2,673 research outputs found
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Study of Ceramic Slurries for Investment Casting with Ice Patterns
Ice patterns generated by rapid freeze prototyping or a molding process can be used to
make ceramic investment molds for metal castings. Due to the use of ice, the ceramic slurries
must be poured around the pattern and cured at sub-freezing temperatures. Success of this
process depends greatly on the mold strength after the gelation of the slurries. This paper
describes the experimental results of the mold strength after the gelation of the slurries under
different compositions. The parameters considered include mixing time, alumino-silicate vs.
fused silica ratio, volume of binder, and volume of catalyst. The strength of the gelled slurries is
examined by breaking test bars on a four-point bending apparatus. Weibull modulus for each
trial is calculated based on the breaking strength from four-point bend tests. Analysis of variance
for breaking strength and Weibull analysis is performed to evaluate the significance of the effect
of each parameter. The casting of a bolt is used to demonstrate that metal castings of complex
geometry can be fabricated using investment casting with ice patterns.Mechanical Engineerin
Modeling and Validation of Temperature and Concentration for Rapid Freeze Prototyping
Rapid Freeze Prototyping is a solid freeform fabrication process that uses water as the main build
material in a cold environment to create three-dimensional parts. A eutectic sugar-water solution
(C6H12O6 – H2O) has been used as a sacrificial material in order to create complex 3D parts with
features such as overhangs. A study of the interaction of the build and support materials is
presented in this paper. The temperature of both materials during deposition and subsequent
cooling is modeled using a semi-empirical model and a theoretical model. A concentration
model is used to predict the concentration in the fabricated parts around the interface of the two
materials with predicted temperatures as input. Experiments are conducted to validate both the
temperature and concentration models.Mechanical Engineerin
Transmitter switch for high-power microwave output
Combiner system can be used for combining output powers of two transmitters or for switching from one to the other. This can be done when pair of transmitters operate on same frequency and carriers are phase coherent as by excitation from single exciter
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Study on Incorporating Support Material in Rapid Freeze Prototyping
Rapid Freeze Prototyping (RFP) is a rapid prototyping method that uses water freezing
into ice to make three-dimensional parts. Each layer of a geometry is deposited and allowed to
freeze before the next layer is added. Using a support material in RFP is a relatively new
addition to the process. Validating the successful use of a support material in conjunction with
the main build material of water is presented in this paper. The support material selected for use
is a eutectic sugar solution. The selection criteria, properties, and characteristics of the support
material are discussed. Of particular interest is the diffusion between the support and main build
material, which must be minimized to an acceptable level for producing good quality,
reproducible, complex parts.Mechanical Engineerin
Shell Cracking In Investment Casting with Laser Stereolithography Patterns
This paper presents an investigation of ceramic shell cracking during the burnout process
in investment casting with internally webbed laser stereolithography patterns. We hypothesize
that shell cracking will occur when the rupture temperature of the ceramic shell is lower than
both the glass transition temperature of the pattern material and the web· link buckling
temperature. The hypothesis is validated by our experimental observations which confirm the
numerical predictions from our fillite element analysis. This provides a basis for design of the
internal web geometry of a lithography pa.tternand evaluation ofthe burnolltprocesswithsuch a
pattern. We show the shell cracking and web link buckling temperatures to be functiollsofthe
pattern geometry (including the cross~sectional dimensions and span length of the web link) and
the shell thickness.Mechanical Engineerin
Interruption of Medium-Voltage Direct-Currents by Seperation of Contact Elements in Mineral Oil Using an Ultra Fast Electro-Magnetic Actuator
The increasing usage of medium-voltage direct-current in upcoming electrical energy grid topologies requests novel solutions for MVDC switching. The interruption of direct-currents is accomplished by enforcing a current zero crossing by adequate means and preventing reignition due to the recovering dc voltage. This paper evaluates the rapid separation of the contact elements in mineral oil leading to a liquid flow around the contact elements and the switching arc. The energy turnover of dielectric liquids interacting with an electric arc is considerably higher leading to heavily increased arc voltages compared to dielectric gases. This paper confirms results of earlier publications and carries them further towards a possible usage in an MVDC switching or protection device. Thus a contact arrangement surrounded by mineral oil in combination with an ultra fast electro-magnetic actuator is introduced and performed measurements are discussed
Templated self-assembly of sub-10 nm quantum dots
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references.Patterned templates can guide the self-assembly of nanoparticles into ordered arrays. Our motivation in pursuing templated self-assembly is to develop a robust method for the creation of ordered structures at length scales below ten nanometers. The basic process entails creating surface relief templates via electron-beam lithography, and spin-coating a suspension of colloidal nanoparticles onto the template. As the solvent evaporates, the quantum dots self-assemble primarily through the capillary forces created by the dewetting of the template. We demonstrate this technique at sub-10nm length scales by spin-coating a solution of organically-capped CdZnS semiconducting quantum dots onto nanopatterned grating structures on silicon substrates. We observe the geometric confinement of the quantum dots via physical templating and capillary forces into well-ordered monolayer aggregates with defined lattice orientations. While recent research has demonstrated the ability to self-assemble sub-10nm metallic nanoparticles via capillary forces into physical templates of similar size, this work is unique in the demonstration of lattice orientation control via physical templating at sub-10nm length scales.by Joshua C. Leu.S.M
Freeform Extrusion of High Solids Loading Ceramic Slurries, Part I: Extrusion Process Modeling
A novel solid freeform fabrication method has been developed for the manufacture of
ceramic-based components in an environmentally friendly fashion. The method is based on the
extrusion of ceramic slurries using water as the binding media. Aluminum oxide (Al2O3) is
currently being used as the part material and solids loading as high as 60 vol. % has been
achieved. This paper describes a manufacturing machine that has been developed for the
extrusion of high solids loading ceramic slurries. A critical component of the machine is the
deposition system, which consists of a syringe, a plunger, a ram actuated by a motor that forces
the plunger down to extrude material, and a load cell to measure the extrusion force. An
empirical, dynamic model of the ceramic extrusion process, where the input is the commanded
ram velocity and the output is the extrusion force, is developed. Several experiments are
conducted and empirical modeling techniques are utilized to construct the dynamic model. The
results demonstrate that the ceramic extrusion process has a very slow dynamic response, as
compared to other non-compressible fluids such as water. A substantial amount of variation
exists in the ceramic extrusion process, most notably in the transient dynamics, and a constant
ram velocity may either produce a relatively constant steady-state extrusion force or it may cause
the extrusion force to steadily increase until the ram motor skips. The ceramic extrusion process
is also subjected to significant disturbances such as air bubble release, which causes a dramatic
decrease in the extrusion force, and nozzle clogging, which causes the extrusion force to slowly
increase until the clog is released or the ram motor skips.Mechanical Engineerin
Freeform Extrusion of High Solids Loading Ceramic Slurries, Part II: Extrusion Process Control
Part I of this paper provided a detailed description of a novel fabrication machine for high solids
loading ceramic slurry extrusion and presented an empirical model of the ceramic extrusion
process, with ram velocity as the input and extrusion force as the output. A constant force is
desirable in freeform extrusion processes as it correlates with a constant material deposition rate
and, thus, good part quality. The experimental results in Part I demonstrated that a constant ram
velocity will produce a transient extrusion force. In some instances the extrusion force increased
until ram motor skipping occurred. Further, process disturbances, such as air bubble release and
nozzle clogging that cause sudden changes in extrusion force, were often present. In this paper a
feedback controller for the ceramic extrusion process is designed and experimentally
implemented. The controller intelligently adjusts the ram motor velocity to maintain a constant
extrusion force. Since there is tremendous variability in the extrusion process characteristics, an
on-off controller is utilized in this paper. Comparisons are made between parts fabricated with
and without the feedback control. It is demonstrated that the use of the feedback control reduces
the effect of process disturbances (i.e., air bubble release and nozzle clogging) and dramatically
improves part quality.Mechanical Engineerin
Aqueous-Based Extrusion Fabrication of Ceramics on Demand
Aqueous-Based Extrusion Fabrication is an additive manufacturing technique that
extrudes ceramic slurries of high solids loading layer by layer for part fabrication. The
material reservoir in a previously developed system has been modified to allow for
starting and stopping of the extrusion process on demand. Design pros and cons are
examined and a comparison between two material reservoir designs is made. Tests are
conducted to determine the optimal deposition parameters for starting and stopping the
extrudate on demand. The collected test data is used for the development of a deposition
strategy that improves material deposition consistency, including reduced material
buildup at sharp corners. Example parts are fabricated using the deposition strategy and
hardware design.Mechanical Engineerin
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