| 1. |
- Akhtar, Farid, et al.
(author)
-
TiB 2 and TiC stainless steel matrix composites
- 2007
-
In: Materials letters (General ed.). - : Elsevier BV. - 0167-577X .- 1873-4979. ; 61:1, s. 189-191
-
Journal article (peer-reviewed)abstract
- Stainless steel matrix composites reinforced with TiB2 or TiC particulates have been in situ produced through the reactive sintering of Ti, C and FeB. X-ray diffraction analysis confirmed the completion of reaction. The TiB2, TiC and steel were detected by X-ray diffraction analysis. No other reaction product or boride was found, indicating the stability of TiB2 and TiC in steel matrix. The SEM micrographs revealed the morphology and distribution of in situ synthesized TiB2 and TiC reinforcements in steel matrix. During sintering the reinforcements TiB2 and TiC grew in different shapes. TiB2 grew in hexagonal prismatic and rectangular shape and TiC in spherical shape.
|
|
| 2. |
- Binbin, Song, et al.
(author)
-
Oxidation properties of self-propagating high temperature synthesized niobium disilicide
- 2014
-
In: Corrosion Science. - : Elsevier BV. - 0010-938X .- 1879-0496. ; 85, s. 311-317
-
Journal article (peer-reviewed)abstract
- NbSi2 monoliths were prepared by self-propagating high temperature synthesis (SHS) and hot pressing (HP) and their oxidation behavior was investigated at various temperatures (823-1123 K) in air. The combustion mode of SHS reaction was steady state combustion, and the combustion product was single-phase NbSi2. Oxidation studies show that the highest mass gain was 0.95675 kg m(-2) at 1023 K. In cyclic oxidation, the oxidation rate was reduced and the mass gain was only 0.15507 kg m(-2). A dense protective amorphous SiO2 scale formed at 823 K and 923 K whereas a porous multilayer SiO2 and alpha/beta-Nb2O5 oxide scales formed at and above 1023 K and spalled off. Pest oxidation of NbSi2 monoliths was not observed in hot pressed NbSi2 monoliths.
|
|
| 3. |
- Akhtar, Farid, et al.
(author)
-
Enhanced sintering, microstructure evolution and mechanical properties of 316L stainless steel with MoSi(2) addition
- 2011
-
In: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 509:35, s. 8794-8797
-
Journal article (peer-reviewed)abstract
- Sintering 316L stainless steel to near full density with an appropriate sintering additive can ensure high mechanical properties and corrosion resistance. We present here a sintering approach which exploits the dissociation of ceramics in steels at high temperatures to activate sintering densification to achieve near full dense 316L stainless steel materials. MoSi(2) ceramic powder was used as a sintering additive for pre-alloyed 316L stainless steel powder. Sintering behavior and microstructure evolution were investigated at various sintering temperatures and content of MoSi(2) as sintering additive. The results showed that the sintering densification was enhanced with temperature and MoSi(2) content. The distribution of MoSi(2) was characterized by XMAPs. It was found that MoSi(2) dissociated during sintering and Mo and Si segregated at the grain boundaries. Excess Mo and Si were appeared as separate phases in the microstructure. Above 98% of theoretical density was achieved when the specimens were sintered at 1300 degrees C for 60 min with 5 wt.% MoSi(2) content. The stainless steel sintered with 5 wt.% MoSi(2) exhibited very attractive mechanical properties.
|
|
| 4. |
- Akhtar, Farid, et al.
(author)
-
Microstructure and property evolution during the sintering of stainless steel alloy with Si 3 N 4
- 2008
-
In: Materials Science & Engineering. - : Elsevier BV. - 0921-5093 .- 1873-4936. ; 472:1-2, s. 324-331
-
Journal article (peer-reviewed)abstract
- This paper describes the sintering of a martensitic stainless steel alloy with addition of Si3N4. Sintering behavior was studied at different sintering temperatures ranging from 1250 to 1400 °C with different holding times (20–80 min) and with varying Si3N4. Results showed that the samples were densified rapidly via liquid phase sintering mechanism. Nearly full density was obtained at 1300 °C after 60 min of holding time with 5 wt% Si3N4. Temperature above 1350 °C and Si3N4 content 10 wt% caused slumping of the samples. Two weight percent Si3N4 was found chemically stable in steel alloy. Above 2 wt% Si3N4 dissolved in the steel matrix. The distribution of dissolved Si and N was characterized by XMAP. When N content reached much above its solubility limit in steel alloy it diffused out leaving pores in steel alloy with considerable decrease in the sintered density. The mechanical properties of the sintered product with varying Si3N4 were measured. A maximum ultimate tensile strength of 1011 MPa was achieved with 2 wt% Si3N4 sintered at 1300 °C after 60 min of holding time. Fracture morphologies of tensile samples are also reported.
|
|
| 5. |
- Akhtar, Farid, et al.
(author)
-
TiC-maraging stainless steel composite: microstructure, mechanical and wear properties
- 2006
-
In: Rare Metals. - 1001-0521 .- 1867-7185. ; 25:6, s. 630-635
-
Journal article (peer-reviewed)abstract
- Particulate TiC reinforced 17-4PH and 465 maraging stainless steel matrix composites were processed by conventional powder metallurgy (P/M). TiC-maraging stainless steel composites with theoretical density >97% were produced using conventional P/M. The microstructure, and mechanical and wear properties of the composites were evaluated. The microstructure of the composites consisted of (core-rim structure) spherical and semi-spherical TiC particles depending on the wettability of the matrix with TiC particles. In TiC-maraging stainless steel composites, 465 stainless steel binder phase showed good wettability with TiC particles. Some microcracks appeared in the composites, indicating the presence of tensile stresses in the composites produced during sintering. The typical properties, hardness, and bend strength were reported for the composites. After heat treatment and aging, an increase in hardness was observed. The increase in hardness was attributed to the aging reaction in maraging stainless steel. The specific wear behavior of the composites strongly depends on the content of TiC particles and their interparticle spacing, and on the heat treatment of the maraging stainless steel.
|
|
| 6. |
- Cai, Xiaoping, et al.
(author)
-
Fabrication of Highly Porous CuAl Intermetallic by Thermal Explosion Using NaCl Space Holder
- 2018
-
In: JOM. - : The Minerals, Metals, and Materials Society. - 1047-4838 .- 1543-1851. ; 70:10, s. 2173-2178
-
Journal article (peer-reviewed)abstract
- A high-porosity CuAl-based intermetallic compound with composition Cu-50 at.% Al has been successfully prepared by thermal explosion (TE) using NaCl as space holder. The results showed that the NaCl particles were completely removed from the green compact by water leaching. The temperature of the specimen during the TE and the evolution of the porous microstructure were investigated. The TE was ignited at 560°C, and the specimen temperature increased to 775°C in 3 s, resulting in formation of intermetallic CuAl and CuAl2 phases in the final product. A porous CuAl-based intermetallic compound with up to 62 vol.% open porosity was produced when adding 60 vol.% NaCl. The compound exhibited a bimodal pore size structure, including large pores (200 μm to 300 μm) that replicated the NaCl particles and small pores (5 μm to 10 μm) interspersed in the pore walls. Moreover, the large pores were interconnected by channels and formed an open CuAl-based intermetallic cellular structure, having great potential for use in heat exchange and filtration applications.
|
|
| 7. |
- Cai, Xiaoping, et al.
(author)
-
Oxidation Resistance of Highly Porous Fe-Al Foams Prepared by Thermal Explosion
- 2018
-
In: Metallurgical and Materials Transactions. A. - : Springer. - 1073-5623 .- 1543-1940. ; 49A:8, s. 3683-3691
-
Journal article (peer-reviewed)abstract
- Open-cell Fe-Al intermetallic foams were successfully prepared by a simple and energy-saving thermal explosion (TE) process. The effects of the Fe/Al molar ratio (Fe-(40–50) at. pct Al) and thermal treatment temperature on the TE temperature profile, phase composition, pore characteristics, and oxidation resistance of the prepared foams were investigated. The results showed that the Al content significantly influenced the ignition (Tig) and combustion (Tc) temperatures of the TE process; in particular, as the Al content decreased, Tig increased gradually from 623 °C to 636 °C and Tc decreased from 1059 °C to 981 °C. FeAl was identified as the dominant phase in the thermally treated foams. The Fe-Al intermetallic foams displayed an open porosity of 60 vol pct, with pores connected with each other to form an open pore structure. The formation of the pores was attributed to the expansion of interparticle pores in the pressed body during the TE reaction. X-ray photoelectron spectroscopy analysis of the Fe-50Al foam showed that the Al 2p and O 1s binding energies were 74.5 eV and at 531.4 eV, respectively. The formation of a surface alumina layer in the early stages of the oxidation process resulted in the parabolic oxidation rate law, and the Fe-50Al foams exhibited an excellent resistance to oxidation at 650 °C in air. These results suggest that the synthesized Fe-Al foams represent promising materials for applications involving an oxidizing environment and high temperatures.
|
|
| 8. |
- Cao, Zhejian, 1991-, et al.
(author)
-
Calcium/strontium chloride impregnated zeolite A and X granules as optimized ammonia sorbents
- 2022
-
In: RSC Advances. - : Royal Society of Chemistry. - 2046-2069. ; 12:54, s. 35115-35122
-
Journal article (peer-reviewed)abstract
- Calcium chloride (CaCl2) impregnated zeolite A and strontium chloride (SrCl2) impregnated zeolite A and X composite granules were evaluated as ammonia sorbents for automotive selective catalytic reduction systems. The SrCl2-impregnated zeolite A granules showed a 14% increase in ammonia uptake capacity (8.39 mmol g(-1)) compared to zeolite A granules (7.38 mmol g(-1)). Furthermore, composite granules showed 243% faster kinetics of ammonia sorption (0.24 mmol g(-1) min(-1)) compared to SrCl2 (0.07 mmol g(-1) min(-1)) in the first 20 min. The composite CaCl2/SrCl2 impregnated zeolite A granules combined the advantages of the zeolites and CaCl2/SrCl2, where the rapid physisorption from zeolites can reduce the ammonia loading and release time, and chemisorption from the CaCl2/SrCl2 offers abundant ammonia capacity. Moreover, by optimizing the content of SrCl2 loading, the composite granules maintained the granular form with a crushing load of 17 N per granule after ammonia sorption-desorption cycles. Such structurally stable composite sorbents offer an opportunity for fast ammonia loading/release in automotive selective catalytic reduction systems.
|
|
| 9. |
- Cao, Zhejian, et al.
(author)
-
Carbon-reinforced MgCl2 composites with high structural stability as robust ammonia carriers for selective catalytic reduction system
- 2020
-
In: Journal of Environmental Chemical Engineering. - : Elsevier. - 2213-3437. ; 8:1
-
Journal article (peer-reviewed)abstract
- Novel carbon-MgCl2 composites were designed as robust ammonia carriers for selective catalytic reduction (SCR) system, with graphite (Gt) and graphene nanoplatelets aggregates (GNA) as additives to MgCl2. The cylindrically pelletized composites manifested high structural stability above the melting temperature of MgCl2 with 95 % mass retention, whereas the pure MgCl2 pellets completely lost their structural integrity. With the support of carbon additives, molten MgCl2 in the composites was isolated and retained the sample-to-holder angle of 90°, contrary to pure MgCl2 of 5.7° contact angle at 1073 K. Furthermore, the composites demonstrated rapid ammonia sorption and desorption kinetics, due to the enhanced surface area and creation of additional microporosity. Our results demonstrated that 20 wt.% GNA-80 wt.% MgCl2 (GNA20) composite presented 83 % faster kinetics in ammonia sorption and 73% faster in the first-2-minutes of desorption compared to the pure MgCl2. The enhancement of both structural stability and sorption kinetics makes the GNA20 composite a robust ammonia carrier.
|
|
| 10. |
- Cao, Zhejian, 1991-
(author)
-
Structured Ammonia Carriers for Selective Catalytic Reduction
- 2021
-
Doctoral thesis (other academic/artistic)abstract
- Air quality has been one of the long-term focuses in society and has raised people’s concern regarding its amelioration in the post coronavirus disease 2019 (COVID-19) pandemic era. Nitrogen oxides (NOx), including NO and NO2, as one of the most harmful air pollutants, have been stringently monitored in most countries due to their devastating impact on the environment and human health. The transport sector, as the primary source of NOx, therefore, is regulated with ever-evolving NOx emission standards for vehicles. One typical approach to abate NOx from vehicle exhaust is using ammonia (NH3) to reduce NOx and produce environmentally friendly nitrogen (N2) and water (H2O) by selective catalytic reduction (SCR). Conventional urea-based SCR systems using urea as an indirect ammonia source have presented a series of problems, including low conversion efficiency with the lowering of exhaust temperature, freezing of urea solution in low-temperature regions, and emission of carbon dioxide (CO2) as a by-product.Solid SCR systems have emerged as a new direction for NOx reduction (DeNOx) in both industry and research, owing to their high NOx converting efficiency at low exhaust temperatures with direct ammonia dosing. In solid SCR systems, the ammonia storage and delivery unit is a critical part influencing DeNOx performance. The most popular ammonia carriers in solid SCR systems are alkaline earth metal halides (AEMHs), such as MgCl2, CaCl2, and SrCl2. AEMHs face two main challenges as ammonia carriers: (1) low kinetics of ammonia absorption and desorption for urban driving and engine idle scenarios; (2) poor structural stability in terms of thermal melting spread due to heat accumulation and dramatic volume expansion/shrinkage during ammonia absorption-desorption cycles.In this thesis, various physisorbents and chemisorbents, including metal-organic frameworks (MOFs), zeolites, and carbon-reinforced AEMHs, are designed, fabricated, and evaluated as optimized ammonia carriers for solid SCR systems.MOFs [M2(adc)2(dabco)] (M = Co, Ni, Cu, Zn) in this research have demonstrated physisorption of ammonia and superior kinetics of adsorption and desorption compared to MgCl2. Among the synthesized MOFs, Ni2(adc)2(dabco) possessed the highest ammonia uptake capacity, resulting from its high specific surface area. Ni2(adc)2(dabco) released 6 times the mole fraction of ammonia in the first 10 minutes compared to Mg(NH3)6Cl2, indicating that physisorbents can offer a solution to shorten the buffer time for ammonia dosing in SCR. To combine the physisorption of microporous materials with the chemisorption of AEMHs, SrCl2-impregnated zeolite granules as well as three-dimensional (3D) printed zeolite units to carry AEMHs were designed. By optimizing the parameters in the ion exchange and impregnation process, the fabricated SrCl2-impregnated zeolite granules showed two stages of ammonia sorption, including a rapid adsorption stage from the zeolites and an abundant absorption stage from SrCl2. The SrCl2-impregnated zeolite A granules retained 73% of the compressive strength of the pristine CaA granules after ammonia cycles, indicating excellent structural stability of the composite granules. The feasibility of applying 3D printing technology to co-structure AEMHs and zeolites was examined by designing zeolite NaX units to carry MgCl2. A 3D-printed NaX scaffold was successfully fabricated with an optimal formulation of zeolite NaX ink after rheological studies.Carbon materials were selected to form composites with AEMHs, including graphite (Gt), graphene nanoplatelets aggregates (GNA) as additives, and graphene networks as the scaffold. The pelletized carbon-MgCl2 composites containing 20 wt% Gt/GNA presented high structural integrity up to 800 °C above the melting point of MgCl2. Besides, the introduction of nanopores from GNA could promote ammonia diffusion in the MgCl2, resulting in enhanced kinetics of ammonia sorption and desorption. A porous SrCl2 structure scaffolded by graphene networks was fabricated by freeze-casting. The optimized porous SrCl2 with 80 wt% SrCl2 loading maintained its macro- and micro-structure, accommodating the volume swing after 20 ammonia sorption–desorption cycles without disintegration. Furthermore, the porous SrCl2 demonstrated superior kinetics of ammonia sorption and desorption by possessing more surface sites for ammonia adsorption and a shorter diffusion length in the SrCl2 particles. This structuring approach was verified with other AEMHs, including MgCl2 and CaCl2.The results from this thesis offer several solutions to structure AEMHs and their composites as ammonia carriers for SCR, with rapid kinetics and enhanced structural stability. Potential directions for optimizing the ammonia carriers are suggested, such as combining physisorbents (MOFs, zeolites, etc.) and chemisorbents (AEMHs) in flexible networks and optimizing the volumetric ammonia uptake capacity while maintaining the structural stability of the ammonia carriers.
|
|
