Low Melting Temperature Metal Research Articles

Vacuum package using anodic bonding assisted by the reflow of low-melting temperature metal

In this paper, we present a highly effective vacuum seal method, which is based on anodic bonding assisted by the reflow of lowmelting temperature metal. To form sacrificial gap for evacuating the packaged cavity before hermetically sealing, the reflow process of Au/Sn/Cr posts due to low-melting temperature Sn metal was introduced. Using this technique to form the evacuation gap, the micro-precise alignment between packaging wafers was obtained. The package method enabled to eliminate contaminant caused by package process. The diaphragm structure sensing pressure was used to investigate the proposed package method. The vacuum level in the packaged cavity was obtained lower than 5 Pa. In comparison to the conventional package method without using the reflow process to form the evacuation gap, the vacuum level has been improved by a factor of 8. The yield and uniformity of the package method were also confirmed to be higher than the conventional package method without using the reflow process to form the evacuation gap. This vacuum package method can be implemented to remove air damping for optimal performance of mechanical oscillators such as accelerometers, gyroscopes, energy harvesters, and micro-mirrors.

Liquid Metal as a Heat Transport Fluid for Thermal Solar Power Applications

In order to increase the thermal efficiency and produce process heat for hydrogen production, the operating temperature of the heat transfer fluid in thermal solar plants needs to increase. In addition reaching 900°C would also increase the heat storage density and the efficiency of the thermodynamic cycle by using a combined cycle for electricity production. The benefits of hydrogen (e.g., for fuel cells) and a more efficient thermodynamic cycle would allow a plant to have a higher energy output per square acre of land use, thereby increasing its economic competiveness.Today, solar thermal plants do not operate at these high temperatures due to the fact that conventional heat transport fluids begin to disintegrate around 600°C [1,2]. For non-solar applications, low melting-temperature metals, such as wood's metal and lead- bismuth eutectic alloy, have been examined as heat-transport media, because of the large temperature ranges over which they remain liquid. Lead-bismuth eutectic alloy (LBE; 45% Pb, 55% Bi) melts at 125°C and does not boil until 1670°C, making it an ideal heat-transfer medium for application in thermal solar power [3]. The main obstacle to using LBE is finding structural materials that can withstand the harsh corrosion environments at high temperatures. In this work the key issues of materials exposed to liquid metal are described while initial data on carious steels tested in liquid metal are provided. While corrosion is a significant issue in this environment, mechanical failure of steels in liquid metal are discussed as well.

Development of a Fused Deposition Modeling System for Low Melting Temperature Metal Alloys

This research focused on extending the applications of fused deposition modeling (FDM) by extrusion and deposition of low melting temperature metal alloys to create three-dimensional metal structures and single-layer contacts which may prove useful for electronic interconnects. Six commercially available low melting temperature solder alloys (Bi36Pb32Sn31Ag1, Bi58Sn42, Sn63Pb37, Sn50Pb50, Sn60Bi40, Sn96.5Ag3.5) were tested for the creation of a fused deposition modeling for metals (FDMm) system with special attention given to Sn–Bi solders. An existing FDM 3000 was used and two alloys were successfully extruded through the system's extrusion head. Deposition was achieved through specific modifications to system toolpath commands and a comparison of solders with eutectic and non-eutectic compositions is discussed. The modifications demonstrate the ability to extrude simple single-layer solder lines with varying thicknesses, including sharp 90 deg angles and smooth curved lines and showing the possibility of using this system for printed circuit board applications in which various connections need to be processed. Deposition parameters altered for extrusion and the deposition results of low melting temperature metal alloys are introduced.

The Influence of Austenitization Temperature on the Mechanical Properties of a Prehardened Mould Steel

The many practical difficulties and longer lead time with heat treatment of mould steels after machining have led to increased demand for steels in prehardened condition, typically ~ 40HRC. At this hardness the steel possesses an optimal combination of high strength and machinability. The steels used for moulds require a wide range of demanding properties, among which high enough strength and toughness are the primary necessities in order to resist any deformation and dimensional change in mould during use. Uddeholm Impax HH which resembles the modified AISI P20 has been widely used for moulding of plastics and die-casting of low melting temperature metals. The common hardening process for Impax HH is conventional quenching and tempering. Hence, investigating the effect of hardening parameters on the required properties upon this steel grade is beneficial in improving it for better performance as a prehardened mould steel. In the present work, the effect of changes in austenitization temperature and consequently the prior austenite grain and martensite packet sizes on the tensile properties and impact toughness of Uddeholm Impax HH at the hardness of ~40HRC is studied. The results have shown reduction in impact toughness but no considerable change in yield and ultimate tensile strength upon increasing the austenitization temperature.

Thermo-compressive transfer printing for facile alignment and robust device integration of nanowires

Controlled alignment and mechanically robust bonding between nanowires (NWs) and electrodes are essential requirements for reliable operation of functional NW-based electronic devices. In this work, we developed a novel process for the alignment and bonding between NWs and metal electrodes by using thermo-compressive transfer printing. In this process, bottom-up synthesized NWs were aligned in parallel by shear loading onto the intermediate substrate and then finally transferred onto the target substrate with low melting temperature metal electrodes. In particular, multi-layer (e.g. Cr/Au/In/Au and Cr/Cu/In/Au) metal electrodes are softened at low temperatures (below 100 °C) and facilitate submergence of aligned NWs into the surface of electrodes at a moderate pressure (∼5 bar). By using this thermo-compressive transfer printing process, robust electrical and mechanical contact between NWs and metal electrodes can be realized. This method is believed to be very useful for the large-area fabrication of NW-based electrical devices with improved mechanical robustness, electrical contact resistance, and reliability.

Growth of Ultra Long Multiwall Carbon Nanotube Arrays by Aerosol-Assisted Chemical Vapor Deposition

Using a home-made aerosol nebulizer, we developed a new aerosol-assisted chemical vapor deposition (AACVD) process that made it possible to synthesize vertically-aligned carbon nanotube (VACNT) arrays with heights over a few millimeters routinely. An essential part of this technique was in-situ formation of metal catalyst nanoparticles via pyrolysis of ferrocene-ethanol aerosol right before CNT synthesis. Through the optimization of aerosol supply and CVD process parameters, we were able to synthesize clean VACNT arrays as long as 4.38 mm with very low metal contents in 20 min. Furthermore, it is worthy noting that such an outstanding height is achieved very quickly without supporting materials and water-assistance. By taking advantage of almost complete inhibition of CNT growth on low melting-temperature metals, we were able to fabricate patterned VACNT arrays by combining AACVD process with a conventional photolithograpic patterning of gold lines. Characterizations of as-grown nanotubes such as morphology, purity, and metal contents are presented.

Work function engineering in low-temperature metals

Semiconductor devices require conducting electrodes with disparate work functions for their operation. Of recent interest are fluidic processing approaches for large-area devices, which present unique challenges in the identification of materials having disparate work functions but similar melting temperatures. Such materials may be engineered by alloying with low-melting temperature metals. As a demonstration, the work function of tin and four binary tin alloys is measured by ultraviolet photoemission spectroscopy and Kelvin probe method. We demonstrate the control of metal work function by 600 meV through alloying while keeping the melting temperature within a 140 °C range.

Development of silver–metal oxide reactive air braze alloys for electroding PZT ceramics

Existing multi-layer devices using lead-based piezoelectric ceramics utilize an internal electrode that does not bond the ceramic layers. Improvements in device performance and processing could be gained if the electrode also acted as a bond between the ceramic layers. In the current work, the feasibility of brazing lead zirconate titanate (PZT) in ambient conditions utilizing silver-based alloys containing low melting temperature metal oxides was investigated. Wettability, joint fracture strength, and microstructural analyses were conducted for various PZT/silver–metal oxide systems. The metal oxide additions included copper (II) oxide, vanadium pentoxide, lead (II) oxide, and eutectic lead oxide-titanium (IV) oxide. The silver–copper oxide (Ag–CuO) system demonstrated the most potential; exhibiting an apparent contact angle of approximately 64° and an average braze joint fracture strength that was approximately 62% of the monolithic PZT strength. In addition, no significant reaction product formation was observed at the silver/PZT interface. However, a preliminary investigation of multi-layer devices electroded with Ag–CuO alloys indicated a decrease in the resistivity of the brazed PZT by several orders of magnitude.

Low sintering temperature of MgTiO 3 for type I capacitors

Magnesium titanate MgTiO 3 is a well-known compound for type I multilayer ceramic capacitors. Nevertheless, the sintering temperature of the pure ilmenite MgTiO 3 is around 1350 °C. Such a high sintering temperature together with the high sensitivity of the dielectric material to reduction when heated in a low-oxygen containing atmosphere implies that MgTiO 3-based MLCC include palladium-rich inner electrodes. The high level of variation of both the costs of Pd and Ag justifies research leading to the use of cheaper metals such as silver or base metals such as nickel or copper. When using low-melting temperature metals (silver melts at 960 °C and copper at 1085 °C), the sintering temperature of the dielectric material has to be lowered. We report here on our investigations into the use of fluorine containing additives for the reducing of the sintering temperature of magnesium titanate, showing the ability of this material to be sintered at temperatures much lower than 1000 °C. Both dielectric and electric properties of such ceramics are compatible with type I capacitors requirements.

High-temperature instrumented microscale compression molding of Pb

Compression molding of Pb plates with LiGA (Lithographie, Galvanoformung, Abformung) fabricated Ni microscale mold inserts was carried out in the temperature range of 100–300 °C. In-situ measurements of compressive molding force, demolding friction force and insert displacement were carried out with a custom-built, high-vacuum, high-temperature, instrumented molding machine. Microscale features generated on Pb plates and insert condition after repeated molding runs were examined. The in-situ force monitoring capability enabled compressive stresses needed for molding and frictional stresses generated by demolding to be measured as a function of temperature. Our results demonstrate that, in cases where no significant metal/insert chemical interactions exist, present LiGA fabricated Ni inserts possess adequate mechanical properties for repeated micromolding of low melting temperature metals.

Liquid metal embrittlement of the martensitic steel 91: influence of the chemical composition of the liquid metal.: Experiments and electronic structure calculations

In previous works [Scripta Mater. 43 (2000) 997; J. Nucl. Mater. 296 (2001) 256], we showed that the martensitic steel 91 is prone to liquid metal embrittlement (LME) by liquid lead provided that some metallurgical conditions are fulfilled. In this work, we report results of LME of the steel 91 in contact with Pb–Bi and other low melting temperature metals such as Sn and Hg. Our experimental results can be interpreted within the framework of the surface energy reduction models for LME. To account for the experimental observations, we performed electronic structure calculations to assess the chemical interaction between low melting temperature metal atoms and iron surfaces. Our results allow to establish a simple criterion that can give trends on the embrittlement power of a liquid metal in contact with iron.

Room Temperature Deposition of High Dielectric Constant, High Density Ceramic Thin Films

ABSTRACTFuture high-performance integrated circuits and electronic packaging technology require the integration of many passive components, including high storage capacitors, on the systems. Because of the low melting temperature metal lines and polymer dielectrics in these electronic systems, one cannot employ most of the existing high temperature deposition techniques to grow thin film components. In this paper, we will report our recent work on the room temperature deposition of amorphous ceramic thin films, including BaxTi 2−xOy and SiOx, using the newly developed partially ionized beam technique. This technique utilizes a small percent (<3%) of selfions derived from the depositing materials to bombard the surface growth front during deposition. It is shown that a dramatic control of the density (and therefore the leakage current) and uniformity of the film can be achieved using this deposition technique without post annealing. Al or Cu was used as the electrodes in our multilayer MIM (metal-insulator-metal) test structure. As deposited thin film capacitors with a capacitance ranging from 25 to 100 nF/cm 2 have been fabricated with tan δ <0.01, leakage current of <l.μA/cm2 at 0.5 MV/cm, and breakdown field strength of several MV/cm. These ceramic capacitors do not show any dispersion up to 1 GHz.

Preparation of monosized metal powders by pulsated orifice injection: Y. Kuroki et al (Tohoku University, Sendai, Japan)

Drop-on-demand jetting of metals offers a fully digital manufacturing approach to surpass the limitations of the current generation powder-based additive manufacturing technologies. However, research on this topic has been restricted mainly to near-net shaping of relatively low melting temperature metals. Here it is proposed a novel approach to jet molten metals at high-temperatures (>1000 °C) to enable the direct digital additive fabrication of micro- to macro-scale objects. The technique used in our research – “MetalJet” - is discussed by studying the ejection and the deposition of two example metals, tin and silver. The applicability of this new technology to additive manufacturing is evaluated through the study of the interface formed between the droplets and the substrate, the inter-droplets bonding, the microstructure and the geometrical fidelity of the printed objects. The research shows that the integrity of the samples (in terms of density as well as metallurgy) varies dramatically in the two investigated materials due to the different conditions that are required to melt the interface of the stacked droplets. Nevertheless the research shows that by a careful choice of the jetting strategy and sintering treatments 3D structures of various complexity can be formed. This research paves the way towards the next generation metal additive manufacturing where various printing resolutions and multi-material capabilities could be used to obtain functional components for applications in printed electronics, medicine and the automotive sectors.

“dual phase” thin films prepared by deposition of aluminum on liquid nucleant gallium layers

Aluminum thin films have been universally employed as interconnections in integrated circuits for the last quarter of a century. However, during this time Al metallization has never been totally immune from assorted reliability problems. This present research involves preparation and characterization of “dual phase” thin films, comprised of Al (or a transition metal) vacuum evaporated onto a liquid Ga (or other low melting temperature metal) nucleant layer. Ga is well known for causing grain boundary embrittlement in structural Al alloys. However, the Ga may well enhance the mechanical properties in Al thin films used as interconnect metallization, where this “dual phase” microstructure will prevent the buildup of stress that has historically resulted in electromigration failures in semiconductor devices. In addition, the presence of a liquid nucleant layer results in thin films (up to 2 μm thick) having surface roughness of the order of the film thickness, which in turn would enhance the bondability of such films.

Control of teeming rate of steel by rotary type electromagnetic stirrer.

In continuous casing of steel, control of the teeming rate of steel from the tundish to the mold is carried out by changing the cross-sectional area of the flow channel by means of a stopper or slide gate. However, nozzle clogging or air suction occasionally occur in the controlled section, and this interferes with the degree of control of the teeming rate of steel or deteriorates the cleanness of the steel.To improve this conventional method, some methods using a linear motor-type electromagnetic stirrer were proposed or tested, though, these methods are not yet practicable because of the difficulty of using a linear motor-type stirrer for control of the teeming rate of steel. In contrast, a rotary type stirrer is considered to be more suitable to the control of the teeming rate molten steel, because a higher intensity of stirring can be obtained through shorter affected zone by the rotary-type stirrer than by a linear motor-type stirrer.On the basis of this idea, an electromagnetic valve using rotary-type stirrer was devised, and a series of experiments using low melting-temperature metal and molten steel were carried out to investigate the characteristics of the control and to improve the design of the electromagnetic valve. By increasing the rotational speed, the flow rate was reduced to about half of that of non-stirring when the depth of the molten steel in the tundish is 500 mm. The response of control was quick enough to apply it to steel or another high melting-temperature metal teeming as a non-contact control method.