Pressed Samples Research Articles

Influence of poly(ethylene oxide) sample preparation on the results of thermogravimetric analysis

Aim: To investigate whether the sample preparation process of poly(ethylene oxide) (PEO) affects kinetic analysis of the thermal degradation process. Kinetic analysis was performed to describe the course of a chemical reaction regardless of the reaction conditions and the reaction system complexity. One differential method, the Friedman method, and one integral Kissinger-Akahira-Sunose method (KAS), were applied in this work. Methods: The PEO sample was prepared in 4 different ways. Thermogravimetric analysis was performed to determine the thermal degradation of prepared samples. Infrared spectroscopic analysis was performed during the preparation of the PEO film obtained by casting from the solution. Results: Dynamic thermal decomposition of PEO, regardless of the method of preparation, takes place through a single decomposition stage, which is manifested by the appearance of one peak on derivative thermogravimetric (DTG) curve. During the preparation of the PEO film, the procedure was carried out at a temperature higher than its melting temperature (Tm=65°C). After the cooling, the obtained sample didn’t solidify and it had an intense odor of acetic acid, which was confirmed by infrared spectroscopic analysis. Samples III and IV were re-prepared at a temperature lower than the melting point of PEO, obtaining samples of satisfactory quality. Conclusion: In order to prepare poly(ethylene oxide) films by solution casting technique, drying should be carried out at temperatures below the melting point of PEO. If TG analysis of pure PEO powder is compared with the results of hot pressed samples and solution cast samples, it can be concluded that the preparation of the sample doesn’t affect the thermal stability of the PEO. The dependence of activation energy calculated by the differential Friedman and integral KAS method on conversion is constant for all samples in a broad conversion range, regardless of how the samples were prepared. The hot pressed samples and solution cast samples have lower activation energy than the commercial PEO powder.

Effect of zirconium oxide particulate composites with Al-Si on the microstructural and mechanical properties of hot pressed, spray forming and stir casting methods

ABSTRACT This research work aims to investigate the aluminum and silicon alloy reinforced with the ZrO2 powder particles, which has been fabricated using stir casting and spray deposition method. The mechanical properties, micro hardness and microstructure of the developed metal matrix composite is investigated. The microstructural results indicate that the rich interface among the AlSi-ZrO2 particles and depicts the agglomeration of reinforced phase resulting to poor wettability of ZrO2 and observed decohesion. The mechanical testing results indicate that the tensile strength increases with the percentage of ZrO2. Moreover, as cast composites exhibit reverse tendency in compressive and hardness values. The highest compressive values for as cast and hot-pressed composites were 380 MPa and 337 MPa for 10% ZrO2. The highest tensile strength of 191.83 MPa was obtained for 5% ZrO2 as cast composite and 164 MPa for 15% ZrO2 hot pressed composite. It is to note that as cast composite method represented more homogenous data compared to the hot-pressed composites. Hot pressed samples exhibited the reduction in the porosity compared to the as cast. The developed method proved to be accurate, reduced time and efficient to predict the numerous samples.

Investigation of the effect of addition of boron nitride on microstructure and mechanical properties of cast iron brake materials used in railway vehicles

In this study, characterization of cast iron break shoes used in locomotives and the effect of BN addition and BN addition on microstructure and mechanical properties by using powder metallurgy (PM) method by adhering to the chemical composition of cast iron break shoes were investigated. In this context, cast iron sabers were chemically analyzed and all elements were supplied in powder form, and then BN added powder compositions were produced by cold and hot pressing technique. A total of 4 different compositions were created without BN, with 0.25, 0.5 and 1 % BN addition. Cold pressing was made in a steel mold with a diameter of 10 mm under 700 MPa pressure under room conditions. Hot pressing was carried out in a graphite mold with a diameter of 15 mm in an argon atmosphere under a pressure of 45 MPa at a temperature of 900 °C. Then the cold and hot pressed samples were sintered at 1 200 °C for 2 hours in argon atmosphere. Density measurement of the produced compositions, microstructure examination, micro hardness measurement were made. Generally, in density measurements, the densities of the samples subjected to hot pressing were higher than the cold pressing process. The hardness value decreased in parallel with the addition of BN. Keywords: Brake shoe; powder metallurgy; microstructure; mechanical properties

Effect of high temperature on mode-I fracture toughness of granite subjected to liquid nitrogen cooling

Liquid nitrogen (LN2) ultra-low temperature fracturing technology has broad application prospects in the storage and transformation of geothermal wells. Therefore, it is of great significance to investigate the fracture toughness of granite samples after elevated temperature treatment and LN2 cooling. In this paper, the Cracked Straight Through Brazilian Disc (CSTBD) granite specimen was heat-treated at different temperatures to study the mode-I fracture toughness of granite after rapid cooling by LN2. Meanwhile, acoustic emission (AE) and digital image correlation (DIC) techniques were used to monitor the deformation and failure process of granite. Our results show that when the heating temperature is more than 400 °C, after LN2 cooling, the internal thermal damage of granite is intensified, the plastic characteristics are gradually enhanced, and the fracture toughness decreases significantly. The DIC results show that, with increasing stress, cracks start to form at the two ends of the pre-crack and propagate to the contact point between the press and rock sample along the loading direction under mode-I loading. Near the peak load, the Von-Mises equivalent strain (Evm) gradually increases, and a high strain band appears along the loading direction. When the heating temperature exceeds 400 °C, the density and width of thermally induced cracks in granite gradually increase after LN2 cooling, the fracture track gradually deviates from the straight line, secondary cracks extend, and the plastic characteristics are enhanced. High temperatures cause thermal damage to the rock, and rapid cooling exacerbates this damage. It can be considered that 400 °C is the critical temperature threshold at which the physical–mechanical behavior and fracture characteristics of CSTBD granite change significantly under the condition of rapid LN2 cooling.

The effect of titanium-carbon mixture mechanical activation on SHS pressing parameters and consolidated titanium carbide microstructure

The paper studies the effect of mechanical activation (MA) modes when stirring a stoichiometric mixture of titanium and soot powders in a ball mill on the properties of mixtures, combustion parameters, relative density, and the microstructure of consolidated titanium carbide samples obtained by SHS. MA conditions for Ti + C reaction mixtures in a ball mill were determined. An increase in the mass of grinding bodies activates the MA mechanism. It was shown that the greatest effect from MA was obtained with a two-stage preparation of mixtures: firstly, the titanium powder was activated separately, then the components were mixed together, and this process included not only their mixing, but also soot powder activation. It was found that combustion behavior is affected by the activation of not only titanium, but also soot. After MA of both components, an anomalous increase in the burning rate (more than 100 cm/s) was found on pressed samples. At the bulk density, there was no effect of MA on the mixture combustion process, since in this case the burning rate of all mixtures was in the range of 1.5–2.5 cm/s. It was revealed that MA of reagents for pressed samples leads to an increase in the combustion temperature, an increase in the relative density of the consolidated refractory product to 93–95 %, and a decrease in the average size of TiC grains. A decrease in the residual porosity of consolidated TiC is due to an increase in the hot pressing temperature and plasticity of the product synthesized during the reaction mixture combustion after MA. The main reason is an increase in the exothermic interaction rate. It was shown that MA when mixing reagents makes it possible to control combustion parameters, the microstructure of consolidated products and opens up new opportunities for obtaining refractory materials featuring a unique structure and properties by SHS pressing.

Optimization of zirconium and yttrium content in 17Cr ferritic ODS steel through Taguchi analysis

The present investigation on an optimum level of zirconium and yttrium content were identified using Taguchi analysis. This effect is the most influencing factor to obtain better mechanical properties and microstructure of 17Cr Ferritic ODS Steel. The 17Cr Ferritic ODS steel alloy was developed from 430L pre-alloyed powder followed by a mechanical alloying process. The 20 h of milled powder were consolidated through vacuum hot pressing (VHP) at 1200 °C and pressure level of 60 MPa with a vacuum level of 10-3 Torr. Hot pressed samples were characterized using TEM analysis. Namely optimization of three parameters such as zirconium, yttrium, and sintering temperature was done in the present study through the design of experiments followed by the Taguchi approach. ANOVA all-encompassing of signal to noise ratio was used to optimize the process parameters through L9 orthogonal array. It has predicted the response variables of density and hardness. The zirconium and yttrium content is the most customary factor subsequently the VHP to improving the density and mechanical properties of 17Cr Ferritic ODS steel. It was confirmed through the regression equation and confirmation test.

Optimization of Mechanical Activation Modes for UHMWPE Dry Mixed Powders and Nanomodificators in the Planetary Ball Mill Pulverisette 7

Introduction. Currently, in various technology areas, bronze, cast iron and other antifriction metals are replaced by polymer composites, which extend significantly service life of tribocoupling. An advanced antifriction polymer is ultra-high-molecular-weight polyethylene. The study deals with determining the optimal specific energy consumption for the mechanical activation of the polymer dry mixed powders and nanomodifiers in the planetary ball mill Pulverisette 7, which ensure the best complex of physico-mechanical and rheological properties of nanocomposites. Materials and Methods. In this work, we used GUR 4120 Ticona ultra-high-molecular-weight polyethylene with a molecular weight of 5 million, a Tuball Matrix Beta concentrate of activated carbon nanotubes at a concentration of 0.1%, calculated with reference to carbon nanotubes, and hydrophobic nanocrystalline silicon dioxide with a dispersion of 20 nm at the same concentration. Mechanical co-activation of polymer powders and nanomodifiers, when varying the specific energy consumption, was carried out in the planetary ball mill Pulverisette 7. The production of films from powders, for studying the elastic-strength and rheological characteristics of nanocomposites, was carried out with the use of the hydraulic press Gibitre. Tests were carried out respectively on the tensile testing machine UAI-7000 M and the rheometer Haake MARS III. Results. It has been established that the best physico-mechanical and rheological properties of nanocomposites are with specific energy consumption for mechanical activation of 3,000‒3,200 J/g that allows us to consider them optimal. The mechanical activation of ultra-high molecular weight polyethylene powder, reducing slightly the elasticity modulus and tensile strength of thermally pressed samples, does not affect the dynamic viscosity of melts at an energy consumption of 650‒4,550 J/g. Discussion and Conclusion. The use of carbon nanotubes and nanocrystalline silicon dioxide at a concentration of 0.1% can significantly improve the physical-mechanical and rheological properties of the polymer with energy costs of 3,000‒3,200 J/g for mechanical activation in planetary ball mills. Nanocrystalline silicon dioxide is a more effective modifier that can be explained by its better dispersion in the polymer matrix due to the lower tendency of nanoparticles to agglomerate.

Vacuum Hot Pressed Novel 21-4N Valve Steel Strengthened by Y-Ti-O Through High-Energy Ball Milling

In recent years, the selection of engine valves materials is based on high temperature performance, longer maintenance life, prominent creep and corrosion and oxidation resistance. Typical engine valve includes materials like 21-2N and titanium alloys. In order to develop a more suitable material to enhance its performance, new nanodispersion strengthened austenitic stainless steels were developed with and without the addition of Y2O3, Ti and MnN from ferro-alloys (ferro-manganese, ferro-nickel and ferro-chrome) to obtain Fe-21Cr-4Ni-9Mn-0.5C-0.4Si as the required composition through powder metallurgy route. These pre-alloyed powders in proper proportion were milled mechanically within a planetary ball mill to obtain the required composition and nanocrystalline structure. The milled powders were subsequently vacuum hot pressed at 1200 °C and 60 MPa by maintaining it at a vacuum level of 10−3 mbar for a period of 2 h. Vacuum hot pressed samples were subjected to densification and hardness studies, and microstructural analysis and characterization studies. Density of vacuum hot pressed samples was 97.9% when compared to its theoretical density. With the addition of Y2O3 and Ti as minor alloying elements, very fine austenitic grain structure was formed with the formation of Y-Ti-O complex oxides. The presence of chromium-rich precipitates was revealed during SEM–EDS analysis at the grain boundaries which was attributed due to lower cooling rate of austenitic stainless steel. TEM analysis revealed homogeneous distribution of Y-Ti-O complex oxides within the metal matrix. A marginal increase in hot pressed density was reported with the inclusion of Y2O3 and Ti in the austenitic matrix along with 28% increase in hardness values.

Identification of type of cement through laser-induced breakdown spectroscopy

The composition of concrete determines its resistance to various degradation mechanisms such as ingress of ions, carbonation or reinforcement corrosion. Knowledge of the composition of the hardened concrete is therefore helpful to assess the remaining service life of an existing structure or evaluate the damage observed during inspections. For example, for most existing concrete structures the type of cement originally used is not known and must therefore be determined afterwards. This paper presents a preliminary study on the application of laser-induced breakdown spectroscopy (LIBS) to identify the type of cement. For this purpose, ten different types of cement were investigated. For every type, three cement paste prisms were produced: (i) prisms dried, ground and pressed into tablets, (ii) prisms dried and (iii) prisms untreated. LIBS measurements were performed with a diode-pumped low energy laser (1064 nm, 3 mJ, 1.5 ns, 100 Hz) in combination with two compact spectrometers which cover the UV and NIR spectral range. A reduced subset of spectral features was used to build a classification model based on linear discriminant analysis. The results show that the classification of homogenized pressed cement powder samples provides a high accuracy, however, factors such as a different sample matrix and moisture content can affect the accuracy of the classification. The study demonstrates that LIBS is a promising tool to identify the type of cement.

Effect of Hot Rolling on the Microstructure and Properties of Nanostructured 15Cr ODS Alloys with Al and Zr Addition.

Oxide dispersion strengthened (ODS) alloys with Al and Zr addition have excellent radiation tolerance, high-temperature strength, and corrosion resistance. The 15Cr-Al-Zr-ODS alloys are processed by mechanical alloying (MA), hot isostatic pressing (HIP), subsequent hot rolling to large strains of 70%, and further annealing. The effect of hot rolling on the microstructure, and the properties of nanostructured 15Cr ODS alloys with Al and Zr addition, were investigated. The microstructure after hot rolling and annealing showed obvious anisotropy. The cubic texture (φ1 = 0°, Φ = 0°, φ2 = 0°) {0 0 1} <1 0 0> and brass-R texture (φ1 = 0°, Φ = 55°, φ2 = 45°) {1 1 1} <1 1 0> were observed. The similar size distribution of precipitates was obtained for the comparison of the hot rolling samples with the hot isostatic pressed samples, which can be attributed to excellent thermal stability. After hot rolling, the alloy showed higher yield strength but did not lose too much plasticity.

Opinions of Saudi elites towards the credibility of electronic press sites and their relationship to degree of dependency: مصداقية المواقع الصحفية الإلكترونية لدى النخبة الأكاديمية السعودية

The study aimed to identify the credibility level of electronic press sites and their correlation with the reliability level as seen by the Saudi elite. To verify the aim, the study followed the descriptive approach. The study sample consisted of (102), representing the audience of the Saudi academic elite and media who use Internet as general and electronic press sites in particular, (Media faculty members of Saudi universities), the study was applied to the Saudi newspaper websites such as (Okaz newspaper, Middle East newspaper, Riyadh newspaper, and Sabk newspaper). The researcher conducted special criteria for the study of website reliability standards of the newspaper. The field study, survey applying, and data collection were conducted in October and November 2018. This study showed the following results: There is a statistically significant correlation between the exposure intensity of the Saudi elite to the study sample websites and their reliability level on it. This means that the greater the exposure of the Saudi elite to the electronic press sites, the greater the level of their reliability on it, and proved a statistically significant correlation between the intensity of the exposure of the Saudi elite to the electronic press sites of the study sample and the credibility level. It also showed a significant correlation between the intensity of the exposure of the Saudi elite to the website’s contents of the study press sample and the motivations of its pursuance in the future. The study concludes a number of recommendations, the most important of which is that the press websites should be concerned with the standard of interaction in terms of communication with the author, and conduct studies on the contact persons who working in the field of newspaper websites to find out the most important pressures and factors affecting their credibility of transferring information.

Technological scheme for lithium-substituted ferrite production under complex high-energy impact

The paper presents technological schemes developed for synthesis of lithium-zinc ferrite powders and sintering of ferrite ceramics, including mechanical activation of the initial mixtures of reagents in a planetary ball mill and subsequent radiation-thermal heating of powders by pulsed or continuous electron beams. Their structural and electromagnetic properties, such as density, porosity, average grain size, Curie temperature, saturation magnetization, initial magnetic permeability, and electrical resistivity, are studied. The technological scheme developed for synthesis of lithium-substituted ferrites using complex effects can be used to produce ferrite powders of homogeneous phase composition with high saturation magnetization at a significantly lower temperature and synthesis time without preliminary compaction of samples. The technological scheme for sintering ferrite ceramics through single-stage radiation-thermal heating of press samples from mechanically activated reagents increases the sample density by ~5%, initial magnetic permeability by ~10%, and electrical resistivity by (2.5–3) times, decreases porosity by ~60% and reduces the duration of ferrite production as compared to that of the two-stage production of ferrite ceramics.

Strength enhancement of magnesium alloy through equal channel angular pressing and laser shock peening

AM80 magnesium alloy was processed by Equal Channel Angular Pressing up to 2 passes under route BC and C, to study the effect of change in microstructure. Microstructures were examined under optical microscope. Samples processed by route BC showed heterogeneous grain structure with good tensile strength compared to that processed by route C. Tensile tests of 2-pass equal channel angular pressed samples showed high tensile strength under route BC. Laser shock peening without coating was carried out on route BC sample for further grain refinement on the surface. Laser pulses with power density of 8 GW cm−2, under different percentages of cover, were used to peen the surface at high strain rate. Microstructures were analysed through scanning and transmission electron microscope, and fine grains of less than 100 nm were observed. Tensile tests indicated that the laser shock peened samples had increased strength and ductility. Fracture details from tensile tested specimens, were examined under SEM. Mixed mode of brittle and ductile fractures was observed in ECAP processed samples. Fracture surface of Laser shock peening without coating treated on equal channel angular pressed samples showed small dimples near the peened surface and intensity of dimples increased with increase in percentage of cover. Wear test was carried out on before and after Laser shock peening without coating processed samples, on pin-on-disc wear test machine. Increase in friction coefficients and wear rate was observed due to roughness induced by peening effect and it decreased after increase in sliding distance due to increase in hardness. Nano indentation experiments were carried out to examine the mechanical characterization at nano level, and it expose the effect of LSPwC in terms of increase in hardness at peened region.

Analysis of thermal stabilization process for organomorphic preforms made of oxidized polyacrylonitrile

Simulation and analysis of thermophysical processes in PAN-fiber-based pressed samples that occurs during thermal stabilization were carried out. It is significant fact from the heat transfer point of view that the heat stabilization process proceeds with a definite exothermic effect. Herewith a strongly pronounced coupling between both the intensity of heat generation and the temperature level and heating rate at each point of the sample was observed. The influence of the main factors that effects on the sample temperature state, such as the density and thermal conductivity of the preform material, also the rate constants of the thermal stabilization process, was studied. The experimental data of the sample temperature state in the thermal stabilization process is presented and was compared with the analysis results.

Influence on mechanical properties of hot pressed, solution treated and age hardened 21-4N ODS alloy developed through pre-alloyed powders

In the current study, mechanical properties of hot pressed, solution treated and aged austenitic stainless steels (21-4N) for were evaluated by varying the percentage of yttria addition. The milled powders were vacuum hot pressed at 1,200°C with a pressure level of 56 MPa with a holding period of 2 hrs under the vacuum level of 10-2 torr. Hot pressed samples were solution treated at 1,200°C for a period of 2 hrs and immediately subjected to water quenching to dissolve more grain boundary carbides. Further, to achieve the desired hardness samples were aged at 750°C held for 4 hrs followed by air cooling. Optical micrographs and SEM-EDS studies reveal austenitic grain with grain boundary carbides in the hot pressed condition. After solution treatment these precipitates disappears which was confirmed through SEM-EDS. Austenitic stainless steel containing 0.3 wt% yttria has attained higher tensile strength of 598 MPa in the age hardened condition.

The influence of oxygen on the chemical composition and mechanical properties of Ti-6Al-4V during laser powder bed fusion (L-PBF)

In Laser powder bed fusion (L-PBF), metal powders, sensitive to humidity and oxygen, like AlSi10Mg or Ti-6Al-4 V are used as starting material. Titanium-based materials are influenced by oxygen and nitrogen due to the formation of oxides and nitrides, respectively. During this research, the oxygen concentration in the build chamber was controlled from 2 ppm to 1000 ppm using an external measurement device. Built Ti-6Al-4 V specimens were evaluated regarding their microstructure, hardness, tensile strength, notch toughness, chemical composition and porosity, demonstrating the importance of a stable atmospheric control. It could be shown that an increased oxygen concentration in the shielding gas atmosphere leads to an increase of the ultimate tensile strength by 30 MPa and an increased (188.3 ppm) oxygen concentration in the bulk material. These results were compared to hot isostatic pressed (HIPed) samples to prevent the influence of porosity. In addition, the fatigue behavior was investigated, revealing increasingly resistant samples when oxygen levels in the atmosphere are lower.

Zinc oxide pieces obtained by pressing and slip casting: physical, structural and photocatalytic properties

ABSTRACT Photocatalysis is a promising alternative for the decontamination of effluents. In this paper, immobilized ZnO-based photocatalysts were obtained by pressing and by slip casting. The cylindrical pieces were heat-treated at 800°C. The samples were characterized by the method based on the principle of Archimedes, XRD, FTIR, Raman, diffuse reflectance and SEM. The samples obtained by slip casting presented lower apparent density (3.12 ± 0.04 g/cm3), higher apparent porosity (44.87 ± 0.47%) and smaller grain size (0.48 ± 0.05 µm) when compared to the pressed samples, with mean apparent density of 5.37 ± 0.08 g/cm3, apparent porosity of 1.56 ± 0.10% and grain size of 0.64 ± 0.02 µm. The performances of the samples were attested by the photocatalytic degradation of Rhodamine B (RhB) under UV-C irradiation (maximum intensity at 254 nm). The samples obtained by slip casting showed photocatalytic degradation between 80% and 90%, while the pressed samples showed degradation between 40% and 60%. The reuse of the photocatalysts was evaluated over five cycles of photocatalytic degradation, in which there was no loss of performance of the samples obtained by slip casting; however, the pressed samples showed a loss of photocatalytic efficiency starting from the third-cycle. Photocatalytic assays were carried out with different dye concentrations, in which the slip casting samples showed better photocatalytic efficiency (degradation of 80% for a RhB concentration of 10 mg/L) due to higher porosity and surface area compared to pressed samples, and there was a loss of performance in higher concentrations.

Effect of Content and Mechanical Activation on the Combustion of a Ni–Al–C System

Effect of carbon content in a Ni–Al–C system and preliminary mechanical activation on a burning rate, sample elongation, and mixture yield after mechanical activation, as well as on the structural features of combustion products is under study. Combustion in pressed samples from a mechanically preactivated Ni + Al mixture at room temperature could not be activated. The addition of carbon [2, 4, and 6% (by weight)] into the reacting Ni + Al mixture allows for combustion in Ni–Al–C based pressed samples. Preliminary mechanical activation of the reacting Ni + Al + xC mixture expands a carbon content limit in the mixture at which the combustion of pressed samples is possible. In addition, mechanical activation, as well as an increase in the carbon content in the Ni–Al–C system reduces the burning rate. The observed relationships are explain.

Space weathering by simulated micrometeorite bombardment on natural olivine and pyroxene: A coordinated IR and TEM study

We studied space-weathering effects caused by micrometeorite bombardment simulated by pulsed intense infrared laser, generating ∼15 mJ per pulse in high vacuum. For our investigation, we selected a natural olivine (San Carlos olivine (Fo91)) and a natural pyroxene (Bamble orthopyroxene (En87)) as important rock forming minerals of the Earth upper mantle as well as key planetary minerals. Irradiated areas of powdered pressed samples were examined by optical reflection spectroscopy in a broad optical and infrared wavelength range (visible-, near-, and mid-infrared) and transmission electron microscopy to identify changes due to micrometeorite impacts. The present study aims to investigate especially the effects of micrometeorite bombardment on reflectance spectra in the mid-IR in preparation for future space missions, as well as for the MERTIS experiment onboard the BepiColombo mission.For both irradiated samples, we found a reduction in albedo and in the reflectance of characteristic Reststrahlen bands and an increase of the transparency feature. VIS and NIR spectra of both minerals show the typical darkening and reddening as described for other space-weathered samples. TEM studies revealed that space-weathered layers in olivine and pyroxene differ in their respective thickness, ∼450 nm in olivine, 100-250 nm in pyroxene, as well as in developed “nanostratigraphy” of laser-ablated material, like nanophase iron (npFe).In conclusion, our spectral and structural findings were compared to samples in which space weathering was caused by different processes. A comparison with these data demonstrates that there is no difference in optical reflectance spectroscopy, but a significant difference in the microstructure of minerals due to the weathering source in space, as there are solar wind and solar flares cause other structural and chemical changes as the bombardment with micrometeorites.

Comparative study of hot and cold press and their effect on superconducting properties of coronene added bulk MgB2

In this study, the superconducting properties of 4 wt% coronene (C24H12) added polycrystalline bulk MgB2 samples prepared by hot pressing and cold pressing, were reported in detail. The results were compared to determine one of which pressing methods is more useful to substitute of carbon released from C24H12 into MgB2 structure and to increase the superconducting properties of MgB2. The lattice strain, crystallite size, residual resistivity, superconducting transition temperature (Tc), critical current density (Jc), flux pinning force density (Fp) and vertical/lateral levitation force (Fz, Fx) were analysed for the hot and cold pressed samples. The a-lattice parameter and Tc value reduced sharply for 4 wt% C24H12 added cold pressed sample, resulting in a notable decrease of increment ratio of Jc and Fp values, while the depressing of Tc is lower for 4 wt% C24H12 added hot pressed sample. The cold pressing strongly increased the lattice strain, confirming more C substitution, while the hot pressing ensured smaller crystallite size. The Jc and Fp values were respectively found as 4.6 × 103 A/cm2, 1.3 × 104 A/cm2, 5.8 × 103 A/cm2 and 1.8 × 108 N/m3, 5.1 × 108 N/m3, 2.3 × 108 N/m3 for the pure sample and, 4 wt% C24H12 added hot and cold pressed samples at 20 K, 4 T. The maximum repulsive and attractive levitation force values at 20 K in ZFC and FC regimes were obtained for the 4 wt% C24H12 added hot pressed sample. Finally, the hot pressing provides an effective way to substitute C into MgB2 lattice without a remarkable decrease of Tc value, ensuring more homogenous and compacted structure, more electron–phonon interaction and strong flux pinning force.