Proceedings of IGTI:
ASME Turbo Expo
June 2002, Amsterdam
GT-2002-30548
FEASIBILITY OF A HIGH-TEMPERATURE POLYMER-DERIVED-CERAMIC TURBINE
FABRICATED THROUGH MICRO-STEREOLITHOGRAPHY
C. Walsh, L. An, J. S. Kapat, L. C. Chow
Mechanical, Materials, and Aerospace Engineering Department, ENGR 307
University of Central Florida, Orlando, FL, 32816-2450
ABSTRACT
Efficient micro turbines are expected to play a major role in
power generation in the coming years. One of the biggest
challenges in significantly increasing the system efficiency from
the currently achievable values is the availability of high
temperature materials that can be micro-fabricated with a low
value of relative tolerance. This paper suggests a possible
solution for both the material and the fabrication technique by
which this goal can be achieved.
Thermodynamic analysis shows that high turbine inlet
temperatures, high isentropic efficiencies, and a recuperator
with high effectiveness and low pressure losses are imperative
to improve system efficiency, particularly in a micro turbine. An
excellent relative tolerance with a high temperature ceramics is
one way to achieve these improvements.
The paper introduces new polymer-derived-ceramics
(PDC), which could be used in turbine and recuperator designs.
These materials can be used in micro-fabrication techniques to
produce absolute tolerances of a few microns, and some of them
are thermally stable up to 1800°C in air.
The paper presents an enhancement to the typical microstereolithography technique, by which PDC can be microfabricated to form parts up to a few centimeters in overall linear
dimensions and yet to have a relative tolerance that is
comparable to or better than that in large-sized conventional
parts. It is projected that for a turbine with 10 cm rotor outside
diameter and 5 mm blade height, a tip-gap-to-blade-height ratio
of better than 0.28% can be achieved by this proposed
enhancement. This technique appears to be quite promising for
next generation micro turbines, and hence requires further
investigation and development.
NOMENCLATURE
DMD digital mirror device
LCD liquid crystal display
PDC polymer derived ceramics
rpl
dimensionless recuperator pressure for each fluid
INTRODUCTION
One of the biggest challenges for the development of micro
turbines with high efficiency is the availability of high
temperature materials that can be used in a precise fabrication
process. Obtaining higher efficiency in a micro turbine can be
dependent on two major factors:
higher turbine inlet
temperature and a recuperator with high effectiveness and lowpressure loss. Typical metal-based manufacturing methods can
produce components with a low tolerance-to-size ratio.
However these components are limited to lower temperature
operations. Ceramic components can be used, but their usage is
limited due to the lack of precise fabrication processes that can
produce a low value for relative tolerance for relatively small
linear dimensions. A highly effective recuperator with a low
pressure loss and a better performing turbine require a ceramic
material with a small value for relative tolerance, that is the
ratio of absolute tolerance and critical linear dimension(s).
With the advent of MEMS technology, different possible
micro-fabricable materials and the corresponding microfabrication techniques have received increased attention in the
recent years from the research community. One such group of
materials can be classified as polymer-derived ceramics (PDC).
As have been shown by the MEMS community, PDC
components can be micro-fabricated by micro-casting or by
lithography technique [1]. Both methods can provide ceramic
parts with absolute tolerance of single-digit microns. Moreover,
when PDC components are fabricated by one of these methods,
a fully dense, amorphous alloy of silicon, carbon and nitrogen
(SiCN) is obtained. The PDC may also contain boron and is
designated as SiBCN. This ceramic material is thermally stable
up to 1500°C for SiCN and 1800°C for SiBCN in air.
Overall linear dimensions of PDC components microfabricated at different laboratories by one of the above two
methods are quite small, typically less than 1 cm. However, a
typical micro turbine requires components with outside linear
dimensions of at least a few cm to greater than 10 cm.
Moreover, for efficient operation turbines must have 3-
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Copyright © 2002 by ASME
dimensional blade profiles, which cannot be fabricated by the
micro-casting method. Also, overall fabrication process must be
relatively inexpensive in order to find wide-spread uses.
This paper reports current efforts by the authors in
extending the current micro-stereolithography technique [1].
The proposed technique can provide PDC components with
linear dimensions of about 10 cm and with complex internal and
external geometry. Fabrication speed is also enhanced over the
current alternatives for micro-stereolithography, and the
eventual fabrication cost is expected to be quite competitive to
the conventional alternatives with inferior relative tolerance
and/or poor material properties for the same overall size. With
further development, this method has the potential in the future
to create highly efficient turbines and recuperators for micro
turbines.
This paper starts with a model micro turbine. Results are
then presented from the existing literature in order to relate a
low value for relative tolerance with the recuperator and turbine
performance, and thus with system efficiency. The rest of the
paper concentrates on the properties of PDC and the existing
methods of micro-fabrication. The paper finally focuses on
description of the proposed extension of the microstereolithography method by which a high-temperature ceramic
material can be fabricated with a low value of relative tolerance
as compared to the currently available alternatives.
Recuperator
4y
244
1
0.5038
4
167
3.81
0.5
2
25
1.00
0.5
1
Air
2x
609
3.58
0.5
Combustor
3
653
1.31
0.5038
900
3.37
0.5038
T[oC]
P[atm]
m[kg/s]
 h  28.4%
Figure 1: A Typical Micro Turbine [2]
Table 1: Micro Turbine Model
Compressor
Combustor
Turbine
Recuperator
A TYPICAL MICRO TURBINE
In order to have a practical context for the discussion in
this paper, a typical micro turbine needs to be considered. Some
of the micro turbines of current interest are the ones developed
by Capstone Turbines, General Electric-Elliott Energy Systems,
and Allied Signal Aerospace Co. These turbines are in the 40 –
80 kW class, use a radial compressor and turbine with a
recuperative cycle arrangement and air mass flow rate of 0.4-1
kg/s, and achieve an average net efficiency of 26 – 30% [2].
The state parameters of such a micro turbine are shown in
Figure 1. It should be noted that a large dilution has been used
here in order to achieve a relatively low flame temperature. The
important cycle parameters are presented in Table 1. This table
defines the baseline model to be used in the results presented in
the next section.
INCREASING THE EFFICIENCY OF A MICRO TURBINE
Many authors (e.g. [3], [4]) have reported parametric
studies of the effect of different turbine and recuperator
parameters on the overall system efficiency of a micro turbine.
Only relevant results from these works are presented here.
These results help to establish requirements on the potential
materials and the manufacturing techniques for the turbine and
the recuperator. First we explore the effect of turbine inlet
temperature on the overall cycle efficiency. The result is
presented in Figure 2.
Natural gas
Exhaust
Cycle
Pressure ratio
Inlet condition
Efficiency
Pressure loss
Outlet temperature
Fuel
Turbine efficiency
Effectiveness
Pressure loss
Cycle efficiency
3.8
25 °C, 1 atm
70%
6%
900 °C
Natural gas
82%
90%
3%
28.4%
Figure 2. Effect of Turbine Inlet Temperature on cycle
efficiency
As expected, this plot clearly shows the need for the turbine
to operate at high inlet temperatures in order for the overall
system to be efficient. Although this is universally true for all
the turbines, this fact is particularly significant for micro
turbines. Unlike large gas turbines for power generation, the
turbine blades and vanes of a micro turbine cannot have internal
cooling channels because of their small size. As a result, such
micro turbines must be operated at a temperature that is
comfortably lower than the material limit. In the absence of
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Copyright © 2002 by ASME
0.40
3%Pressure Loss
6%Pressure Loss
System Efficiency
internal cooling, the high operating temperatures can only be
achieved through the use of ceramic components for hot
sections. That is, if we want to have system efficiencies in
excess of 40% (which is the objective of a recent DOE
program), we must use ceramic turbine and recuperator.
Similarly, it can be shown that overall cycle efficiency is
greatly affected by turbine and compressor isentropic
efficiencies. Although this is quite expected for gas turbines,
this is significant for micro turbines where linear dimensions are
much smaller compared to those in conventional systems for
power generation. In conventional fabrications techniques for
metal, and even more so in fabrication techniques for ceramics,
the relative tolerance deteriorates quickly with decreasing linear
dimensions. However, isentropic efficiency is a strong function
of relative tolerances. Of particular importance is the ratio of
clearance gap and blade height. This is illustrated in Figure 3
([3]). As relative clearance is increased from 0.075 to 0.125 for
turbine blades and from 0.025 to 0.075 for compressor blades,
there is a 12.5% reduction in performance compared to the
baseline case (indicated by the upper dot). Thus, a key enabling
factor in improving overall efficiency of a micro turbine is to
have a low value for the relative tolerance, and hence microfabricable ceramic components can be quite important. Even
though the proposed PDC is the most appropriate for the
turbine, the requirement of an excellent relative tolerance makes
it ideal material for the compressor as well.
9%Pressure Loss
12%Pressure Loss
0.35
0.30
0.25
0.9
0.925
0.95
0.975
Recuperator Effectiveness
Figure 4. Effects of recuperator pressure loss and effectiveness
on cycle efficiency
optimization study [4] produced the results presented in Table 2
for a compact recuperator for a micro turbine with similar
specifications. As can be seen from this table, a low value for
relative tolerance (which is the ratio of wall thickness to overall
width in this case) is a key requirement for the recuperator as
well, and hence the proposed micro-stereolithography technique
for PDC would be an enabling technique to fabricate such
recuperators.
Table 2. Results of recuperator optimization for ducts with
square and equilateral triangle cross-sections
Figure 3. Effect of clearances on cycle efficiency [3]
(A value of 1.0 on the ordinate refers to the baseline case)
Micro turbines typically have smaller pressure ratios
compared to larger systems because of their small sizes. For
these machines, having a recuperator is imperative to obtain
respectable system efficiency. Using a simple system model
developed in [4], we can find the impact of recuperator
parameters. Figure 4 presents the variation of system efficiency
as a function of recuperator effectiveness for four different
values of recuperator pressure losses: 3%, 6%, 9%, and 12%.
It is clear that the recuperator must be fabricated in a way
so as to maximize effectiveness, and minimize pressure loss and
axial conduction. A long recuperator with small ducts and thin
walls is required for high effectiveness, whereas lower pressure
drops require shorter recuperator with larger ducts. This
contradictory requirements call for optimization. A recent
Cross section:
Square
Triangle
System efficiency
39.4%
42.2%
Duct size
1.319 mm
1.39 mm
Wall thickness
0.05 mm
0.05 mm
Number of ducts
66,560
158,800
Effectiveness
0.961
0.985
rpl
2.86%
2.4%
Overall volume
0.125 m3
0.125 m3
Mass flow rate 7.512 x 10-6 kg/s 4.452 x 10-6 kg/s
Flow velocity
9.772 m/s
5.214 m/s
Max stress
0.119 Gpa
0.175 GPa
[Here, duct size corresponds to one side of the square or
triangular cross-section, and flow velocity refers to the average
value in each duct]
Moreover, a metal recuperator requires a large length so
that axial conduction is negligible [5]. It should be noted that
axial conduction is typically not a problem in conventionalsized recuperators because of relative smaller values for the
ratio of wall thickness to overall length. If PDC could be used
to fabricate a recuperator with absolute resolution of about a
few microns, then axial conduction will be negligible even with
a comparatively compact length as thermal conductivity of PDC
is quite low compared to typical metals [5].
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Copyright © 2002 by ASME
POLYMER-DERIVED-CERAMICS
Photo-polymerization is a low cost approach to fabricating
polymer structures. By adding a photo-initiator to a liquid
precursor, solidification can be accomplished by exposure to
UV radiation rather than by heat addition. Thus by performing
photolithography directly to the precursor, solid polymer shapes
may be obtained without the need for a mold [1, 6, 7].
The approach builds on a recently developed polymerprecursor technique that converts a polymer into Si-based
ceramics [8,9]. The technique consists of three steps as
illustrated in Figure 5: (i) solidifying the liquid polymer either
thermally or with UV light, (ii) cross-linking the solidified
polymer under isostatic-pressure, and (iii) pyrolysis of the
polymer component to form monolithic ceramics.
The ceramics obtained in this fashion are fully dense
(Figure 6). The key to the success of this process is the
application of isostatic pressure during cross-linking. The heat
treatment during cross-linking generates gaseous by-products.
The applied isostatic pressure provides an obstacle to the
nucleation of bubbles and micro cracks. The gaseous byproducts would need to build up sufficient pressure to overcome
the applied pressure before bubbles and micro cracks can
nucleate. Due to the open structure of the polymer, the gases
will diffuse out of the sample before they can build up a
pressure that is high enough to overcome the applied isostatic
pressure. Therefore, transparent and defect free cross-linked
polymers can be obtained by the application of isostatic
pressure during cross-linking [8].
Cross-linking
~400 oC, iso-press
solidification
Transparent
solid
Pyrolysis
~ 1000 oC
Transparent
solid (infusible)
Fully dense
ceramics
Figure 5. Process steps to obtain polymer-derived-ceramics
Figure 6. A SEM micrograph showing the fully dense structure
of polymer-derived SiCN
The cross-linked parts are then pyrolyzed at 1000°C for 8 hours
to convert the polymer into a ceramic.
Very low
heating/cooling rates are used: the heating rate is about
1°C/min to 400°C and 25°C/h to 1000°C. The cooling rate is
1°C/min to room temperature [8]. Even though the pyrolysis is
only to 1000°C, the alloys are stable up to 1500°C for SiCN
and 1800°C for SiBCN (Figure 7) in air. Physical properties of
SiCN are presented in Table 3, together with those of SiC and
Si3N4.
Figure 7. Thermal stabilization behavior of polymer-derived
SiCN and SiBCN, compared to Si3N4 (reconstruction from [9])
Table 3. Properties of PDC-SiCN, SiC and Si3N4
Materials
PDC-SiCN
SiC
Si3N4
Density (g/cm3)
2.35
3.17
3.19
E modulus (GPa)
100-150
405
314
Poisson’s Ratio
0.17
0.14
0.24
CTE (x 10-6/K)
3
3.8
2.5
Hardness (GPa)
25
30
28
Strength (MPa)
1000 [10]
418
700
Ratio of Strength-to-Density
468
132
219
Toughness (MPa.m1/2)
3.5
4-6
5-8
Thermal Shock FOM* (K)
2200
270
890
Creep rate at 1350 oC (s-1)
~ 10-9
~ 10-9 ~ 10-9
* FOM = Strength/(E-modulus.CTE)
Most room-temperature properties and creep resistance of
SiCN are in the same range as those of SiC and Si3N4.
However, the ratio of strength-to-density (which is important for
small turbine engine rotors due to the high rotation speed that
results in high centrifugal force), thermal shock resistance and
oxidation resistance of SiCN are superior to those of SiC and
Si3N4[11], indicating longer life and higher reliability.
The polymer-derived ceramic has been used in many
simple manufacturing processes. These include micro casting
and lithography. An example of a micro cast gear is seen in
Figure 8. The limitation of micro-casting technique is that only
simple two-dimensional parts can be created. However, with
modification and further complication of the basic microcasting technique, simple three-dimensional structures can be
made. This modified process is lengthy, time consuming, and
requires special operating conditions. Also each manufacturing
step increases the risk for failure and increases the opportunity
for errors.
This leads to the idea of using the micro-stereolithography
technique. With this technique, much more complicated and
useful parts can be created. This technique involves preprocessing of a CAD model. A computer-controlled system
actually performs the build process. Due to the simplicity and
speed of this technique, it has been the clear front-runner for the
development of 3-dimensional MEMS structures. An example
that is possible on a micro-stereolithography machine at the
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Copyright © 2002 by ASME
Swiss Federal Institute of Technology at Lausanne (EPFL) is
presented in Figure 9 [12]. As this is the technique of choice,
this is discussed in more details in the following sections.
Mold
SiCN gear
Figure 8. A Micro-cast SiCN gear (from [8])
Figure 9. Micro turbine by micro-stereolitography (from [12])
MICRO-STEREOLITHOGRAPHY TECHNIQUE
SL can be implemented to maximize the positive
characteristics of polymer-derived ceramics. Through this
process, parts with absolute resolutions of a few microns have
been demonstrated [1]. This system is composed of four main
parts (Figure 10).
1) A high power laser and optics – used to illuminate the
active layer mask.
2) A reflective liquid crystal display (LCD) or digital micromirror device (DMD) – used as an active mask.
3) A vertical translation stage – used to lower the part.
4) A computer based control system – used to automate the
build process.
A diode pumped solid-state laser is the first major
component in the SL process. The laser adopted in our system
has 3.5 watts of power and operates at a wavelength of 355 nm.
We chose to use this wavelength due to the easy availability of
many different photoinitiators or commercial resins that will
cure at this wavelength. We also chose to use a solid-state
(frequency-tripled Nd:YAG) laser over the alternative, Kr- or
Ar-gas lasers, because it is much more efficient and can operate
from a standard outlet and does not require a large heat
exchanger and an external water cooling system. This would be
an important feature in deciding the cost for this process when
it is commercialized. Moreover, the most suitable strong lines of
Kr- or Ar-gas lasers are around 400 to 410 nm, where not that
many photoinitiators are available. The optical components for
the entire system will be made from fused silica to prevent
damage from the short UV wavelength. The beam will be
polarized and expanded through two lenses in order to
illuminate the active layer mask.
The active layer mask is what separates microstereolithography from traditional stereolithography processes.
The standard process uses a vector-by-vector or raster method.
The raster method takes a solid model from the computer and
calculates instructions for a path that the laser will follow for
each layer. The laser beam scans the area of resin that is to be
solidified in a crosshatch pattern. Once a layer is done, a
vertical translation stage is used to move down the part inside
the vat of polymer. After all layers have been completed the
part is removed from the liquid resin and cleaned. This is the
approach taken in all commercial stereolithography systems
currently available.
The layer-by-layer technique, which is adapted here, uses
an active layer mask that will be either a reflective liquid crystal
display (LCD) or a digital micro mirror device (DMD). The
LCD or DMD screen will show the pattern for the entire layer,
which will allow an entire layer to be created in one shot and
will reduce the residual stresses within the layer itself as well as
save laser time. This approach is not used at present in any
commercially available rapid prototyping machines. It is at
different stages of development at UCF, a few other universities
and an industrial R&D group, SRI. A simple comparison
between the two options for active masks is provided in Table 4
where an LCD and a DMD from typical respective vendors are
compared.
In the LCD technology, individual pixels of a large pixel
array can be addressed and can be switched “on” or “off”
according to the image. Some LCD screens allow the light to
get transmitted through the “on” pixels and are called
transmissive LCD screen. In reflective LCD screens, the light
reflected from the screen picks up the image according to the
state of the pixels. With a transmissive screen a part of each
pixel is blocked by the electrodes that activate the pixel. With
the reflective screen this electrode is attached to the back of the
pixel and reflects the light when the pixel is off. In our system,
the reflective LCD screen is chosen over a transmissive screen
due to the higher fill ratio, that is the ratio of the active pixel
area to the total area. LCD screens can be made with a 1280 by
1024 array of 13m pitch pixels. In comparison, currently
available DMD arrays have a pixel pitch of 16 m. A typical
DMD array is shown in Figure 11. For either LCD or DMD
screens, a problem arises with the wavelength of the laser used
to illuminate the mask. Wavelengths in the UV range (<400
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Copyright © 2002 by ASME
Figure 10. Schematic of the process set up for micro-stereolithography technique being developed at University of Central Florida
nm) may cause damage to the screen and hence appropriate
protective coating must be used.
Table 4. Comparison of DMD and LCD arrays
Three Five Systems
Texas Instruments
MD 1280 LCD
Resolution
1280 X 1024
Imaged Dimensions 15.36 X 12.29mm2
Pixel Pitch
12 m
Pixel Gap Width
0.6 m
Fill Factor
> 92 %
Operating
Temperature
-1°C to 70°C
DMD
1280 X 1024
20.48 X 16.38mm2
16 m
1 m
>90 %
layer approach with dynamic masks, we have decided to add X
and Y stages to create horizontal motion for the same layer. We
plan to use the dynamic mask as a stamp to create parts that will
be much larger than the screen itself. With this approach the
up to 65°C
For both conventional stereolithography and microstereolithography techniques, a vertical translation stage is
needed. When one layer is completed either by vector-by-vector
method or by using a dynamic mask, the translation stage
lowers the part in the container holding the resin so that next
layer can be worked on.
PROPOSED ENHANCEMENT AND ESTIMATES OF
ACHIEVABLE RELATIVE TOLERANCE
The current design of the system being implemented at the
University of Central Florida plans to combine the vector-byvector method with the layer-by-layer method that uses dynamic
masks. In this modified version, two high-resolution translation
stages are used in addition to the vertical stage. Each stage has
an absolute resolution of 0.1 m. Even if we are using layer-by-
Figure 11. SEM Photograph of a DMD array (from TInst.)
cross section can be split into multiple pieces and we can make
parts that are correspondingly larger, thus obtaining excellent
relative tolerance.
The desired high absolute accuracies require a control
system that is efficient and accurate. The computer program will
control the stages, active layer mask, and laser. It will also
constantly take measurements of resin temperature, resin height,
and laser output power, to adjust the exposure time for each
layer. This control program is an integral component in order to
get high-resolution parts. Also, the entire machine is set up on a
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Copyright © 2002 by ASME
vibration isolated optical table to prevent errors from vibration
of the floor and building.
To save laser time and money, the computer control
program will analyze each layer of the part to be fabricated. It
will then determine the best way to place the screens to create
the part as quickly as possible. For example, the part shown in
Figure 13 needs only three screens to be built for this particular
layer. The procedure will allow this particular microstereolithography technique to make parts of any reasonable
size and will not be limited to one build envelope. The only
limiting factor to how large a part may be is the time the
operator is willing to wait for the part to finish.
Figure 13. Placement of a layer on multiple screens
A summary of comparison between this system and other
known micro-stereolithography systems is presented in Table 5.
Of the systems reported in this Table, only the system soon to
be commercialized by SRI uses PDC other than the UCF
system. However, the SRI system uses silicon nitride whereas
the system described here would use SiCN and SiBCN. As
reported by [9] and presented in Figure 7, Si3N4, SiCN and
SiBCN are thermally stable up to 1400oC, 1500oC and 1800oC.
It should be noted that an increase of turbine inlet temperature
from 1400oC to 1800oC can cause about 10 percentage points
increase in system efficiency (Figure 2), which is very
significant in the power generation industry. Moreover, it is
will act as a barrier coating in the presence of water vapor (as in
combustion gases). However, this prediction is yet to be
substantiated.
It should be noted that machine resolution (which is
imposed by resolutions of the optical system, dynamic mask and
translation stages) is not the only factor that affects achievable
absolute tolerance in a fabricated part. A separate limitation is
imposed by material properties. When any material is photopolymerized, it undergoes some shrinkage. Typically the
average value of this shrinkage is around 3 %. This amount is
known a priori and can always be considered in determining the
dimensions of a part such that the part has the correct
dimensions after shrinkage due to photo-polymerization.
However, the standard deviation in the amount of shrinkage
may not be negligible and will cause random fluctuations in the
dimensions, thus reducing effective absolute tolerance. The
fluctuating portion of the amount of shrinkage is here termed as
random shrinkage, and is around 1% of the average shrinkage
for photo-polymerization of typical materials [13].
For the materials considered in this research, the combined
value for the average shrinkage for the first two steps (that is,
photo-polymerization and cross-linking in Figure 5) has been
found to be around 1% [10]. Even with a random shrinkage that
is 1% of the average shrinkage, the maximum expected random
variation in dimensions would be around 1% X 1% X 10 cm =
10 microns for a part with overall dimensions of 10 cm. This
corresponds to a rather low value of relative tolerance of 0.01%
or 0.0001.
For PDC’s such as SiBCN, the pyrolysis step causes
considerable additional shrinkage, where the average shrinkage
has been noticed to be 28%. However, the random shrinkage
has not been observed to be significant [10]. A more systematic
investigation is currently underway in order to determine the
amount of random shrinkage in these steps, and the results
would be reported in a future paper.
Even if, for the sake of argument, it is assumed that the
random shrinkage is 1% of the average shrinkage in pyrolysis
step, then the expected variations in feature dimensions for a
turbine rotor with outside diameter of 10 cm is expected to be
around 280 microns. If the blades were 5 mm tall (as reported
Table 5. Comparison between alternative micro-stereolithography systems
UCF: Proposed SRI: Direct
3D Systems UCLA: Current
System
Photo Shaping
Viper Si2
System
Resolution in (x, y) < 20 m
48 m - 130 m 75 m +/- 15m m
Resolution in z
10 m
12.7 m
50 m
5 m
3
3
3
Min Build Envelope 0.5x0.5x0.5 in 2.6x1.9x1.0 in 5x5x10 in
1x1x1 mm3
3
3
Max Build Envelope 6 x 6 x 4 in
7.0x5.2x2.0 in
Time Per Layer
< 30 sec.
18 - 43 sec.
Varies
< 1 min.
Method employed dynamic mask dynamic mask raster
dynamic mask
PDC Material
SiCN, SiBCN Silicon Nitride Not PDC
Not PDC
(For consistency, the above numbers for resolutions only include machine resolutions)
expected that boron in SiBCN will lead to oxides of boron that
The attached details are
obtained from:
(1) for the SRI system:
web-site and phone
conversation
(2) for Viper Si2: from
published literature on
the website
(3) for the UCLA system:
from [14]
in Figure 3), the corresponding relative tolerance would be
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Copyright © 2002 by ASME
0.056 or 5.6%. The typical relative tolerance in conventionally
manufactured turbine rotors for micro turbines is around 10%
[3]. Even in this case with a quite pessimistic projection that is
disputed in the following discussion, the achievable relative
tolerance is better than what can be achieved through
conventional methods and conventional materials.
It is expected that a relative tolerance of much better than
5% can be achieved for a turbine rotor with outside diameter of
10 cm and blade height of 5 mm for two reasons. First,
preliminary investigation indicates that random shrinkage is
much smaller than 1%, and perhaps as small as 0.1%, of the
average shrinkage in the cross-linking and pyrolysis steps where
the average shrinkage is quite large. Second, in the fabrication
technique mentioned here, the value of random shrinkage
(which is the standard deviation of the actual shrinkage, as
defined before) in critical dimensions such as blade tip
clearance is not determined by the overall dimensions but by the
local ones such as blade height. In that case the achievable tip
gap clearance would be 14 microns for a blade height of 5 mm,
which corresponds to tip gap clearance to blade height ratio of
0.28%. This value is one order-of-magnitude better than the
range 7.5% to 12.5% mentioned in [3].
CONCLUSION
In this paper we have established that high temperature
ceramics that can be micro-fabricated with micron-level
accuracy and into complex shapes such as turbine blades are
needed in order to have a high system efficiency. We have
introduced a new class of polymer-derived-ceramics that have
the necessary properties and have been used in high-resolution
fabrication. One of these materials can maintain temperature
stability up to 1800°C. Due to the fact that the initial polymer
becomes a solid with a photo initiator, it can be used in
stereolithography processes that can make complex shapes.
In order to reach the desired resolution on parts with linear
dimensions of a few centimeters, we proposed a variation of the
typical micro-stereolithography process. Here x,y-translational
stages are added in addition to a dynamic mask. With further
refinement, this process has the ability to make very accurate
high temperature parts with PDC for use in micro turbines. It is
projected that for a turbine with 10 cm rotor outside diameter
and 5 mm blade height, a tip-gap-to-blade-height ratio of better
than 0.28% can be achieved by this proposed enhancement.
This technique appears to be quite promising for next
generation micro turbines, and hence requires further
investigation and development.
ACKNOWLEDGEMENT
The authors acknowledge a DURIP grant made available
from BMDO (Ballistic Missile Defense Organization) that has
made part of this research possible.
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8
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