Determination of the Origin of Crystal Orientation for Nanocrystalline Bismuth Telluride-Based Thin Films Prepared by Use of the Flash Evaporation Method

We have investigated the origin of crystal orientation for nanocrystalline bismuth telluride-based thin films. Thin films of p-type bismuth telluride antimony (Bi–Te–Sb) and n-type bismuth telluride selenide (Bi–Te–Se) were fabricated by a flash evaporation method, with exactly the same deposition conditions except for the elemental composition of the starting powders. For p-type Bi–Te–Sb thin films the main x-ray diffraction (XRD) peaks were from the c-axis (Σ{00l}/Σ{hkl} = 0.88) whereas n-type Bi–Te–Se thin films were randomly oriented (Σ{00l}/Σ{hkl} = 0.40). Crystal orientation, crystallinity, and crystallite size were improved for both types of thin film by sintering. For p-type Bi–Te–Sb thin films, especially, high-quality structures were obtained compared with those of n-type Bi–Te–Se thin films. We also estimated the thermoelectric properties of the as-grown and sintered thin films. The power factor was enhanced by sintering; maximum values were 34.9 μW/cm K² for p-type Bi–Te–Sb thin films at a sintering temperature of 300°C and 23.9 μW/cm K² for n-type Bi–Te–Se thin films at a sintering temperature of 350°C. The exact mechanisms of film growth are not yet clear but we deduce the crystal orientation originates from the size of nano-clusters generated on the tungsten boat during flash evaporation.     

Carboxylate-Passivated Silver Nanoparticles and Their Application to Sintered Interconnection: A Replacement for High Temperature Lead-Rich Solders

Lead-free silver nanoparticle pastes have been tested as a replacement for high temperature lead-rich solders used in electronic manufacturing. The pastes contain a small amount of solvent, and primarily consist of submicron-silver powder and passivated silver nanoparticles. The nanoparticles were synthesized from Ag₂CO₃ and a long-chain alcohol by a method that produced a passivating layer consisting almost exclusively of the carboxylate of the reactant alcohol. The pastes were used to connect a silicon diode chip to copper bases without applied pressure when sintered at 350°C under nitrogen. Diode packages made with sintered silver interconnects had electrical and thermal properties equal to those with lead-soldered interconnects, even after 3000 thermal cycles between −55°C and +150°C. The mechanical strength was half that of lead-rich solder joints, but still strong enough for practical use.     

Electromigration in Sintered Nanoscale Silver Films at Elevated Temperature

Sintered nanoscale silver is a promising interconnection material for semiconductor devices because it provides improved joint properties compared with solder and wire bonds. It has higher electrical and thermal conductivity and is capable of higher operating temperature. Joints with die shear strength above 20 MPa can be formed at around 250°C even without applied pressure. Sintered silver joints were also found to be an order of magnitude more reliable than solder joints and wire bonds. In this work, the electromigration behavior of sintered nanosilver material under conditions of high applied current density and elevated temperature was investigated. Thin strips of sintered nanosilver formed on ceramic substrates were tested under current densities exceeding 150 kA/cm² at temperatures of 150°C and above. Results based on the percentage change in sample resistance showed that the sintered silver lasted at least ten times longer than aluminum wire bonds. Examination of failed strips revealed that hairline cracks formed during sintering were the main cause of failure. Otherwise, defect-free samples exhibited a 10-fold increase in lifetime over wire bonds under similar conditions.     

Effects of Reaction Conditions on the Properties of Spherical Silver Powders Synthesized by Reduction of an Organometallic Compound

Silver powders were synthesized by reducing a silver organometallic compound, silver 2-ethylhexanoate, with di-n-octylamine. The effects of preparation conditions on the characteristics of the powders were investigated. Silver powders prepared from silver 2-ethylhexanoate and di-n-octylamine in the ratio 2:1 (MA21) at 150°C for 3 h had the best characteristics (average particle size 277 nm, narrow particle-size distribution, high tap density of 4.0 g/cm³), and were also obtained in high yield (98%). Use of an excessive amount of di-n-octylamine resulted in intense thermolysis and a low yield of silver powders of irregular morphology with a wide particle-size distribution. As the proportion of silver 2-ethylhexanoate was increased, the silver powders obtained had a bimodal particle-size distribution and a relatively low tap density. Silver films seemed to have high resistivity when the temperature used for synthesis of the silver powders was too low or reaction time was insufficient. The electrical resistivities of silver films prepared from MA21 powders and sintered at 300°C and 500°C for 30 min were 3.8 × 10^?6 Ω cm and 2.3 × 10^?6 Ω cm, respectively, close to that of bulk silver.     

Effect of Sintering Temperature on the Properties of Fused Silica Ceramics Prepared by Gelcasting

Fused silica ceramics were fabricated by gelcasting, by use of a low-toxicity N′N-dimethylacrylamide gel system, and had excellent properties compared with those obtained by use of the low-toxicity 2-hydroxyethyl methacrylate and toxic acrylamide systems. The effect of sintering temperature on the microstructure, mechanical and dielectric properties, and thermal shock resistance of the fused silica ceramics was investigated. The results showed that sintering temperature has a critical effect. Use of an appropriate sintering temperature will promote densification and improve the strength, thermal shock resistance, and dielectric properties of fused silica ceramics. However, excessively high sintering temperature will greatly facilitate crystallization of amorphous silica and result in more cristobalite in the sample, which will cause deterioration of these properties. Fused silica ceramics sintered at 1275°C have the maximum flexural strength, as high as 81.32 MPa, but, simultaneously, a high coefficient of linear expansion (2.56 × 10^?6/K at 800°C) and dramatically reduced residual flexural strength after thermal shock (600°C). Fused silica ceramics sintered at 1250°C have excellent properties, relatively high and similar flexural strength before (67.43 MPa) and after thermal shock (65.45 MPa), a dielectric constant of 3.34, and the lowest dielectric loss of 1.20 × 10^?3 (at 1 MHz).     

New Method for Evaluating the Peltier Coefficient Based on Temperature Measurements in a Thermoelectric Module

A new method for determining the Peltier coefficient of thermoelectric devices has been developed. The Peltier coefficient has been evaluated by measuring the temperature distribution along the junction of two dissimilar materials X and Y. The energy balance has been used to link the Peltier coefficient with the hot and cold temperatures of the metallic blocks of a thermoelectric module (TEM), thus enabling the evaluation of this coefficient. Data on the thermal conductance of the pellets are also needed. The experimental device used in this paper is a TEM composed of N?=?71 couples of bismuth telluride, suitably doped to provide individual n and p elements. Using nominal values given by the manufacturer for the Seebeck coefficient of the TEM, the Onsager reciprocal relation has been confirmed.     

The Effect of Heat Treatment on Structural and Multiferroic Properties of Phase-Pure BiFeO3

Phase-pure multiferroic BiFeO₃ (BFO) was prepared by the coprecipitation technique using diverse precursors bismuth oxide at temperature as low as 400°C. The dependence of structural, microstructural, thermal, electrical (AC and DC), and magnetic properties on sintering temperature was systematically investigated. Uniaxially pressed samples (Ø8 mm) were sintered in air at 500°C to 800°C for 4 h. X-ray diffraction analysis was used to determine the amorphous and perovskite nature of as-synthesized and calcined/sintered samples, respectively. The crystallite size of sintered powders increased from 47 nm to 67 nm. Scanning electron microscopy showed grain growth during sintering, which improved intergranular connectivity and decreased porosity in the samples. The ferroelectric to paraelectric Curie transition temperature (T _C) of pure BFO powder was detected by differential scanning calorimetry analysis and found to be 820°C ± 1°C. The samples exhibited high AC resistivity and dielectric constant, and low loss tangent values. The samples exhibited weak ferromagnetic behavior with an unsaturated magnetization versus magnetic field hysteresis loop at room temperature. Ferroelectric behavior and variation in remnant polarization and coercivity were observed from polarization versus electric field loops. Enhanced capacitance in the magnetic field revealed the magnetoelectric effect in the samples.     

Enhanced Thermoelectric Properties of Antimony Telluride Thin Films with Preferred Orientation Prepared by Sputtering a Fan-Shaped Binary Composite Target

p-Type antimony telluride (Sb₂Te₃) thermoelectric thin films were deposited on BK7 glass substrates by ion beam sputter deposition using a fan-shaped binary composite target. The deposition temperature was varied from 100°C to 300°C in increments of 50°C. The influence of the deposition temperature on the microstructure, surface morphology, and thermoelectric properties of the thin films was systematically investigated. x-Ray diffraction results show that various alloy composition phases of the Sb₂Te₃ materials are grown when the deposition temperature is lower than 200°C. Preferred c-axis orientation of the Sb₂Te₃ thin film became obvious when the deposition temperature was above 200°C, and thin film with single-phase Sb₂Te₃ was obtained when the deposition temperature was 250°C. Scanning electron microscopy reveals that the average grain size of the films increases with increasing deposition temperature and that the thin film deposited at 250°C shows rhombohedral shape corresponding to the original Sb₂Te₃ structure. The room-temperature Seebeck coefficient and electrical conductivity range from 101 μV K^?1 to 161 μV K^?1 and 0.81 × 10³ S cm^?1 to 3.91 × 10³ S cm^?1, respectively, as the deposition temperature is increased from 100°C to 300°C. An optimal power factor of 6.12 × 10^?3 W m^?1 K^?2 is obtained for deposition temperature of 250°C. The thermoelectric properties of Sb₂Te₃ thin films have been found to be strongly enhanced when prepared using the fan-shaped binary composite target method with an appropriate substrate temperature.     

Effect of Sintering Temperature on Microstructure, Electrical Properties, and Thermal Expansion of Perovskite-Type La0.8Ca0.2CrO3 Complex Oxides Synthesized by a Combustion Method

Perovskite-type La_0.8Ca_0.2CrO₃ complex oxides were synthesized by a combustion method. Microstructural evolution, electrical properties, and thermal expansion behavior of the ceramics were investigated in the sintering temperature range of 1250°C to 1450°C. It was found that the electrical conductivity (σ _e) remarkably improved with increasing sintering temperature from 1250°C to 1400°C, ascribed to the development of microstructural densification, whereas it declined slightly above 1400°C due to generation of excessive liquid. The specimen sintered at 1400°C had a maximum conductivity of 31.6 S cm^?1 at 800°C, and lowest activation energy of 0.148 eV. The improvement of the thermal expansion coefficient (TEC) with increasing sintering temperature was monotonic as a result of the microstructural densification of the materials. The TEC of La_0.8Ca_0.2CrO₃ sintered at 1400°C was about 10.5 × 10^?6 K^?1, being consistent with other components as high-temperature conductors. With respect to microstructure, electrical properties, and thermal expansion, the preferable sintering temperature was ascertained to be about 1400°C, which is much lower than for the traditional solid-state reaction method.     

Good Thermal Stability,High Permittivity,Low Dielectric Loss and Chemical Compatibility with Silver Electrodes of Low-Fired BaTiO<Subscript>3</Subscript>–Bi(Cu<Subscript>0.75</Subscript>W<Subscript>0.25</Subscript>)O<Subscript>3</Subscript> Ceramics

(1 ? x)BaTiO₃–xBi(Cu_0.75W_0.25)O₃ [(1 ? x)BT–xBCW, 0 ≤ x ≤ 0.04] perovskite solid solutions ceramics of an X8R-type multilayer ceramic capacitor with a low sintering temperature (900°C) were synthesized by a conventional solid state reaction technique. Raman spectra and x-ray diffraction analysis demonstrated that a systematically structural evolution from a tetragonal phase to a pseudo-cubic phase appeared near 0.03 < x < 0.04. X-ray photoelectron analysis confirmed the existence of Cu⁺/Cu²⁺ mixed-valent structure in 0.96BT–0.04BCW ceramics. 0.96BT–0.04BCW ceramics sintered at 900°C showed excellent temperature stability of permittivity (Δε/ε _25°C ≤ ±15%) and retained good dielectric properties (relative permittivity ～1450 and dielectric loss ≤2%) over a wide temperature range from 25°C to 150°C at 1 MHz. Especially, 0.96BT–0.04BCW dielectrics have good compatibility with silver powders. Dielectric properties and electrode compatibility suggest that the developed materials can be used in low temperature co-fired multilayer capacitor applications.     

Porous Materials in Synthetic Transverse Thermoelements

Substantial transverse Peltier cooling has been reported for synthetic composites consisting of layers of bismuth telluride and either bismuth or lead. Ideally, the two materials in such a composite should have very different values for the electrical and thermal conductivities. If this condition is satisfied, the transverse figure of merit, Z _ϕ , for the optimum orientation is not much smaller than the longitudinal figure of merit, Z, for the two materials when used as a conventional thermocouple. A second condition is that Z should be as large as possible. We have shown that the two conditions can be met simultaneously if one of the components is porous or otherwise discontinuous. Thus, a highly efficient transverse thermoelement could be made from porous p-type and dense n-type bismuth telluride. Effective transverse thermoelements could also be made from less porous p-type bismuth telluride in conjunction with either single crystal bismuth or YbAl_2.96Mn_0.04. It is shown that a dimensionless transverse figure of merit, Z _ϕ T, in excess of 0.6 should easily be obtained with these configurations. It should be possible to observe temperature depressions in excess of 100° by the use of suitably shaped synthetic transverse thermoelements.     

Synthesis, Structures, and Multiferroic Properties of Strontium Hexaferrite Ceramics

Simultaneous occurrence of large ferroelectricity and strong ferromagnetism has been observed in strontium hexaferrite (SrFe₁₂O₁₉) ceramics. Strontium hexaferrite powders with hexagonal crystal structures have been successfully synthesized through the co-precipitation precursor method using strontium nitrate and ferric nitrate as starting materials. The powders were pressed into pellets and then sintered into ceramics at a temperature range of at 1000°C to 1100°C for 1 h. The structure and morphology of the ceramics were determined using x-ray diffraction and field-emission scanning electron microscopy techniques. Clear ferroelectric hysteresis loops demonstrated large spontaneous polarization in the SrFe₁₂O₁₉ ceramics at room temperature. The maximum remnant polarization of the SrFe₁₂O₁₉ ceramic was estimated to be approximately 15 μC/cm². The FeO₆ octahedron in its perovskite-like hexagonal unit cell and the displacement of Fe³⁺ off the center of the octahedron are proposed to be the origin of electric polarization in SrFe₁₂O₁₉. In our experimental observations, the SrFe₁₂O₁₉ ceramic also revealed strong ferromagnetism at room temperature.     

The shear strength of nano-Ag sintered joints and the use of Ag interconnects in the design and manufacture of SiGe-based thermo-electric modules

Thermo-electric modules (TEMs) can be used to convert heat into electricity by utilizing the Seeback effect. It is now possible to buy BiTe thermo-electric modules that can operate up to temperatures of around 300 °C. However, many applications, such as the harvesting of excess gas turbine heat, may occur at higher temperatures. Therefore, new materials and manufacturing processes need to be developed to produce packaged TEMs that can operate at a maximum operating temperature of 650 °C. Two critical areas in the manufacture of a SiGe TEM are the choice and strength of materials used to both sintered joint the TE material to the rest of the module and the metal used for the interconnects. The interconnection material needs to be sufficiently strong to withstand large temperature fluctuations while maintaining a low contact resistance, as well as being compatible with the nano-Ag sintered joint. Shear force tests of the sintered thermo electrical leg material showed that the joints are brittle when sintered to W metallized AlN substrates are used and ductile fracture behavior when sintered to Cu metallized AlN substrates using the NanoTach K nano silver paste. Almost all of the joints were found to be brittle when using the NachTach X nano silver paste. Shear testing of the sintered joints showed that the X paste joints were variable in strength and stiffness, having a typical Young’s modulus between 10 and 100 MPa at room temperature. The K paste joints were stiffer, but had a similar strength as compared to the X paste joints.     

Effects of Heat-Treatment Temperature on the Microstructure, Electrical and Dielectric Properties of M-Type Hexaferrites

M-type hexaferrite BaCr _x Ga _x Fe_12?2x O₁₉ (x = 0.2) powders have been synthesized by use of a sol–gel autocombustion method. The powder samples were pressed into 12-mm-diameter pellets by cold isostatic pressing at 2000 bar then heat treated at 700°C, 800°C, 900°C, and 1000°C. X-ray diffraction patterns of the powder sample heat treated at 1000°C confirmed formation of the pure M-type hexaferrite phase. The electrical resistivity at room temperature was significantly enhanced by increasing the temperature of heat treatment and approached 5.84 × 10⁹ Ω cm for the sample heat treated at 1000°C. Dielectric constant and dielectric loss tangent decreased whereas conductivity increased with increasing applied field frequency in the range 1 MHz–3 GHz. The dielectric properties and ac conductivity were explained on the basis of space charge polarization in accordance with the Maxwell–Wagner two-layer model and Koop’s phenomenological theory. The single-phase synthesized materials may be useful for high-frequency applications, for example reduction of eddy current losses and radar absorbing waves.     

Microwave Dielectric Properties of BiCu<Subscript>2</Subscript>PO<Subscript>6</Subscript> Ceramics with Low Sintering Temperature

A BiCu₂PO₆ microwave dielectric ceramic was prepared using a solid-state reaction method. As the sintering temperature increased from 800°C to 880°C, the bulk density of BiCu₂PO₆ ceramic increased from 6.299 g/cm³ to 6.366 g/cm³; the optimal temperature was 860°C. The best microwave dielectric properties [permittivity (? _r ) = ～16, a quality factor (Q × f) = ～39,110 GHz and a temperature coefficient of resonant frequency (τ _f ) = ～?59 ppm/°C] were obtained in the ceramic sintered at 860°C for 2 h. Then, TiO₂ with a positive τ _f (～+400 ppm/°C) was added to compensate the τ _f value. The composite material was found to have a near-zero τ _f (+2.7 ppm/°C) and desirable microwave properties (? _r  = 19.9, Q × f = 24,885 GHz) when synthesized at a sintering temperature of 880°C. This system could potentially be used for low-temperature co-fired ceramics technology applications.     

Segmented thermoelectric unicouple for an operating temperature range of 30–320°C

The possibility of increasing the efficiency of a thermocouple in the temperature range of 30–320°C is studied using an approach associated with the development of segmented thermoelectric unicouples. n- and p-type thermoelectric unicouples are constructed from low-temperature thermoelectric materials based on bismuth telluride and the addition of an intermediate-temperature material based on PbTe and GeTe, respectively. The thermoelectric unicouples are fabricated by spark plasma sintering (SPS). This method provides a contact resistance of ≤10 μΩ cm. The properties of segmented and conventional unicouples are compared. The efficiency of segmented unicouples in comparison with conventional ones increases by almost 70% and attains 5.3% in the operating range of 30–320°C.     

0.75 V supply nanowatt resistorless sub-bandgap curvature-compensated CMOS voltage reference

This work presents a resistorless self-biased and small area sub-bandgap voltage reference that works in the nano-ampere consumption range with 0.75 V of power supply. The circuit applies a curvature compensation technique that allows an extended temperature range without compromising the temperature stability. The behavior of the circuit is analytically described, and a design methodology is proposed which allows the separate adjustment of the bipolar junction transistor bias current and its curvature compensation. Simulation results are presented for a 180 nm CMOS process, where a reference voltage of 469 mV is designed, with a temperature coefficient of 5 ppm/°C for the ?40 to 125 °C extended temperature range. The power consumption of the whole circuit is 16.3 nW under a 0.75 V power supply at 27 °C. The estimated silicon area is 0.0053 mm².     

Non-ohmic effect and pulse aging behavior of V/Mn/Co/Bi/Dy co-doped ZnO semiconducting varistors with sintering processing

We examined the effect of sintering on the microstructure, non-ohmic properties, clamping characteristics, and pulse aging behavior of V/Mn/Co/Bi/Dy codoped ZnO semiconducting varistors. The average grain size increased from 4.7 to 10.4 µm and the densities of the sintered pellets decreased from 5.47 to 5.37 g/cm³ with the increase in sintering temperature. The maximum non-ohmic coefficient (35.3) was obtained at a sintering temperature of 900 °C. Varistors sintered at 900 °C exhibited the best clamp characteristics, a clamp voltage ratio of 1.74–2.54 at a pulse current of 1–25 A. Varistors sintered at 925 °C exhibited the strongest electrical stability; variation rates for the breakdown field measured at 1.0 mA/cm², for the non-ohmic coefficient, and for the leakage current density were 3.4%, 6.6%, and −11.2%, respectively, after application of a pulse current of 100 A.     

Phase Content Influence on Thermoelectric Properties of Manganese Silicide-Based Materials for Middle-High Temperatures

The higher manganese silicides (HMS), represented by MnSi _x (x = 1.71 to 1.75), are promising p-type leg candidates for thermoelectric energy harvesting systems in the middle-high temperature range. They are very attractive as they could replace lead-based compounds due to their nontoxicity, low-cost starting materials, and high thermal and chemical stability. Dense pellets were obtained through direct reaction between Mn and Si powders during the spark plasma sintering process. The tetragonal HMS and cubic MnSi phase amounts and the functional properties of the material such as the Seebeck coefficient and electrical and thermal conductivity were evaluated as a function of the SPS processing conditions. The morphology, composition, and crystal structure of the samples were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction analyses, respectively. Differential scanning calorimetry and thermogravimetric analysis were performed to evaluate the thermal stability of the final sintered material. A ZT value of 0.34 was obtained at 600°C for the sample sintered at 900°C and 90 MPa with 5 min holding time.     

Microwave Sintering of Bi2Te3- and PbTe-Based Alloys: Structure and Thermoelectric Properties

Microwave sintering is well known as an expeditious process in applications involving ceramics and biomaterials. For powders in the nanometer range, rapid microwave heating could reduce material exposure to elevated temperatures, thus preserving nanostructures in the resulting materials. To investigate the potential of this technique for thermoelectric (TE) materials, we have prepared samples of bismuth-telluride- and lead-telluride-based alloys from powders, for both materials, having sizes of partially agglomerated particles distributed from 0.15 μm to 7 μm. Sintering of the cold-pressed powders was carried out in a microwave furnace for 900 s at temperatures in the range of 583 K to 623 K for bismuth telluride and 793 K to 813 K for lead telluride specimens. For optimized sintering times and temperatures, the samples obtained showed relative densities of almost 95%. Scanning electron microscopy shows some residual porosity and a reduction of grain size, up to a factor of 5 for PbTe, compared with optimized hot-extruded specimens. For bismuth telluride samples, the TE performance in the range of 300 K to 460 K is poor, which is attributed to the arbitrary texture obtained from cold pressing of a highly anisotropic alloy prior to its sintering. In contrast, PbTe exhibits isotropic properties, hence deficiency of texturing is not expected to have a negative impact on its TE properties. Harman measurements show a value of ZT = 0.42 at 617 K for PbTe p-type sintered samples, which is comparable to hot-extruded alloys from similar powders. The present work demonstrates that microwave sintering is a promising alternative to other powder consolidation techniques for polycrystalline materials exhibiting isotropic TE properties.