Effect of Heating on the Microstructure of NiAl + CrB2 Coatings Deposited by Mechanical Embedding in a Ball Mill

The thickness of NiAl + CrB2 coatings, produced by the mechanical embedding of powders, is limited due to the increasing brittleness of processed materials with milling time. Only the NiAl grain growth and resultant softening of the coating matrix could overcome this problem. Therefore, the effect of heating up to 750°C on the microstructure of NiAl + CrB2 coatings deposited in a ball mill rotating at 350 rpm was investigated through in situ TEM observations. The performed observations proved that defect annihilation starts at ~400°C in large intermetallic grains, which are first attached to the substrate. The growth in NiAl nanocrystallites forming most of the coating is activated only above ~600°C. The average crystallite size was measured to be 5, 14, and 19 nm at RT, 650°C, and 750°C, respectively. The first stage of nano-crystallite growth is relatively fast and connected with the reconstruction of crystallite boundaries using up the amorphous material accumulated in between them. The second stage is slower and involves the expansion of larger crystallites at the expense of smaller ones. The performed experiment proved that heating up to 750°C allows the microstructure recovery and grain coarsening of coatings to be activated.

Crystallite growth, phase transition, magnetic properties, and sintering behaviour of nano-CuFe2O4 powders prepared by a combustionlike process

The synthesis of nano-crystalline CuFe2O4 powders by a combustion-like process isdescribed herein. Phase formation and evolution of the crystallite size during thedecomposition process of a (CuFe2)-precursor gel were monitored up to 1000 °C. Phase-purenano-sized CuFe2O4 powders were obtained after reaction at 750 °C for 2 h resulting in acrystallite size of 36 nm, which increases to 96 nm after calcining at 1000 °C. The activationenergy of the crystallite growth process was calculated as 389 kJ mol−1. The tetragonal -cubic phase transition occurs between 402 and 419 °C and the enthalpy change (dH) wasfound to range between 1020 and 1229 J mol−1 depending on the calcination temperature. The optical band gap depends on the calcination temperature and was found between 2.03 and1.89 eV. The shrinkage and sintering behaviour of compacted powders were examined. Denseceramic bodies can be obtained either after conventional sintering at 950 °C or after a twostepsintering process at 800 °C. Magnetic measurements of both powders and correspondingceramic bodies show that the saturation magnetization rises with increasing calcination-/sintering temperature up to 49.1 emu g−1 (2.1 μB f.u.−1), whereas the coercivity and remanencevalues decrease.

Investigations of BaFe0.5Nb0.5O3 nano powders prepared by a low temperature aqueous synthesis and resulting ceramics

A facile method to prepare nanoscaled BaFe0.5Nb0.5O3 via synthesis in boiling NaOH solution is describedherein. The nano-crystalline powder has a high specific surface area of 55 m2 g−1 and a crystallite sizeof 15 nm. The as-prepared powder does not show any significant crystallite growth up to 700 ◦C. Theactivation energy of the crystallite growth process was calculated as 590 kJ mol−1. Dense ceramics can beobtained either after sintering at 1200 ◦C for 1 h or after two-step sintering at 1000 ◦C for 10 h. The averagegrain sizes of ceramic bodies can be tuned between 0.23 µm and 12 µm. The thermal expansion coefficientwas determined as 11.4(3)·10−6 K−1. The optical band gap varies between 2.90(5) and 2.63(3) eV. Magneticmeasurements gave a Néel temperature of 20 K. Depending on the sintering regime, the ceramic samplesreach permittivity values between 2800 and 137,000 at RT and 1 kHz.

Investigations on Bi25FeO40 powders synthesized by hydrothermal and combustion-like processes

The syntheses of phase−pure and stoichiometric iron sillenite (Bi25FeO40) powdersby a hydrothermal (at ambient pressure) and a combustion−like process are described.Phase−pure samples were obtained in the hydrothermal reaction at 100 °C (1), whereas thecombustion−like process leads to pure Bi25FeO40 after calcination at 750 °C for 2 h (2a). Theactivation energy of the crystallite growth process of hydrothermally synthesized Bi25FeO40was calculated as 48(9) kJ mol−1. The peritectic point was determined as 797(1) °C. Theoptical band gaps of the samples are between 2.70(7) eV and 2.81(6) eV. Temperature andfield−depending magnetization measurements (5−300 K) show a paramagnetic behaviour with a Curie constant of 55.66·10−6 m3·K·mol−1 for sample 1 and C = 57.82·10−6 m3·K·mol−1for sample 2a resulting in magnetic moments of μmag = 5.95(8) μB·mol−1 and μmag = 6.07(4)μB·mol−1. The influence of amorphous iron−oxide as a result of non-stoichiometric Bi/Feratios in hydrothermal syntheses on the magnetic behaviour was additionally investigated.