Identification and characterization of diffusion barriers for Cu/SiC systems

The promise of CuSiC metal matrix composites (MMCs) as a thermal management material is to provide increased power density and high reliability for advanced electronic systems. CuSiC will offer high thermal conductivity between 250 and 325 W/mK with corresponding adjustable thermal expansion coefficient between 8.0 and 12.5 ppm/^∘C. The major challenge in development of these materials is control of the interface interactions. Cu and SiC react at high temperatures between 850 and 1150^∘C, needed for fabrication of the CuSiC material, with an expected decrease in thermal conductivity of the CuSiC MMCs as the Si product of reaction dissolves into the Cu. The application of barrier coatings onto SiC was observed to control chemical reaction of Cu and SiC. In the current study, the effectiveness of four barriers to prevent Cu diffusion and reaction with SiC were evaluated between 850 to 1150^∘C. Immersion experiments were conducted at 1150^∘C to understand the reaction between copper and silicon carbide. Reaction products were identified with transmission electron microscopy (TEM) and electron diffraction. Laser flash thermal diffusivity measurements confirmed thermal conductivity to decrease with increasing silicon content of the copper as determined by induction coupled plasma mass spectrometry (ICPMS) and glow discharge mass spectrometry (GDMS). An erratum to this article is available at .     

Dielectric response of an epoxy resin when exposed to high temperatures

Epoxy resins are popular insulators that are used for the encapsulation of integrated circuits and for the fabrication of printed circuits boards. As such, it is important to evaluate their reliability when exposed to an environmental stress. This work reports on the influence of a high-temperature thermal stress (400 ^∘C) on the dielectric properties of an acid-anhydride cured DGEBA resin. Gold/epoxy/gold capacitors are used as a test structure. The studied electrical properties are the dielectric constant and the loss factor in the 10⁻¹ Hz100 kHz range, and the DC resistivity. It is found that cycling the sample for several minutes to temperatures as high as 400 ^∘C has almost no effect on its dielectric properties. It is believed that the gold electrodes prevent the thermo-oxidative degradation of the underlying epoxy. Electrical properties are also studied at high temperatures in the 200 ^∘C-400 ^∘C range. Above 200 ^∘C the DC resistivity is considerably increased, as well as the loss in the low-frequency part of the spectrum (< 1kHz). At high frequencies (> 1kHz) the epoxy maintains good dielectric performances up to 400 ^∘C.     

Structural and electrical properties of perovskite ruthenate-based lead-free thick film resistors on alumina and LTCC

CaRuO₃ perovskite-based lead-free thick-film resistors (TFRs) were prepared on 96%-alumina and Low Temperature Co-fired Ceramic (LTCC) substrates. The microstructure evolution, possible interactions, and electrical properties of resistors were investigated. The hot and cold TCR values of all the resistors were measured in the temperature range (+20 to +120 ^∘C) and (+20 to −120 ^∘C), respectively. TFRs with 25% by vol. of CaRuO₃ on alumina exhibit a sheet resistance R_s = 5kΩ/sq. with hot and cold TCR of 225 and 470 ppm/^∘C respectively, whereas the same composition gives 1.2 kΩ/sq., 16.5 kΩ/sq. and 0.7 kΩ/sq. for co-fired, post-fired resistors on LTCC and buried resistors, respectively. The hot (HTCR) and cold (CTCR) values were evaluated; HTCR = 190 ppm/^∘C and CTCR = 314 ppm/^∘C were found for co-fired structures; HTCR = 216 ppm/^∘C and CTCR = 205 ppm/^∘C for post-fired samples and HTCR = 520 ppm/^∘C and CTCR = 350 ppm/^∘C for buried in LTCC structures.     

Aqueous supercapacitors on conductive cotton

Wearable electronics offer the combined advantages of both electronics and fabrics. In this article, we report the fabrication of wearable supercapacitors using cotton fabric as an essential component. Carbon nanotubes are conformally coated onto the cotton fibers, leading to a highly electrically conductive interconnecting network. The porous carbon nanotube coating functions as both active material and current collector in the supercapacitor. Aqueous lithium sulfate is used as the electrolyte in the devices, because it presents no safety concerns for human use. The supercapacitor shows high specific capacitance (˜70–80 F·g⁻¹ at 0.1 A·g⁻¹) and cycling stability (negligible decay after 35,000 cycles). The extremely simple design and fabrication process make it applicable for providing power in practical electronic devices.     

Factors affecting the resistance of cementitious materials at high temperatures and medium[0] heating rates

The resistance of mortars made of dolomite and quartz aggregate with and without polypropylene fibers has been studied at a nominal heating rate of 30^∘C/min from room temperature up to 1000^∘C. It is demonstrated that the key parameters that affect the performance of plain mortars are volume instability, phase transformation, aggregate dissociation, and permeability. Experimental results point at the major role of aggregate type on mass loss, porosity, volume instability, microstructure, cracking pattern, and mechanical properties. Three thermal zones are identified; low (up to about 300^∘C), intermediate (about 300 to 600^∘C), and high (>600^∘C). It is shown that in the low thermal zone, the mechanical properties are about the same or better than those at room temperature. The intermediate thermal zone is characterized by a moderate decline of mechanical properties, whereas a rapid decline is registered in the high thermal zone. Explosive spalling due to pressure built-up of volatiles took place at temperatures over 200^∘C. Addition of polypropylene fibers prevents spalling due to the occurrence of porous and permeable interface between the fibers and the matrix rather than fiber shrinkage or melting. An erratum to this article can be found at     

Investigation of microstructures and tensile properties of a Sn-Cu lead-free solder alloy

In recent years, the pollution of environment from lead (Pb) and Pb-containing compounds in microelectronic devices attracts more and more attentions in academia and industry, the lead-free solder alloys begin to replace the lead-based solders in packaging process of some devices and components. In this work, microstructures and mechanical properties of the lead-free solder alloy Sn99.3Cu0.7(Ni) are investigated. This paper will compare the mechanical properties of the lead-based with lead-free solder alloys (Sn99.3Cu0.7(Ni) and 63Sn37Pb). The tensile tests of lead-based and lead-free solder alloys (Sn99.3Cu0.7(Ni) and Sn63Pb37) were conducted at room and elevated temperature at constant strain rate; the relevant tensile properties of Sn99.3Cu0.7(Ni) and Sn63Pb37 were obtained. Specifically, the tensile strength of this lead-free solder- Sn99.3Cu0.7(Ni) in 25^∘C, 50^∘C, 75^∘C, 100^∘C, 125^∘C was investigated; and it was found that tensile strength of the lead-free solder decreased with the increasing test temperature at constant strain rate, showing strong temperature dependence. The lead-free solder alloy Sn99.3Cu0.7(Ni) was found to have favorable mechanical properties and it may be able to replace the lead-based solder alloy such as Sn63Pb37 in the packaging processes in microelectronic industry.     

Effect of pH, temperature and Cl− concentration on electrochemical behavior of NiTi shape memory alloy in artificial saliva

The cooperation of pH, temperature and Cl⁻ concentration on electrochemical behavior of NiTi shape memory alloy in artificial saliva was studied using orthogonal test method. The results showed that the pitting potential for NiTi in artificial saliva decreased at low and high pH; at 25^∘C, the pitting potential was the lowest compared to those at 10^∘C, 37^∘C and 50^∘C; when the Cl⁻ concentration was not less than 0.05 mol/L the pitting potential decreased with the increase of Cl⁻ concentration. The free corrosion potential of austenitic NiTi was lower than that of mixture of austenite and martensite.     

Superplasticity and associated activation energy in Ti-3Al-8V-6Cr-4Mo-4Zr Alloy

The high temperature deformation characteristics of a commercial β -titanium alloy Ti-3Al-8V-6Cr-4Mo-4Zr have been studied in the temperature range 830–925∘C. The alloy exhibited superplasticity in a narrow temperature and strain rate range i.e. 850–865∘C and 5× 10^{− 5}–3× 10^{− 3} s^{− 1} respectively, with a maximum elongation of 634% at 855∘C. The superplastic behaviour in the alloy is considered to arise as a result of subgrain formation at the higher strain rates (region III) which enhances diffusional creep at lower strain rates (region II). The activation energy values for regions II and III were found to be close to the lower of the two activation energy values (129.2 KJ/mole) proposed to describe self diffusion in β -phase suggesting that the rate controlling mechanism during high temperature deformation of the alloy was that for lattice diffusion.     

Use of high-purity AlxGa1−x as layers in epitaxial structures for high-power microwave field-effect transistors

The fabrication of high-purity layers of Al_xGa_1−x As solid solutions in the range 0⩽x⩽0.38 by molecular beam epitaxy is reported. The low-temperature photoluminescence spectra of these layers reveal predominantly the free exciton recombination line (X). The narrow width of the X line, the high intensity ratio of this line to that of the band-acceptor transition line, and the linear dependence of the X line intensity on the excitation power density in the range between 1×10⁻⁴ and 100V·cm⁻² indicate a low concentration of background impurities in these layers. Using this material in pseudomorphic AlGaAs/InGaAs/GaAs heterostructures for high-power microwave transistors produced devices with a specific saturated output power of 0.9 W/mm at 18 GHz. Pis’ma Zh. Tekh. Fiz. 25, 8–15 (August 12, 1999)     

Superconducting Performance of Ex-situ SiC Doped MgB2 Monofilamentary Tapes

The superconducting performance of the ex-situ SiC doped MgB₂ monofilamentary tapes are reported. Polycrystalline powders of MgB₂ doped with 5 and 10 wt% SiC were synthesized by a conventional solid-state reaction route and characterized for their superconducting performances. It was found that the superconducting parameters viz., upper critical field (H _c2), irreversibility field (H _irr) and critical current density (J _c) were improved significantly with SiC addition. It was also found that relatively lower synthesis temperature resulted in further improved superconducting parameters in comparison to higher synthesis temperature. Thus, synthesized powders are used for the fabrication of ex-situ powder-in-tube (PIT) monofilamentary tapes. The superconducting performance in terms of critical current density (J _c), being determined from both magnetization (J _cm) and transport (J _ct) measurements, was improved significantly. In particular, the SiC doped MgB₂ tapes (fabricated using 700 °C heat treated bulk powder) exhibited the transport J _ct of nearly 10⁴ A/cm² under applied fields of as high as 7 Tesla. Further, it was found that the J _ct anisotropy decreases significantly for SiC doped tapes. Disorder due to substitution of C at B site being created from broken SiC and the presence of nano SiC respectively in SiC added ex-situ MgB₂ tapes was responsible for decreased anisotropy and improved J _c(H) performance.     

The Effect of flux types on the formation of green light emitting phosphor particles with spherical shape and filled morphology

(CeTb)MgAl₁₁O₁₉ (CTMA) phosphor particles were prepared by high-temperature spray pyrolysis from spray solutions with various types of flux materials. The particles prepared from spray solutions with ammonium dihydrogen phosphate and lithium carbonate fluxes had spherical shape and filled morphology at temperatures of 900^∘C and 1650^∘C. On the other hand, the particles prepared from spray solutions without flux material had hollow and fractured morphology at temperatures of 900^∘C and 1650^∘C. The melting of flux material formed the spherical intermediate particles with filled morphology. These spherical intermediate particles were formed from spray solutions with flux material that transformed into spherical CTMA phosphor particles with filled morphology at a high-preparation temperature. The phosphor particles prepared by spray pyrolysis from the spray solution with appropriate flux materials at 1650^∘C had high photoluminescence intensities, spherical shape, and filled morphology.     

Complex impedance analysis of NaLaTiO4 electroceramics

A layered perovskite type ceramic oxide, NaLaTiO₄, has been prepared by a standard high temperature solid-state reaction route. Material formation has been confirmed by x-ray diffraction (XRD) studies. Complex impedance analysis on this system has been carried out, to investigate its electrical properties in details. The impedance analysis results have indicated the electrical conduction process to be governed by the contribution of both the grain (bulk) and grain boundaries above a temperature of 350 ^∘C with typical negative temperature coefficient of resistance (NTCR) type behaviour like that of a semiconductor. The d. c. conductivity of the material as evaluated from the impedance analysis has been observed to be of the order of ∼ 10⁻⁹ Scm⁻¹ at room temperature and ∼ 10⁻⁶ Scm⁻¹ at 500 ^∘C. The conductivity variation shows a cross over from Mott-type hopping behaviour at lower temperatures to a thermally activated Arrhenius behaviour at higher temperatures.     

Dielectric properties of BSTO/MgO ceramic composites

Ba_0.55Sr_0.45TiO₃/MgO ceramic composites with 50wt% MgO content were studied in terms of the phase distribution, microstructure and electric properties, prepared by traditional ceramic process-solid phase synthesis. The results show that a small amount of Mg²⁺ ions have entered BSTO lattices to substitute for Ti⁴⁺ and behave as electron acceptors, contributing to the decrease of the dielectric loss. Ba_0.55 Sr_0.45TiO₃/MgO composites in the paraelectric state at room temperature still have a high tunability because of a field-induced hardening of the soft phonon and the presence of micro- or nano-polar phases in the paraelectric phase. For Ba_0.55Sr_0.45TiO₃/MgO ceramic composites, the microwave loss at 2.5 GHz is 7.1 × 10⁻³ and the value of tunability is 13.8% with the external DC field 4 kV ⋅ mm⁻¹. It can basically meet the requirements of phase shifters working at microwave frequencies. The phase shift of 122^∘ was obtained at 9.2 GHz using the designed phase shifter with the bias voltage 2 kV ⋅ mm⁻¹.     

Physicomechanical and thermophysical properties of SiC-based ceramics

Fine-grain SiC-based ceramics have been produced via infiltration of molten silicon into preforms fabricated from SiC and graphite powders, with a phenol-formaldehyde resin as a binder. The materials thus prepared have a density of 2.70–3.15 g/cm³, dynamic modulus of elasticity from 200 to 400 GPa, compressive strength from 800 to 1900 MPa, bending strength from 150 to 315 MPa, thermal expansion coefficient (KTE) of 4.1 × 10⁻⁶ K⁻¹, and thermal conductivity of 140–150 W/(m K). Their properties are compared to those of known silicon carbide materials fabricated by other processes. The results indicate that the density and physicomechanical properties of the silicon carbide ceramics depend little on the fabrication process and are determined primarily by the SiC content. Increasing the SiC content from 20 to 99.5 wt % increases the density of the ceramics from 2.2 to 3.15 g/cm³ and leads to an exponential rise in their physicomechanical parameters: an increase in modulus of elasticity from 95 to 430 GPa, in compressive strength from 120 to 4200 MPa, and in bending strength from 70 to 410 MPa. The thermal conductivity of the ceramics depends very little on the fabrication process, falling in the range 100–150 W/(m K) over the entire range of SiC concentrations. Their KTE decreases slightly, from 4.3 × 10⁻⁶ to 2.4 × 10⁻⁶ K⁻¹, as the SiC content increases to 99–100 wt %.     

Influence of Microstructure on The Resistance to Salt Crystallisation Damage in Brick

In the article we study the variation of brick durability and, more specifically, its resistance to salt crystallisation produced by changes in its microstructure during firing. For this purpose, the evolution of both mechanical and pore structure properties are studied within a wide range of temperatures (700–1100^∘C). An increase in the firing temperature produces a more homogeneous and resistant brick, measured using ultrasound velocity and uniaxial compressive strength. This result is obtained thanks to the vitrification process and changes in the brick's pore structure: larger, rounder pores, which are quantified by their roundness and fractal dimension. As a result of these changes, an excellent durability is achieved in the bricks studied when fired at temperatures above 1000^∘C. Considering that few differences are noted in pore structure and brick strength between 1000 and 1100^∘C, the recommended firing temperature is, for raw materials with a similar composition and production process, 1000^∘C, as this involves a lower production cost than firing at 1100^∘C.     

Influence of Li<Superscript>+</Superscript> dopants on the properties of the ZnO piezoelectric films

ZnO piezoelectric films with the preferred 002-orientation were prepared by sol-gel method. The annealing temperature was 600^∘C and the resistivity of the ZnO film was 1 × 10⁶ Ω ⋅ cm. Li₂CO₃ and LiCl were added respectively into ZnO precursor as source of Li⁺-ion. The molar ratio of [Li⁺]/[Zn^{2 +}] was 0.05. It is observed that the annealing temperature for forming preferred 002-orientation of ZnO films decreases from 660 to 550^∘ C after Li₂CO₃ being doped. When Li₂CO₃ and LiCl are doped, the resistivity of ZnO films increases to 10⁸Ω ⋅ cm and 10⁹Ω ⋅ cm, respectively, with an annealing temperature of 550^∘ C. When annealing temperature is 600^∘ C, the resistivity of the ZnO film with LiCl dopant increases to 10⁷Ω cm. The influence mechanism of the two dopants on the properties of the ZnO films is analyzed.     

Composites between hydroxyapatite and poly(ε-caprolactone) synthesized in open system at room temperature

The hydroxyls present on the surface of hydroxyapatite (HA) granules, annealed at 700 ∘, 900 ∘ and 1100 ∘C, are able to initiate the polymerization of ε-caprolactone (CL), not only at 185 ∘C under vacuum, but also at room temperature in open system. A polymer layer ionically linked to the substrate is formed on HA surface, enhancing the compatibility between the organic phase and the inorganic one in composite biomaterials. We studied the characteristics of the polymer, produced by the reaction carried out at room temperature in open system, as well as the percentages of the poly(ε-caprolactone) (PCL) ionically bonded to the HA structure and of the “free” one. Both percentages appear very dependent on the annealing temperature; in particular, HA annealed for 1 h at 1100 ∘C is the most efficient initiator of the reaction leading to ionically bonded PCL. The percentages of “free” polymer are much higher than at 185 ∘C under vacuum. Its formation is attributed to the role of water in opening the CL rings, and to the presence of CO₃²⁻ and HPO₄²⁻ ions in the HA annealed at lower temperatures. The presence of water appears to be the limiting factor for the production of PCL not bonded to the HA structure.     

A Rosen-type piezoelectric transformer employing lead-free K0.5Na0.5NbO3 ceramics

Lead-free K_0.5Na_0.5NbO₃–K_5.4Cu_1.3Ta₁₀O₂₉–MnO₂ (KNN–KCT–Mn) ceramics have been prepared by a conventional ceramic sintering technique. The ceramics show excellent piezoelectric properties for application in power devices, and the optimum properties measured are as follows: piezoelectric constant d ₃₃ = 90 pC/N, planar electromechanical coupling factor k _p = 0.40, mechanical quality factor Q _m = 1900, remanent polarization P _r = 11.8 μC/cm², coercive field E _c = 0.85 kV/mm. A Rosen-type piezoelectric transformer with a dimension of 21 mm × 6 mm × 1.2 mm was fabricated using the KNN–KCT–Mn ceramics. Properties of the piezoelectric transformer operating in the first and second modes have been characterized. For the first mode, the transformer has a maximum output power of 0.7 W with a temperature rise of 14 °C. For the second mode, the maximum output power of the transformer is 1.8 W with a temperature rise of 33 °C. KNN–KCT–Mn ceramics have shown to be a potential lead-free candidate to be used in high-voltage–low-current devices.     

Fractal character of in situ heat treated metal-compound semiconductor contacts

The heat treatment of metallized (Au) compound semiconductors (InP) was studied by in situ scanning electron microscopy combined with mass spectrometry. Correlation was found between the change in the surface morphology and the volatile component loss caused by the material interactions taking place during the heat treatment. Our experiments proved that the surface morphology can be characterized by its fractal dimension at the maximum value of the volatile component loss. In this paper the dependence of the fractal dimension of the surface pattern on the heat treatment temperature (in a given temperature range), on the volatile component loss and on metal thickness is described. Changes of the surface morphology on the analyzed samples begin at different temperatures. The evaluated patterns were created describing contour lines of the metal islands on the surface. This island formation known as balling-up phenomenon is due to the heating up of the samples. In the case of the 10 and 30 nm Au/InP(100) samples the fractal behavior appeared nearly at the same temperature (470^∘C α 490^∘C). The examined thick Au(85 nm)/InP(100) contact showed a fractal character at a lower (385^∘C) temperature. Although fractal dimension values could be obtained in a rather wide temperature range the surface had a real fractal character at only those temperatures where volatile component loss took place.     

Microstructure and microwave dielectric characteristics of CaO–B2O3–SiO2 glass ceramics

CaO–B₂O₃–SiO₂ (CBS) glass powders are prepared by traditional glass melting method, whose properties and microstructures are characterized by Differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that the pure CBS glass ceramics possess excellent dielectric properties (ε _r = 6.5, tan δ = 5 × 10⁻³ at 10 GHz), but a higher sintering temperature (>900 °C) and a narrow sintering temperature range (about 10 °C). The addition of a low-melting-point CaO–B₂O₃–SiO₂ glass (LG) could greatly decrease the sintering temperature of CBS glass to 820 °C and significantly enlarge the sintering temperature range to 40 °C. The CBS glass ceramic with 30 wt% LG glass addition sintered at 840 °C exhibits better dielectric properties: ε _r ≈ 6, tan δ < 2 × 10⁻³ at 10 GHz, and the major phases of the sample are CaSiO₃, CaB₂O₄ and SiO₂.