Jorge J. Santiago-Avilés

Overview of low temperature co-fired ceramics tape technology for meso-system technology (MsST)

For certain applications low temperature co-fired ceramic (LTCC) tape materials used in multi-layer packages offers the potential of emulating a great deal of silicon sensor/actuator technology at the meso scale level. The goal of this review is to describe meso-system technology (MsST) using LTCC, thick film and silicon technologies. A mayor MST application being addressed today is fluid handling for miniaturized chemical analytical systems. For larger MST-3D applications, in the meso-size (from 10 to several hundred microns), it would be desirable to have a material compatible with hybrid microelectronics, with suitable thermal, mechanical and electrical properties, easy to fabricate and inexpensive to process. Such a material is the LTCC tape multilayer system. One of the important features of LTCC technology is the possibility of fabricating 3D structures using multiple layers. In this review, we want to emphasize sensors and actuators for meso-systems exploring LTCC Tape possibilities in the following ways: Sensors for proximity measurement; Fluid media realization of vias, holes, cavities, channels and manifolds; Sensors for flow measurement; Actuators for hybrid microvalves & micropumps. # 2001 Elsevier Science B.V. All rights reserved.

The utilization of low temperature co-fired ceramics (LTCC-ML) technology for meso-scale EMS, a simple thermistor based flow sensor

For certain applications the small size characteristics of MEMS structures can be traded off for low cost production and high manufacturing yields. This is the realm of meso-scale (intermediate scale) electromechanical systems. Low temperature co-fired ceramic tape materials offers the potential of emulating a great deal of silicon MEMS technology at the meso scale level. In this article we show the design, construction, characterization and modeling of a thermistor based flow sensor totally constructed using LTCC tapes and the hybrid microelectronics technology.

Conductivity measurement of electrospun PAN-based carbon nanofiber

Although electrostatic generation, or electrospinning, of ultrafine fibers was invented as early as in 1930s [1], the technique has been used to produce conductive polymer fibers only recently [2]. Reneker and Chun et al. [3] electrospun polyaniline fibers from sulfuric acid into a coagulation bath. Chun et al. [4] electrospun polyacrylonitrile (PAN) nanofibers, and pyrolyzed them into carbon nanofibers. Because of their obvious high specific surface area, electrospun ultrafine fibers are expected to be used as high performance filters, or scaffolds in tissue engineering [5]. For the same reason, one may expect that chemisorbed gases may modulate the electrical conductivity, leading to the fabrication of sensor devices. Unfortunately, the electrical transport properties of these electrodeposited materials have caught the interest of few researchers except for Norris et al. [5], who used the indirect four-point probe method to measure the conductivity of the electrospun non-woven ultra-fiber mat of polyaniline doped with camphorsulfonic acid blended with polyethylene oxide (PEO). As the non-woven mat is highly porous and the “fill factor” of the fibers is less than that of a cast film, the measured conductivity tends to be lower than that of bulk [5]. In this paper, a direct method is used to measure the I -V curve and conductivity of the electrospun PAN-based carbon nanofibers. Commercial PAN powder and N,N-Dimethyl Formamide (DMF), in a ratio of 600 mg PAN to 10−5 m3 DMF, were used to prepare a solution. This mixture was vigorously stirred by an electromagnetically driven magnet at room temperature before it became a homogeneous polymer solution. The electrospinning was conducted in a homemade setup shown in Fig. 1. The DC power supply was an ES30-0.1P Model HV

Pyrolysis temperature and time dependence of electrical conductivity evolution for electrostatically generated carbon nanofibers

Carbon nanofibers were produced from polyacrylonitrile/N, N-Dimethyl Formamide (PAN/DMF) precursor solution using electrospinning and vacuum pyrolysis at temperatures from 773-1273 K for 0.5, 2, and 5 h, respectively. Their conductance was determined from I-V curves. The length and cross-section area of the nanofibers were evaluated using optical microscope and scanning probe microscopes, respectively, and were used for their electrical conductivity calculation. It was found that the conductivity increases sharply with the pyrolysis temperature, and increases considerably with pyrolysis time at the lower pyrolysis temperatures of 873, 973, and 1073 K, but varies, less obviously, with pyrolysis time at the higher pyrolysis temperatures of 1173 and 1273 K. This dependence was attributed to the thermally activated transformation of disordered to graphitic carbon.

Surface compositional and topographical changes resulting from excimer laser impacting on YBa2Cu3O7 single phase superconductors

Superconducting pressed pellets of YBa2Cu3O7 (Tc=90 K), which were used as ablation targets for laser‐induced vapor deposition of high Tc(85 K) superconducting thin films, have been analyzed by secondary electron microscopy, scanning Auger microscopy, energy dispersive x‐ray analysis, and x‐ray diffractometry. The elemental distribution of Y, Ba, and Cu appears reasonably uniform at depths corresponding to that probed by energy dispersive x‐ray analysis (∼1 μm). However, scanning Auger microscopy analysis of the laser‐impacted area shows a significant depletion of Cu and spatial redistribution of Y, Ba, Cu, and O on the target surface. X‐ray diffractometry of the laser‐impacted area shows the appearance of a new broad peak at a diffraction angle 2θ=29.7°, characteristic of BaY2O4 and a poorly defined peak at 2θ=29.3°, that can be attributed to BaCuO2. A possible influence of the laser‐induced bulk superconductor compositional changes on the film composition is discussed in relation to recently reported ex...

Growth of pinhole‐free epitaxial yttrium silicide on Si(111)

This paper reports the growth of pinhole‐free epitaxial YSi2−x layers on Si(111) as thin as 30 A. This has been accomplished by depositing both Y and Si at room temperature and then annealing to 500–900 °C. Use of the template method allows for the growth of thicker films also free of pinholes. Deposition of yttrium metal only onto Si(111) requires a temperature ∼300 °C for nucleation of the silicide reaction between the Y overlayer and Si substrate. Such a process creates small pinholes ∼500 A in diameter, randomly distributed throughout the film. These pinholes increase in size with higher annealing temperature, resulting from a raised interface free energy intrinsic to the nucleation controlled growth.

Large negative magnetoresistance and two-dimensional weak localization in carbon nanofiber fabricated using electrospinning

The carbon nanofibers used in this work were derived from polyacrylonitrile/N,N-dimethyl formamide precursor solution using electrospinning and vacuum pyrolysis techniques. Their conductivity σ was measured at temperatures between 1.9 and 300 K and transverse magnetic field between −9 and 9 T. Zero magnetic field conductivity σ(0, T) was found to increase monotonically with the temperature with a convex σ(0, T) vs T curve. The conductivity increases with the external transverse magnetic field, revealing negative magnetoresistance at temperatures between 1.9 and 10 K, with a maximum magnetoresistance of −75% at 1.9 K and 9 T. The magnetic field dependence of the conductivity can be explained using the two-dimensional (2D) weak localization effect. The temperature dependence of σ(0, T) can be explained using a modified simple two-band model with temperature-dependent carrier mobility, and a correction introduced by 2D weak localization.

Synthesis and characterization of tin oxide microfibres electrospun from a simple precursor solution

Tin oxide (SnO2) microfibres in the rutile structure were synthesized using electrospinning and metallorganic decomposition techniques. Fibres were electrospun from a precursor solution containing 20 mg poly(ethylene oxide) (molecular weight 900 000), 2 ml chloroform and 1 ml dimethyldineodecanoate tin, and sintered in the air for 2 h at 400, 600 and 800 °C, respectively. Scanning electron microscopy, x-ray diffraction and Raman microspectrometry were used to characterize the sintered fibres. The results showed that the synthesized fibres are composed of SnO2.

Meso (Intermediate)-Scale Electromechanical Systems for the Measurement and Control of Sagging in LtcC Structures

Sagging of suspended or laminated structures is a common problem in the processing of Low Temperature Co-Fired Ceramics (LTCC). These glass-ceramic composites are susceptible to plastic deformation upon lamination, or under the stress of body forces once the glass transition temperature of the glass binder is reached during processing. We have designed and fabricated, using the conventional methods of LTCC fabrication, meso-scale structures (ranging in size from 100 mm to 1 cm) to quantify and seek control strategies for this problem. We have implemented bridge structures and membranes to emulate most of the conventional structures encountered during packaging or sensor (actuator) device fabrication. We have observed that when an LTCC tape with holes larger than 400 µm in diameter is laminated, the tapes above and below deform into the cavity. For smaller diameters, deformation is negligible. Bridging structures can be compensated for the potential effect of body forces by screen-printing a thick film over-layer which exert internal tensile stresses on sintering. This can often yield straight bridges. The use of fugitive phase materials, which disappear or flow during firing, is another way of supporting bridging structures. Several of these strategies have been explored, and results are presented.

Optical bandgap and photoconductance of electrospun tin oxide nanofibers

Optical and photoconductive properties of transparent SnO2 nanofibers, made from C22H44O4Sn via electrospinning and metallorganic decomposition, were investigated using Fourier transform infrared and ultraviolet (UV)/visible spectrometry and the two-probe method. Their optical bandgap was determined from their UV absorption edge to be 3.95–4.08eV. Their conductance responds strongly to UV light for a wavelength of 254nm: in air its steady-state on-to-off ratios are 1.31–1.56 (rise) and 1.25–1.33 (fall); its 90% rise and fall times are 76–96 and 71–111s, respectively. In a vacuum of about 10−4torr, its on-to-off ratios are higher than 35.6 (rise) and 3.4 (fall), respectively, and its 90% rise and fall times are longer than 3×104s.