Effect of various parameters on the microstructure and magnetic properties of sintered Sr-hexaferrite

Sr-hexaferrite powders with Fe/Sr molar ratio of 11 was synthesized via conventional ceramic route. Powders were milled for 5–25 h, then pressed to make green compacts by applying a pressure of 100–400 MPa. In other series, K2CO3 was used in starting materials. K2CO3/SrCO3 mixing molar ratio was 0.05 and 0.5. First series were calcined at 1250 °C, and the second series were calcined at 1000 °C for 2 h. The XRD characterization results revealed Sr-hexaferrite structure has been produced in both series; Crystallite size is constant then increases while lattice strain rises by variation of milling time from 5 h to 25 h. (BH)max and Br increase by increasing milling time to 25 h and (BH)max reaches to optimized value of 24.75 kJ/m3. SEM was used for microstructure investigation; particle morphologies are hexagonal shape. Addition of slight amount of K2CO3 makes SrFe12O19 crystallites get less platelet shape and more regular shape. By increasing pressure morphology of the grain enhances in regularity. Magnetic properties were measured via permagraph; By increasing sintering temperature from 1150 °C to 1200 °C, density increases from 3.38 g/cm3 to 4.85 g/cm3. Maximum energy product varies from 9.54 kJ/m3 to 25.27 kJ/m3. Between the sintering temperatures of 1200–1250 °C maximum energy product stays at the highest values, But after 1250 °C drops to 3.82 kJ/m3. As density increases, remanence (Br) and maximum energy product [(BH)max] first increase at the pressure range of 100–200 MPa until it reaches to a optimize value of 15.20 kJ/m3 at 200 MPa. Keywords: Sr-hexaferrite, Lattice strain, Williason-Hall, Salt-melt, Sinter-aid, Milling time, Demagnetization curv

Surface modification strategies and the functional mechanisms of gold nanozyme in biosensing and bioassay

Gold nanozymes (GNZs) have been widely used in biosensing and bioassay due to their interesting catalytic activities that enable the substitution of natural enzyme. This review explains different catalytic activities of GNZs that can be achieved by applying different modifications to their surface. The role of Gold nanoparticles (GNPs) in mimicking oxidoreductase, helicase, phosphatase were introduced. Moreover, the effect of surface properties and modifications on each catalytic activity was thoroughly discussed. The application of GNZs in biosensing and bioassay was classified in five categories based on the combination of the enzyme like activities and enhancing/inhibition of the catalytic activities in presence of the target analyte/s that is realized by proper surface modification engineering. These categories include catalytic activity enhancer, reversible catalytic activity inhibitor, binding selectivity enhancer, agglomeration base, and multienzyme like activity, which are explained and exemplified in this review. It also gives examples of those modifications that enable the application of GNZs for in vivo biosensing and bioassays