Uğur Kölemen

Indentation Size Effect and Microhardness Study of β-Sn Single Crystals

The load dependence of apparent microhardness of ?-Sn single crystals having different growth directions is investigated. The measurements are performed on (110) planes of these crystals in the load range from 10 to 50?mN. It is found that the degree of the microhardness anisotropy decreases for higher indentation test loads. The examined materials exhibit the behaviour of indentation size effect (ISE), i.e., the apparent hardness increases with decreasing indentation load. Neither Meyers law nor the proportional specimen resistance (PSR) model can fully explain the nonlinear variation of microhardness with load. Instead, preference is given to modified the PSR model based on the consideration of the effect of machining-induced residually stressed surface on the hardness measurement.

Analysis of load-penetration depth data using Oliver–Pharr and Cheng–Cheng methods of SiAlON–ZrO2 ceramics

Depth sensing indentation tests were carried out to obtain load–penetration depth curves of SiAlON–ZrO2 ceramics under different peak loads ranging from 200 to 1800 mN. The values of indentation hardness (HOP, HCC) and reduced elastic modulus (EOP, ECC) were calculated by using Oliver–Pharr (OP) and Cheng–Cheng (CC) methods. The experimental results revealed that the indentation hardness and reduced elastic modulus exhibit peak-load dependence, i.e. indentation size effect. Both indentation hardness and reduced elastic modulus values were obtained by considering indenter tip roundness and machine compliance. These values obtained from the OP method are higher than from the CC method. On the other hand, hf/hm ratios were used to clarify the indentation deformation behaviour of the SiAlON–ZrO2 ceramic. The experimental hf/hm values were lower than the reported critical value (0.7) for all applied test loads, which is attributed to work-hardening behaviour of SiAlON–ZrO2 composites.

Synthesis of sol–gel derived nano-crystalline ZnO thin films as TCO window layer: effect of sol aging and boron

The present paper reports synthesized nano-sized ZnO thin films for use as front window electrodes in PV applications, and their behavior after different sol aging times (1 h, 24 h, 3 days and 21 days) for undoped and 1% B amount doping. The highly nanocrystalline ZnO thin films were synthesized by the sol–gel method and B was used as a guest material to examine its role in structural, optical, electrical and morphological properties in this process. X-ray diffraction studies revealed that all the prepared films exhibited the ZnO hexagonal wurtzite structure with preferential orientation along the (0 0 2) crystal plane. Analyses of X-ray diffraction patterns demonstrated that the sol aging can improve the degree of preferred crystal orientation along the c-axis, and it did not change the crystal growth orientation. Average crystallite sizes of the undoped and B doped ZnO films were increased from 16 to 31 nm and from 19 nm to 32 nm with increasing sol aging from 1 h to 21 days. Moreover, AFM results showed that the RMS values increased from 5 nm to 12 nm as an indication of a good film formation on the surface. The minimum value of resistivity was found to be 7.24 × 10−3 Ω m with B and a high optical transmittance of ∼98% in the visible region was observed. The films produced at optimum conditions are suitable for optical and electrical applications owing to their low resistivity, high optical transmittance, optimum band gap, good crystallization and surface formation.

Observations of a hysteresis loss valley in yttrium barium copper oxide superconductors

We present measurements of the ac susceptibility of sintered yttrium barium copper oxide pellets prepared by a melt processed solid state reaction method, with different amounts of ZnO added. The measurements were performed versus temperature T for several different ac magnetic field amplitudes Hac in the presence of static bias magnetic fields Hb directed along Hac. For all our samples, for each choice of Hac, the imaginary part of the ac susceptibility versus T traces a peak, denoted by χ″max, centered at a temperature denoted by Tmax.We also find that plots of the values of χ″max vs Hac and of χ″max vs Tmax generate curves that display a valley. Calculations of hysteresis losses, exploiting a critical-state model where the critical current density jc=α(T)∕Bn, reproduce the experimental data quite well.

Nanoindentation study on Gd-deposited YBaCuO superconductor

Nanoindentation technique was used to characterize the mechanical properties of Gd-deposited bulk YBaCuO superconductors fabricated by solid-state reaction method. In order to determine the hardness and reduced modulus of the samples, load-displacement data were analysed by using the Oliver–Pharr method. The hardness values exhibited significant peak load-dependence especially at lower peak loads, while the reduced modulus values were found to be nearly constant at studied loading range. In order to find true hardness of the samples, the peak load-dependency of hardness was analysed by using Meyer’s law, minimum resistance model, elastic/plastic deformation model, energy balance model, Nix–Gao model and Mukhopadhyay approach. Of the aforementioned models, energy balance model and Mukhopadhyay approach were found to be the most effective models to explain the load-dependency of hardness.

Interpolated mapping and investigation of environmental radioactivity levels in soils and mushrooms in the Middle Black Sea Region of Turkey

ABSTRACT The activity concentration of natural (238U, 232Th, and 40K) and artificial (137Cs) radionuclides was determined in 50 samples (obtained from the same station) from various species of mushrooms and soil collected from the Middle Black Sea Region (Turkey). The activities of 238U, 232Th, 40K, and 137Cs were found as 84 ± 16, 45 ± 14, 570 ± 28, and 64 ± 6 Bq kg–1 (dry weight), respectively, in the mushroom samples and as 51 ± 6, 41 ± 6, 201 ± 11, and 44 ± 4 Bq kg–1, respectively, in the soil samples for the entire area of study. The results of all radionuclide activity measurements, except those of 238U and 232Th in the mushroom samples, are consistent with previous studies. In the soil samples, the mean values of 238U and 232Th are above the world mean, and the activity mean of 40K is below the world mean. Finally, the activity estimation was made with both the soil and mushroom samples for unmeasured points within the study area by using the ordinary kriging method. Radiological distribution maps were generated.