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    Effects of different milling conditions on the properties of lanthanum hexaboride nanoparticles and their sintered bodies
    (Elsevier Sci Ltd, 2019) Agaogullari, Duygu; Balci, Ozge; Akçamlı, Nazlı; Duman, Ismail; Ovecoglu, M. Lutfi
    In this study, the preparation and consolidation of nanocrystalline LaB6 powders originating from powder blends of La2O3, B2O3 and Mg were reported. A consecutive route of mechanochemical synthesis (MCS) and purification was utilized for the achievement of nano-sized LaB6 powders. As-synthesized powders were leached out from intermediate reaction products or impurities. Then, a sequential step of cold pressing (uniaxial pressure at 800 MPa) and pressureless sintering (at 1700 degrees C for 5 h under Ar gas flow) were utilized for the consolidation of the purified LaB6 powders. The type of mill (vibratory and planetary high-energy ball mills) was employed as a MCS parameter to reveal its effect on the physical, microstructural and mechanical properties of the LaB6 powders, and their bulk structures. Compositional, physical and microstructural properties of the products after powder processing were determined via X-ray diffractometer (XRD), particle size analyzer (PSA), differential scanning calorimeter (DSC), stereomicroscope (SM), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS) coupled with both SEM and TEM, and vibrating sample magnetometer (VSM). The bulk properties of the LaB6 consolidated from nanocrystalline powders with a minimum 99.99% purity, and similar to 62 nm (for vibratory ball mill) or similar to 74 nm (for planetary ball mill) average particle size were compared according to various properties. LaB6 powders were synthesized in planetary mill at an approximately six times longer duration than that of in vibratory mill. According to the results, density, surface area and mean particle size values of the vibratory ball-milled samples (containing paramagnetic powders) are better than those of planetary ball-milled (containing diamagnetic powders) ones. However, mechanical properties such as hardness, surface roughness, wear rate, friction coefficient, and also electrical conductivity were improved in the planetary ball-milled LaB6 bulks.
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    Mechanochemical synthesis and consolidation of nanostructured cerium hexaboride
    (Univ Novi Sad, Fac Technology, 2019) Agaogullari, Duygu; Balci, Ozge; Akçamlı, Nazlı; Suryanarayana, Challapalli; Duman, Ismail; Ovecoglu, Mustafa Lutfi
    This study reports on the mechanochemical synthesis (MCS) and consolidation of nanostructured CeB6 powders of high purity. CeB6 powders were prepared via MCS by milling CeO2, B2O3 and Mg powders in a high-energy ball mill for different milling times. The effects of milling time on the formation, microstructure and thermal behaviour of the synthesized powders were investigated and the optimum MCS duration was determined. Purified powders were obtained after HCl leaching by removing MgO by-product. The prepared powders were characterized by a number of techniques including X-ray diffraction, stereomicroscopy, scanning and transmission electron microscopy coupled with energy dispersive spectrometry, differential scanning calorimetry, atomic absorption spectrometry, particle size analysis, surface area analysis and vibrating sample magnetometry. The high-purity CeB6 powders having an average particle size of 86 nm were consolidated by cold-pressing followed by pressureless sintering at 1700 degrees C for 5 h. The bulk CeB6 specimen was investigated for its microstructure, density, electrical resistivity, surface roughness and some mechanical properties (microhardness and wear behaviour). The relative density, electrical resistivity, microhardness and wear rate of the bulk CeB6 were determined as 95.2% TD, 57.50 mu W.cm, 11.65 GPa and 1.46 x 10(-4) mm(3)/N.m, respectively.
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    Synthesis of bulk nanocrystalline HfB2 from HfCl4-NaBH4-Mg ternary system
    (Springer, 2017) Akçamlı, Nazlı; Agaogullari, Duygu; Balci, Ozge; Ovecoglu, M. Lutfi; Duman, Ismail
    This study reports on the synthesis and consolidation of pure HfB2 powders starting from HfCl4-NaBH4-Mg blends via autoclave processing, annealing and purification followed by pressureless sintering (PS, with 2 wt% Co aid) or spark plasma sintering (SPS). During autoclave reactions conducted at 500 A degrees C for 12 h under autogenic pressure, excess amounts of NaBH4 were utilized to investigate its effects on the reaction products and mechanism. A subsequent washing (with distilled water), annealing (at 750, 1000 and 1700 A degrees C) and acid leaching (HCl) were applied on the as-synthesized products. Pure HfB2 powders with an average particle size of 145 nm were obtained after autoclave synthesis in the presence of 200 wt% excess NaBH4, washing, annealing at 1000 A degrees C for 3 h and 6 M HCl leaching. SPS sample has higher relative density and microhardness values (94.18% and 20.99 GPa, respectively) than those of PS sample (90.14% and 14.85 GPa). Relative wear resistance was improved considerably (8.2 times) by employing SPS technique.
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    Synthesis of HfB2 powders by mechanically activated borothermal reduction of HfCl4
    (Elsevier Sci Ltd, 2016) Akçamlı, Nazlı; Agaogullari, Duygu; Balci, Ozge; Ovecoglu, M. Lutfi; Duman, Ismail
    HfB2 powders were synthesized via a borothermal reduction route from mechanically activated HfCl4 and B powder blends. Mechanical activation of the powder blends was carried out for 1 h in a high-energy ball mill using hardened steel vial and balls. Mechanically activated powders were subsequently annealed at 1100 degrees C for 1 h under Ar atmosphere. Then, purification processes such as washing with distilled water and leaching in HCl solution were applied for the elimination of the undesired boron oxide (B2O3) phase and the probable Fe impurity. The effect of boron amount on the microstructure of the resultant powders was investigated. The boron amount in the starting blends plays an important role in the formation of the HfO2 phase. HfB2 powders without any detectable HfO2 were prepared by adding 20 wt% excess amount of boron. Microstructural analyses of the mechanically activated, annealed and purified powders were performed using X-ray diffractometer (XRD), particle size analyzer (PSA), stereomicroscope (SM), scanning electron microscope/energy dispersive spectrometer (SEM/EDS) and transmission electron microscope (TEM). (C) 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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    Synthesis of triclinic and hexagonal SmBO3 powders by mechanically activated annealing of Sm2O3 and B2O3 blends
    (Elsevier Sci Ltd, 2016) Akçamlı, Nazlı; Agaogullari, Duygu; Balci, Ozge; Ovecoglu, M. Lutfi; Duman, Ismail
    Samarium borate (SmBO3) powders were fabricated from oxide raw materials by a two-step solid-state synthesis method including mechanical activation and annealing. Blends containing stoichiometric amounts of samarium oxide (Sm2O3) and boron oxide (B2O3) were mechanically activated in a high-energy ball mill and subsequently annealed in air. Afterwards, mechanically activated and annealed powders were washed with distilled water in order to remove probable unreacted B2O3 phase. The effects of mechanical activation duration (15 min, 1 h, 3 h and 9 h) and annealing temperature (700-1250 degrees C) on the resultant powders were investigated. Compositional, microstructural, physical, thermal and optical properties of the powders obtained throughout the different process steps were characterized by using an X-ray diffractometry (XRD), particle size analysis (PSA), stereomicroscopy (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), gas pycnometry, differential scanning calorimetry (DSC), heating stage microscopy (HSM), atomic absorption spectrometry (AAS), Fourier transform infrared (FTIR) spectrometry and ultraviolet-visible spectrophotometry (UV-vis) techniques. Fine-grained and pure SmBO3 powders were successfully synthesized via a simple, feasible and scalable route, yielding both triclinic and hexagonal crystal structures. Triclinic SmBO3 powders were synthesized after mechanical activation for 1 h and annealing at 700 degrees C for 2 h. The polymorphic transformation temperature of SmBO3 powders from triclinic to hexagonal is about 1080 degrees C. Due to the effect of mechanical activation, the synthesis of triclinic SmBO3 phase and its transformation to hexagonal form were found to take place at similar to 50-100 degrees C lower temperatures than those reported in other methods. Mainly hexagonal SmBO3 powders were obtained after annealing at 1150 degrees C in the presence of a very small amount of triclinic SmBO3. The resultant powders showed intense UV absorptions in the range between 1025 and 1150 nm with minimum reflectivity of 0.57% (triclinic SmBO3 phase) and 0.68% (hexagonal SmBO3 phase) depending on their crystal structures. (C) 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.