SBA-15. These materials were characterized by N₂ physisorption, TEM, XRD, ²⁷Al NMR and their surface chemical properties were probed by DRIFT, cumene cracking and MoO₃ thermal spreading. The results show that the chemical nature of the Al-grafted materials varies continuously with the number of grafting from pure silica to pure alumina. Typically, after three graftings, this original method of synthesis allows one to prepare ordered mesoporous aluminas with specific surface areas above 300 m² g⁻¹ and a narrow pore size distribution centered on ca. 60 Å. In addition to the characterizations, different models were developed to understand the evolution of the specific surface area and to discard a possible pore blocking.