nachum frage
Ben Gurion University of the Negev, Materials Engineering, Faculty Member
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Taguchi’s method was applied to investigate the effect of the main HEBM parameters: milling time (MT), ball to powder weight ratio (BPWR) and milling speed (MS) on the dissolved AlN fraction in TiN. The settings of HEBM parameters were... more
Taguchi’s method was applied to investigate the effect of the main HEBM parameters: milling time (MT), ball to powder weight ratio (BPWR) and milling speed (MS) on the dissolved AlN fraction in TiN. The settings of HEBM parameters were determined by using the orthogonal experiments array (OA). The as-received and milled powders were characterized by X-ray diffraction (XRD). The optimum milling parameter combination was determined by using the analysis of signal-to-noise (S/N) ratio. According to the analysis of variance (ANOVA) the milling speed is the most effective parameter and the optimal conditions for powder synthesis are: MT 20h, MS 600rpm, BPWR 50:1. The result of the experiment conducted under optimal conditions (AlN was completely dissolved during experiment) confirmed the conclusions of the statistical analysis.
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Evolution of precipitates in aluminum 6061 alloy, quenched after 2 h hold at 550ºC (super-saturated solid solution state – SSSS) and aged at 145ºC (for up to 960 min), was studied by routine, ex-situ, transmission electron microscopy... more
Evolution of precipitates in aluminum 6061 alloy, quenched after 2 h hold at 550ºC (super-saturated solid solution state – SSSS) and aged at 145ºC (for up to 960 min), was studied by routine, ex-situ, transmission electron microscopy (TEM). In parallel, initial stages of the precipitation hardening process (after few-minute ageing) were studied by in-situ TEM. It was An appearance of short-living (~8 min) loose, disordered, spherical ~6 nm in diameter precipitates was captured. These precipitates rearranged after relatively long interval of time (~100 min, corresponding to 240 min ageing in the ex-situ tests) into long-range ordered rod-like Guinier–Preston (GP) zones (presuming GPI). The dimensions of the latter, 20 nm×2.1 nm, was determined based on the ex-situ TEM images.-Longer, 960 min, ageing results in GPI → GPII transformation accompanied by an increase of dimensions and strength of the rod-like precipitates. Determined geometrical parameters are in a reasonable agreement with corresponding parameters assessed previously using shock-wave technique. This fact implies that shock-wave technique can be considered as a useful tool for studying dislocation/defect interactions in a wide variety of strengthened alloys.
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ABSTRACT
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The present communication is concerned with the fabrication of a composite material consisting of a two-phase ceramic skeleton, i.e. boron carbide and in situ generated titanium diboride with excess free carbon, reaction bonded to silicon... more
The present communication is concerned with the fabrication of a composite material consisting of a two-phase ceramic skeleton, i.e. boron carbide and in situ generated titanium diboride with excess free carbon, reaction bonded to silicon carbide that was formed subsequently to the infiltration of the skeleton with molten silicon. The material displays high rigidity, extremely high hardness and low density, providing potential for applications ranging from light armor to precision machining equipment. Its advantage resides in the significantly lower sintering temperature than is necessary for pressureless sintering of boron carbide. The advantage of this approach resides in the several degrees of freedom that allow some latitude in designing required properties.
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ABSTRACT The morphology and structure of the weld interface in magnetic pulse welding of similar and dissimilar metals were investigated. The interface zone of dissimilar metal couples such as Al–Mg, was studied in comparison to Al–Al... more
ABSTRACT The morphology and structure of the weld interface in magnetic pulse welding of similar and dissimilar metals were investigated. The interface zone of dissimilar metal couples such as Al–Mg, was studied in comparison to Al–Al welds. It was found that intermetallic phases (IMP) of different compositions are created during welding of the Al–Mg couple by rapid solidification of a thin melted layer at the interface. According to the calculated energy balance of magnetic pulse welding (MPW), there is enough energy to melt a thin interfacial layer and create IMP. Intensive characterisation techniques were used, including the focused ion beam (FIB) method that was used to prepare a cross-section of the Al–Mg interface for TEM characterisation. It was established that the jet action plays an important role in the melting process at the bonding zone.
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The inter-diffusion of carbon (originating from a graphite substrate) into a niobium coating and the fabrication of its carbides by heat treatment in the temperature range of 1073–1773 K was studied. The thickness of the Nb2C and NbC... more
The inter-diffusion of carbon (originating from a graphite substrate) into a niobium coating and the fabrication of its carbides by heat treatment in the temperature range of 1073–1773 K was studied. The thickness of the Nb2C and NbC phases formed after heat treatment as well as the inter-diffusion coefficients for the formation of the carbide layers were also studied. It
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Niobium layers of 6 to 8 μm in thickness have been deposited on an ATJ graphite substrate by radio-frequency (rf) magnetron sputtering. After deposition, the layers were annealed under vacuum (6.67×10−4 Pa) at a temperature in the range... more
Niobium layers of 6 to 8 μm in thickness have been deposited on an ATJ graphite substrate by radio-frequency (rf) magnetron sputtering. After deposition, the layers were annealed under vacuum (6.67×10−4 Pa) at a temperature in the range of 1373–2073 K for 0.5 to 3 h. The effects of the negative substrate bias voltage on the deposited niobium layers and
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Research Interests: Materials Engineering, Condensed Matter Physics, Materials Science, Scanning Electron Microscopy, Graphite, and 10 moreThin Film, Activation Energy, Radio Frequency, Magnetron Sputtering, Niobium, Surface and Coatings Technology, X ray diffraction, Growth rate, Low Temperature, and Thermal annealing
The effect of Ti additions on the wetting behavior of CaF2 by non-reactive liquid metals (In, Sn, Ga, Au, Ge) was investigated. Pure metals do not wet CaF2 while minor additions of Ti improve wetting. Small changes of the contact angle... more
The effect of Ti additions on the wetting behavior of CaF2 by non-reactive liquid metals (In, Sn, Ga, Au, Ge) was investigated. Pure metals do not wet CaF2 while minor additions of Ti improve wetting. Small changes of the contact angle were observed in the CaF2/Au–Ti and CaF2/Ge–Ti systems, which are characterized by strong Me–Ti interaction in the melt, while
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ABSTRACT This work reports results of silicon carbide plates, disks, pipes, and pipe–disk couples bonded by a spark plasma sintering apparatus. The joining was conducted at 1900 °C for 30 min with a 35 MPa uniaxial pressure. The samples... more
ABSTRACT This work reports results of silicon carbide plates, disks, pipes, and pipe–disk couples bonded by a spark plasma sintering apparatus. The joining was conducted at 1900 °C for 30 min with a 35 MPa uniaxial pressure. The samples were analyzed by Scanning acoustic microscopy, which in turn revealed a low amount of small defects at the samples’ periphery. Scanning acoustic microscopy results were verified through scanning electron microscopy and nanoindentation. It was concluded that Spark Plasma Sintering technique may serve as a valid and effective tool for diffusion bonding of high-temperature-resistant silicon carbide with different geometries.
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Steffen Rothe, Stefan Hartmann and Nachum Frage 1 Institute of Applied Mechanics Clausthal University of Technology Adolph-Roemer-Str. 2a, 38678 Clausthal-Zellerfeld, Germany email: steffen.rothe@tu-clausthal.de, web page:... more
Steffen Rothe, Stefan Hartmann and Nachum Frage 1 Institute of Applied Mechanics Clausthal University of Technology Adolph-Roemer-Str. 2a, 38678 Clausthal-Zellerfeld, Germany email: steffen.rothe@tu-clausthal.de, web page: http://www.itm.tu-clausthal.de 2 Department of Materials Engineering Ben-Gurion University of the Negev PO.Box 653, Beer-Sheva 84105, Israel email: nfrage@bgu.ac.il web page: http://cmsprod.bgu.ac.il/Eng/engn/mater/
Abstract Ceramic phosphors have great potential as high-power color converters for solid-state lighting. The strive for developing increasingly bright and efficient phosphors stimulates research on scattering effects within phosphor media... more
Abstract Ceramic phosphors have great potential as high-power color converters for solid-state lighting. The strive for developing increasingly bright and efficient phosphors stimulates research on scattering effects within phosphor media to enhance photoluminescence (PL). In the present study, we demonstrated a simple manner to improve PL properties of Ce:YAG transparent ceramic phosphors fabricated by spark plasma sintering. Controlled pore growth was achieved by post-sintering heat treatments, utilizing high-pressured gas trapped within residual pores to induce swelling. After heat treatment, the emission intensity and efficiency increased substantially. This is a promising method for microstructure tailoring to enhance PL and realize more effective ceramic phosphors.
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The present paper is concerned with the static and the dynamic mechanical properties at strain-rates up to 103 s−1, of ceramic composites based n porous B4C infiltrated with molten Si. The static mechanical properties of the infiltrated... more
The present paper is concerned with the static and the dynamic mechanical properties at strain-rates up to 103 s−1, of ceramic composites based n porous B4C infiltrated with molten Si. The static mechanical properties of the infiltrated composites depend on the amount of the residual ilicon. The Young’s modulus and the hardness increase, while the flexural strength and K1C decrease with decreasing fraction of residual silicon. he dynamic strength and the initiation fracture toughness K1D have significantly higher values than the corresponding static properties, and are nsensitive to both the residual silicon fraction and the strain-rate up to ε̇ = 10 s−1. 2007 Elsevier B.V. All rights reserved.
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Neural networks are one of the first major milestones in developing artificial intelligence systems. The utilisation of integrated photonics in neural networks offers a promising alternative approach to microelectronic and hybrid... more
Neural networks are one of the first major milestones in developing artificial intelligence systems. The utilisation of integrated photonics in neural networks offers a promising alternative approach to microelectronic and hybrid optical-electronic implementations due to improvements in computational speed and low energy consumption in machine-learning tasks. However, at present, most of the neural network hardware systems are still electronic-based due to a lack of optical realisation of the nonlinear activation function. Here, we experimentally demonstrate two novel approaches for implementing an all-optical neural nonlinear activation function based on utilising unique light-matter interactions in 2D Ti3C2Tx (MXene) in the infrared (IR) range in two configurations: 1) a saturable absorber made of MXene thin film, and 2) a silicon waveguide with MXene flakes overlayer. These configurations may serve as nonlinear units in photonic neural networks, while their nonlinear transfer fun...
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Spark plasma sintering (SPS) is an advanced one-stage, rapid, near-net shape densification technique combining uniaxial pressure with resistive heating. Various transparent ceramics have been successfully fabricated by SPS, despite the... more
Spark plasma sintering (SPS) is an advanced one-stage, rapid, near-net shape densification technique combining uniaxial pressure with resistive heating. Various transparent ceramics have been successfully fabricated by SPS, despite the existence of inherent carbon contamination and residual pores. Due to the disk-shape of SPS-processed samples, the technique may be suited for producing thin-disk ceramic laser materials. Nevertheless, an in-depth study of these materials has never been reported. With that goal in mind, the major focus of this study was to characterize the laser performance of Nd:YAG ceramics fabricated by one-stage SPS under conventional (60 MPa) and high (300 MPa) applied pressures. In addition to measuring the lasing slope efficiency and threshold, the passive losses associated with each sample were also evaluated. Surprisingly, it was found that in-line transmittance spectra do not provide accurate predictions of laser performance due to the nature of residual por...
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Creep is a time dependent, temperature-sensitive mechanical response of a material in the form of continuous deformation under constant load or stress. To study the creep properties of a given material, the load/stress and temperature... more
Creep is a time dependent, temperature-sensitive mechanical response of a material in the form of continuous deformation under constant load or stress. To study the creep properties of a given material, the load/stress and temperature must be controlled while measuring strain over time. The present study describes how a spark plasma sintering (SPS) apparatus can be used as a precise tool for measuring compressive creep of materials. Several examples for using the SPS apparatus for high-temperature compressive creep studies of metals and ceramics under a constant load are discussed. Experimental results are in a good agreement with data reported in literature, which verifies that the SPS apparatus can serve as a tool for measuring compressive creep strain of materials.
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Abstract Contrary to the characteristic strengthening of polycrystalline ceramics with a decrease in grain size, extremely fine nanocrystalline ceramics exhibit softening, increased plasticity and an inverse Hall-Petch relation. Despite... more
Abstract Contrary to the characteristic strengthening of polycrystalline ceramics with a decrease in grain size, extremely fine nanocrystalline ceramics exhibit softening, increased plasticity and an inverse Hall-Petch relation. Despite experimental evidence, questions remain regarding the underlying deformation mechanisms governing this abnormal mechanical behavior. In the present study, an in-depth microstructural examination was performed on nanostructured transparent magnesium aluminate spinel (MgAl2O4) subjected to microhardness tests. Microstructural observations revealed regions strained to various degrees below the point of indentation, containing varying amounts of dislocations and nano-cavities. Furthermore, the residual strain in different areas was estimated by local electron diffraction. These observations and analysis provided evidence for grain boundary (GB) mediated mechanisms (e.g., GB sliding and rotation). Moreover, shear bands formed and were found to be associated with micro-cracking. By combining the microstructural analysis with suitable models, it was concluded that these mechanisms govern plastic deformation. By elucidating how strain is accommodated within nanocrystalline ceramics, a deeper understanding of their unique mechanical behavior is gained.
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Abstract The strengthening of aluminum alloy 6061 (AA6061) by different point defects was determined experimentally using a shock wave technique. Decay of the amplitude of elastic precursor wave τ e l with propagation distance h was... more
Abstract The strengthening of aluminum alloy 6061 (AA6061) by different point defects was determined experimentally using a shock wave technique. Decay of the amplitude of elastic precursor wave τ e l with propagation distance h was studied in four groups of AA6061 samples, namely, that in the super-saturated solid solution state (SSSS), and those strengthened by atomic clusters (short-range order), by Guinier-Preston zones I and by Guinier-Preston zones II after, respectively 7.5, 240 and 960 min aging of the SSSS samples at 145 °C. The dependences τ e l ( h ) were found to be kinked at stress τ * , corresponding to the transition of the control of dislocation motion from phonon viscous drag at τ e l > τ * to thermally activated obstacle passage at τ e l τ * . The values of strengthening by atomic clusters, GP-I, and GP-II zones were found equal to τ sro = 67 , τ GP - I = 67 , and τ GP - II = 124 MPa , respectively. Activation volumes corresponding to the interaction of dislocations with the studied defects, estimated based on τ e l τ * segments of the measured dependences τ e l ( h ) , were found to be in reasonable agreement with existing concepts of dislocation/defect interactions.
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Abstract High-pressure spark plasma sintering of Si 3 N 4 with Y 2 O 3 , Al 2 O 3 and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si 3 N 4 phase and 8% β-Si 3 N 4 phase. The... more
Abstract High-pressure spark plasma sintering of Si 3 N 4 with Y 2 O 3 , Al 2 O 3 and LiF additives was employed to fabricate high quality dense ceramics comprising approximately 92% α-Si 3 N 4 phase and 8% β-Si 3 N 4 phase. The relatively high pressure applied (up to 650 MPa) had a substantial effect on densification by enhancing particle rearrangement, making it possible to obtain dense Si 3 N 4 at a significantly lower sintering temperature (1350 °C). Consequently, virtually no α to β phase transformation transpired during the liquid phase sintering process. The LiF additive had an indispensable influence on the densification process by lowering the viscous glass formation temperature, which also contributed to enhanced particle rearrangement. The nearly fully dense samples (theoretical density ≥99%) obtained displayed a good combination of mechanical properties, namely elastic modulus (304–316 GPa), hardness (1720–1780 HV2) and fracture toughness (6.0 MPa m 1/2 ).
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Abstract Mechanical properties (tensile strength and creep) of AlSi10Mg specimens fabricated by selective laser melting (SLM) in the Z-direction were investigated in the 25-400 °C temperature range. Specimens were tested after stress... more
Abstract Mechanical properties (tensile strength and creep) of AlSi10Mg specimens fabricated by selective laser melting (SLM) in the Z-direction were investigated in the 25-400 °C temperature range. Specimens were tested after stress relief treatment. The results revealed that yield stress (YS) significantly decreases and the elongation increases at temperatures higher than 200 °C. The ultimate tensile stress (UTS) continuously decreases with temperature. The creep parameters, namely stress exponent n and apparent activation energy Q, were found to be 25 ± 2 and 146 ± 20 kJ/mole, respectively. It was shown that plastic deformation during creep is governed by dislocation movements in primary aluminum grains. The tested material is actually an aluminum composite reinforced by sub-micron Si particles. The creep resistance of AlSi10Mg alloy fabricated by selective laser melting is close to that for aluminum matrix particles reinforced composites.
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Abstract Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method... more
Abstract Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.
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Polycrystalline magnesium aluminate (MgAl O ) spinel (PMAS) exhibits a unique combination of physical, chemical, mechanical, and optical properties, which makes it useful for a wide range of applications, including UV lenses for... more
Polycrystalline magnesium aluminate (MgAl O ) spinel (PMAS) exhibits a unique combination of physical, chemical, mechanical, and optical properties, which makes it useful for a wide range of applications, including UV lenses for lithography, electroinsulation, and structural windows for both VIS and IR region radiation and armor applications. Conventional two-stage processing of PMAS involves prolonged pressureless sintering followed by hot isostatic pressing. The costly processing of high-quality transparent PMAS ceramic is the main reason for the limited usage of this material in industry. Spark plasma sintering (SPS) is a relatively novel one-stage, rapid, and cost-effective sintering technique, which holds great potential for producing high-quality optical materials. Here, recent advances in the fabrication of transparent PMAS by the SPS approach, the influence of sintering parameters on microstructure evolution during densification, and their effects on the optical and mechanic...
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Abstract Samples of transparent polycrystalline spinel with average grain size varying from 0.14 to 170 μm were prepared by different sintering approaches. The effect of grain size on the flexural strength, hardness and Hugoniot elastic... more
Abstract Samples of transparent polycrystalline spinel with average grain size varying from 0.14 to 170 μm were prepared by different sintering approaches. The effect of grain size on the flexural strength, hardness and Hugoniot elastic limit (impact loading) was investigated. It was found that values of hardness divided by three for samples with grain size in the 0.14–15 μm range were almost equal to the dynamic yield strength values, estimated based on the Hugoniot elastic limit. This led to the assumption that the onset of inelastic deformation at the Hugoniot elastic limit was brittle rather than ductile. The observed departure of the dynamic yield strength from the hardness divided by three value for ceramics with grain size >15 μm was associated with either impact-induced shear banding or twinning. The feasibility of such banding/twinning intervention in initiating inelastic deformation in the spinel is supported by the values of apparent Hall-Petch coefficients in the corresponding grain size domains.
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Additive manufacturing of metals by selective laser melting (AM-SLM) is hampered by significant limitations in product size due to the limited dimensions of printing trays. Electron beam welding (EBW) is a well-established process that... more
Additive manufacturing of metals by selective laser melting (AM-SLM) is hampered by significant limitations in product size due to the limited dimensions of printing trays. Electron beam welding (EBW) is a well-established process that results in relatively minor metallurgical modifications in workpieces due to the ability of EBW to pass high-density energy to the related substance. The present study aims to evaluate structural properties of EB-welded AlSi10Mg thin-walled pressure vessels produced from components prepared by SLM technology. Following the EB welding process, leak and burst tests were conducted, as was fractography analysis. The welded vessels showed an acceptable holding pressure of ~ 30 MPa, with a reasonable residual deformation up to 2.3% and a leak rate better than 1 × 10−8 std-cc s−1 helium. The failures that occurred under longitudinal stresses reflected the presence of two weak locations in the vessels, i.e., the welded joint region and the transition zone between the vessel base and wall. Fractographic analysis of the fracture surfaces of broken vessels displayed the ductile mode of the rupture, with dimples of various sizes, depending on the failure location.
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Abstract Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as... more
Abstract Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.
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Abstract One of the key factors behind the effect of pulse magneto-oscillation (PMO) on grain size is convection. According to the dendrite fragmentation model, refined equiaxed grains are formed by the increased convection that results... more
Abstract One of the key factors behind the effect of pulse magneto-oscillation (PMO) on grain size is convection. According to the dendrite fragmentation model, refined equiaxed grains are formed by the increased convection that results from PMO treatment. However, when convection is weak, as in experiments within a small crucible diameter or in those conducted at low current intensities, grain size was reduced, although the columnar form was maintained. The observations support the concept that dendrite fragmentation mostly operates during solidification under relatively strong convection conditions. The cavities activation mechanism, which is related to the presence of stabilized solid embryos due to PMO treatment, operates when convection is weak.
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High-pressure spark plasma sintering (HPSPS) was employed to fabricate polycrystalline Nd:YAG specimens with desired functional properties. Specimens fabricated under a uniaxial pressure of 300 MPa at 1300°C at a heating rate of 50°C/min... more
High-pressure spark plasma sintering (HPSPS) was employed to fabricate polycrystalline Nd:YAG specimens with desired functional properties. Specimens fabricated under a uniaxial pressure of 300 MPa at 1300°C at a heating rate of 50°C/min and holding time of 60 min displayed submicrometer microstructure and elevated mechanical properties, including resistance to thermal shock. Optical properties (i.e., spectral transmittance, fluorescence emission spectra and fluorescence lifetime) of the HPSPS-processed specimens were close to those obtained with specimens fabricated by conventional sintering procedure. Specifically, remarkable differences in threshold power and laser slope efficiency were found and attributed to the variance in Nd concentration in the specimens tested. The results of this study indicate that the low cost and timesaving HPSPS process allows for the fabrication of polycrystalline Nd:YAG specimens with optical properties suitable for laser applications.
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In the present study the modeling of the HEBM process is presented. The impact velocity, impact angle, rotation speed, mass of balls, ball-to-powder weight ratio and time of milling have been taken into account in order to calculate the... more
In the present study the modeling of the HEBM process is presented. The impact velocity, impact angle, rotation speed, mass of balls, ball-to-powder weight ratio and time of milling have been taken into account in order to calculate the energy transferred from the balls to the powder. Two different systems, namely, TiN-AlN and polysalicylic acid were experimentally investigated in order to confirm the validity of the model. The calculation results are in a reasonable agreement with the results of experimental research.