1. *
Undergraduate Student – Department of MaterialsScience and Engineering – Lehigh University
Tel.: 610-597-8007
E-mail:gjf210@lehigh.edu
IR Transmission, Hardness, and Fracture Toughness of MgAl2O4
Spinel after doping with MgO
George J. Ferko V*
Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA, 18015, United States
August 13, 2009
Abstract
Magnesiumaluminate spinel has shown promise in the applications of transparent armor, window, and dome design. Recent work has
focused on additives used to alter the sintering kinetics of spinel to make the production of spinel more economically viable and
control its final properties. To test the effect of additives on important spinel properties high purity magnesium aluminate spinel
powder has been doped with 100 ppm of magnesia and tested to determine its hardness,fracture toughness,transmission in the IR
spectrum and microstructure. Evaluation in the SEM has shown that doping with MgO results in increased grain growth and abnormal
large grains. Analysis of mechanical properties by Vickers microindentation has shown that the MgO doped spinel has a decreased
hardness and slightly increased fracture toughness compared to that of undoped spinel. Transmission testing by FT-IR spectroscopy
has determined that doping with 100 ppm of MgO does not significantly reduce the transmission of spinel in the IR spectrum.
1 - Introduction
Polycrystalline magnesium aluminate (MgAl2O4) spinel has
been found to have attractive properties for both transparent
armor and radar window and dome enclosures. In 1960 Hughes
Research Laboratories demonstrated that spinel was capable of
transmission further into the infrared (IR) spectrumthan
sapphire and in 1969 General Electric Co. produced the first
visibly transparent polycrystalline spinel sample. The 30 years
of intense research that followed has yielded many innovations
in spinel processing often with the unfortunate quality of being
irreproducible due to impurities in the available powders [6].
Recent availability of high purity spinel powders has brought
spinel to the forefront of transparent armor, window, and dome
design [11,15]. High purity powders have allowed for spinel
windows and domes to be manufactured with high transparency
in the midwave infrared (MWIR) range making spinel very
attractive for military radome applications. Spinels transmission
in the ultraviolet and visible wave (UV-VIS) spectrums make
spinel attractive for transparent armor as well [1,2,4,5,6,9]. The
relatively low density of spinel compared to other popular
transparent armor materials such as AlON or sapphire makes the
study of spinel an even more attractive venture [14]. The
research done by Technology Assessment & Transfer
Incorporated (TA &T) and Surmet is largely responsible for the
recent success ofspinel. Their production processes have lead to
consistently transparent large scale windows and domes. The
recent efforts of TA & T to improve the economics of their
manufacturing process by removing the hot isostatic pressing
(HIP) step have not been realized [3]. Windows and domes
produced by Surmet face the issue of inhomogeneity in select
properties due to a bimodal grain size distribution. It is the goal
of this research endeavoras well as future research to solve
issues with grain size and economics by introducing additives to
spinel so that they can be produced satisfactorily by hot pressing,
annealing, and surface finishing steps alone. The effects of the
additives on optical properties, mechanical properties, and
thermal properties must also be studied to ensure that an additive
does not negative effect spinels properties and to find any
potential improvements in spinels properties that can be induced
by doping.
2 - Experimental Methods
2.1 - Sample Preparation
Spinel powder of high purity was obtained from the
Baikalox family of products (Baikowki, France). The starting
powder for the MgO doped samples consisted of100 ppm MgO
excess powder introduced by mixing the high purity MgO
powder with the spinel powder in ethanol and heating in a
vacuum until all of the ethanol had evaporated. 0.3-0.5 g of

2. 2
George J. Ferko V
powder was poured into a ultra high purity graphite die obtained
from POCO Graphite, Inc. (Decatur, TX). A diagram of the die
assembly used is shown in figure 1.
Figure 1: Schematic of the die assembly used for all of the
spinel samples produced by hot-pressing.
The powder was hot-pressed in a Thermal Technologies LLC
high temperature graphite hot zone furnace Model: 1000-2560-
FP20 (Santa Rosa, CA) undervacuum. The furnace ramp rate
was set to 7.5oC per minute where the powders were first held at
700oC for 3 to 4 hours for degassing. This degassing
temperature was used to allow volatiles to escape while
preventing the bonding of spinel particles to a degree that would
trap gasses in the sample or result in cracking during the
pressing operation. Whetheror not gas was still evolving from
the sample was checked by turning off the vacuumpump and
waiting 30 seconds to see if the pressure in the hot-zone would
increase. Once the evolution of gas had ceased the ram force in
the hot-press was slowly increased to the desired stress.
Samples that were hot-pressed at a maximum temperature of
1250oC were pressed using a stress of ~70MPa. The
temperature was than increased by 7.5oC per minute to the
desired hot-pressing temperature and held for 2 hours. Upon
removal from the hot-press each sample had ~0.8mm cut off of
each of its 4 sides and ~200µm ground off of each of its 2
surfaces. The cutting and grinding of the samples is necessary to
remove the large amount of carbon contamination that exists at
the surfaces where the graphite die touches the spinel sample.
Following cutting and grinding each sample was cleaned
ultrasonically in the VWR B3500A-DTH (Batavia, IL) using
deionized water, acetone, and ethanol to ensure that no
contaminates remained on the samples. Great care was exerted
to keep the samples clean at all stages of their processing and
analysis because of the importance of maintaining very high
purity samples. Following the cleaning step some of the samples
were annealed in a CM Inc Rapid Temp Box Furnace (Model:
950065, Bloomfield, NJ) underan air atmosphere. The
annealing step was left out for some samples so that they could
be characterized without the added grain growth that occurs
during the annealing process or the potential contamination that
was thought to occur from being in the box furnace. The box
furnace was set to a ramp rate of 15oC per minute and a dwell
temperature of 1000oC. After this step the samples were
polished to a surface finish of 50nm and thermally etched. The
thermal etching of each sample was performed from
temperatures of 1000oC to 1100oC for no longer than 60
minutes.
2.2 - Scanning Electron Microscopy
The Following thermal etching the samples were coated
with Ir to prepare for analysis in the SEM using the Hitachi S-
4300 CFE (Japan). The high resolution images obtained were
analyzed for differences in grain size and grain size distribution
using the line intercept method and percent porosity using the
100 point count method.
2.3 - Transmission
Following SEM analysis the samples were re-polished to
remove the Ir conductive coating and cleaned using the same
cleaning process previously mentioned. The samples were then
mounted on a sample holder over a 4mm diameter pinhole using
two-sided tape. A Varian 7000e FT-IR spectrometer (Palo Alto,
CA) was used to measure the percent transmission of each
sample in both the infrared (IR) spectrumand the ultra-
violet/visible (UV-VIS) spectrum. Samples containing
absorption peaks were run multiple times to ensure that the
peaks were not the result of surface contamination or surface
roughness. The transmission spectra of each sample were
plotted on the same chart to show differences in transmission
and the relevant absorption peaks were labeled.
2.4 - Mechanical Properties

3. 3
George J. Ferko V
Following transmission testing the samples were cleaned
using the same method that has been previously mentioned.
Each sample was then mounted on an aluminum slug using an
amorphous adhesive to prevent any sample movement during
micro-indentation. Micro-indentation was performed using a
LECO M-400 microhardness tester (St. Joseph,MI). The
indenter was equipped with a diamond Vickers indenter tip.
Before the indenter could be used a stereo-micrograph of the
indenter tip was taken to ensure the tip was of the proper
geometry, shown below in figure 2. Due to spinels hygroscopic
nature samples were indented while in immersion oil to prevent
humidity in the air from causing crack growth [20].
Figure 2: Stereo-Micrograph of the Vickers indenter tip
showing that the geometry has not been affected by regular use.
The samples were indented at loads of 300, 500, and 1000gf. It
was reported by Parimal J. Patel, et al. that below a load of
388gf the indentation size effect (ISE) would artificially inflate
the hardness values for the samples [33]. For this reason
indentations made at 300gf were used only to verify that the ISE
was inflating the hardness and not for actual hardness
calculations. Indentations were used to calculate both the
hardness and the fracture toughness ofthe samples using as
described in the literature [20,31,22]. In order for the
indentations to be used in calculations of hardness and fracture
toughness the indentation had to meet the following ideal
conditions. All cracks had to propagate straight out of the
indentation corners. All cracks had to be of the same length. No
secondary cracks could be observed on or below the sample
surface. Each indentation had to be perfectly square [22]. An
ideal crack and indentation is pictured in figure 3.
Figure 3: Light optical micrograph of an ideal indentation and
crack propagation.
In some samples up to 80 indentations were required to obtain
indentations and cracks of an ideal nature. For each sample
three separate ideal indentations were measured five times each
and their values were averaged. The area of the indentation was
used to calculate hardness through equation 1 where P is the
force incident on the sample and A is the area of the indentation.
𝐻𝑉 =
𝑃( 𝑠𝑖𝑛 ( 𝜋
360
68 𝑜))(1000)
𝐴
(
𝑘𝑔𝑓
𝑚𝑚2
) (equ. 1)
Once the hardness was calculated equation 2 was used to
calculate the fracture toughness ofthe sample where Kc is the
fracture toughness,α is a constant equalto 0.016, E is the
Young’s modulus, H is the Vickers hardness,P is the load
applied by the indenter, and c is the length of the crack measured
from the center of the indentation to the tip of the crack.
𝐾𝑐 = 𝛼 (
𝐸
𝐻
)
1
2
(
𝑃
𝑐3/2
) ( 𝑘𝑔𝑓√𝑚𝑚) (equ. 2)
Unfortunately a suitable method of non-destructively
determining the Young’s moduli of the samples was not able to
be performed and the modulus was held constant at 275GPa for
all samples.
3 - Results

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George J. Ferko V
3.1 - Microscopy
A micrograph from sample SsqA7 is shown in figure 1. The
sample SsqA7 has not been doped and has not been densified to
the extent of transparency. This sample was hot-pressed at
1050oC with 100MPa of pressure.
Figure 1: Micrograph of sample SsqA7 showing sub-micron
size grains and a pore size on the order of 10 nm.
The roughness seen on the grains is a result of over etching a
problem which is difficult to solve because the low hot-pressing
temperature limits the thermal etching temperature that can be
used without causing additional grain growth. The porosity is
about 0.3% and the grain size is ***. The small grain size and
angularity of the grains is a desirable feature in spinel for
increased hardness,according to the Hall-Petch relationship, and
increased fracture toughness when the mode of fracture in spinel
is intergranular. The grain size and morphology in this sample
would appear to be desirable; however, a significantly smaller
amount of porosity is needed to achieve transparency.
Shown in figure 2 is a micrograph of sample SsqA3. This
sample has been hot-pressed at 1250oC with 70MPa of pressure.
The result is a sample that has densified to a percent porosity
that is immeasurable by graphical analysis. This sample was
translucent upon removal from the hot-press and after polishing
appeared transparent to the naked eye.
Figure 2: Micrograph of sample SsqA3 showing no noticeable
porosity and a grain size of **.
A slight gray color was observed in the sample suggesting that
some carbon contamination occurred during hot-pressing. For
the purpose of this study and future studies on doping spinel it is
very important to have as little contamination as possible. To
remove the carbon contamination from the hot-pressed samples
it was decided that the carbon could be annealed out of the
sample.
Shown in figure 3 is sample SsqA4. This sample has
undergone hot-pressing at 1250oC with a pressure of 70MPa, just
as sample SsqA3has. This sample has also been annealed at
1000oC for 18 hours in a box furnace with an air atmosphere.
Figure 3: Micrograph of sample SsqA4 after undergoing carbon
decontamination by annealing showing no noticeable porosity
and a grain size of **.

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George J. Ferko V
The unfortunate effect of annealing is that a significant amount
of grain growth has occurred. The largest grains in sample
SsqA4 are more than three times smaller than the reported grain
size for commercially available spinel, such as those from TA &
T. Fortunately annealing produced a sample lacking in gray
coloring, suggesting that the carbon contamination was
successfully removed. The tradeoff between grain size and
contamination was deemed necessary so that a sample of lower
contamination could be used for this study.
Shown in figure 4 is sample SsqA6. This sample has the same
thermal history as sample SsqA4. It has been hot-pressed at
1250oC with 70MPa of pressure and annealed at 1000oC for 18
hours.
Figure 4: Micrograph of sample SsqA6 showing some visible
porosity as well as enhanced grain growth above that which was
expected from the thermal operations performed.
In figure 4 it can be seen that the same hot-pressing and
annealing operations that yielded the sample in figure 3 have
produced a different grain morphology. The micrograph of
sample SsqA4 in figure 3 shows a normal grain size distribution
with an acceptable average and maximum value, however; figure
4 shows sample SsqA6 which has a bimodal grain size
distribution and much higher average and maximum grain size
values. Both samples were also thermally etched using the same
parameters for temperature and time yet the sample in figure 4
appears to have undergone very little etching on select grain
boundaries. The result is a poor image that is useless for image
analysis, but it can still be discerned from the image that the
grain growth kinetics have unexpectedly changed. The residual
porosity that was observed is also a troubling result.
Shown in figure 5 is sample SM100sqA2. The starting powder
for this sample was doped with 100ppm of MgO before hot-
pressing. The hot-pressing and annealing operations for this
sample are the same as those in samples SsqA4 and SsqA6.
Figure 5: Micrographs of sample SM100sqA2 showing very large grains relative to the other samples with a pronounced bimodal
grain size distribution and no noticeable porosity at 5k, 2k, and 800x magnifications.
Doping with MgO has resulted in very large grains and a grain
size distribution that is bimodal. The smaller grains in the
sample appear in clusters and are very similarly sized to those in
the undoped sample seen in figure 3. The smaller grains in the
sample are unfortunately difficult to resolve at magnifications
where the large grains can be imaged. This meant that accurate
grain size measurements were not able to be made by the linear
intercept method. The large grain shown on the far right image
in figure 5 shows one of the large grains in the sample. This
grain has not only undergone a relatively large amount of grain

6. 6
George J. Ferko V
growth compared to that in the undoped samples, but it has also
lost its angular grain morphology. The shapes ofthe larger
grains in the sample shown in figure 5 appear round.
Sample SX100/111sqA2is shown in figures 6 and 7. This sample
is a bi-crystal single crystal polycrystal hybrid with the
polycrystalline regions of the sample sandwiched between two
single crystals. The polycrystalline region that was against the
<111> direction of the single crystal is shown in figure 6 and the
polycrystalline region grown against the single crystal oriented
in the <100> direction is shown in figure 7.
Figure 6: Micrograph of sample SX100/111sqA2showing the
polycrystalline region against the single crystal that was oriented
in the <111> direction with a bimodal grain size distribution.
Figure 7: Micrograph of sample SX100/111sqA2showing the
polycrystalline region against the single crystal that was oriented
in the <100> direction with a bimodal grain size distribution.
The grain size distribution in the two samples is bimodal, not
unlike that of sample SM10sqA2 in figure 5. The average grain
size of the large grains in figure 6 is ** and the average grain
size of the large grains in figure 7 is **. Other images of the
polycrystalline regions indicate that the grains closest to the
<111> oriented single crystal have experienced more grain
growth than those closest to the <100> oriented single crystal. It
should be noted that it is probable that the crystal direction does
not play a role in the grain growth of the sample and the final
grain size. This difference in grain growth effects is likely due
to differing contamination levels on the two different single
crystal faces.
3.2 - Transmission
Shown in figure 8 are the transmission spectra for several
important samples that were tested. The single crystal sample
was polished using the same technique as the othersamples to
ensure that surface finish would not be at fault for any of the
results. Others have reported that uncoated single crystal spinel
should have a transmission of about 87% [5,10,32]. The single
crystal tested reaches a maximum transmission of 85.91% which
indicates that there may be some reflection or beam diffraction
issues and a higher transmission could be obtained from the
same samples with a difference surface finishing technique.
There is some slight noise between the wavelengths of 2.57 and
2.80 µm that is most likely ethanolthat remained on the sample
from cleaning. While measuring the spectra it was observed that
the noise decreased with time and eventually disappeared as a
result of the ethanol on the sample surface evaporating. The two
absorption peaks at 3.50 and 3.42 µm are thought to be from
water or carbon dioxide that was still in the testing chamber.
These peaks also disappeared with time except in the case of
sample SM100sqA2. Sample SM100sqA2 was doped with
100ppm MgO, hot-pressed at 1250oC with 70MPa of pressure,
and annealed at 1000oC for 18 hours. In sample SM100sqA2 not
only did the absorption peaks at 3.50 and 3.42 µm not change in
intensity with time, but the absorption peak at 2.96 µm also
remained at the same intensity over time. To insure that the
peaks weren’t part of some kind of surface contamination the
sample was cleaned with ethanol, placed under a heat lamp for
an hour to obtain a more complete evaporation of the ethanol,
and then tested again. After cleaning and heating intensity of the
absorption peak remained the same.

7. 7
George J. Ferko V
Figure 8: Figure showing the transmission spectra for six samples in the IR range.
The samples in figure 8 that did not appear in the section 3.1
on microscopy are the single crystal, SM100sqA3, SsqA8, and
ST1. The single crystalwas obtained from **. SM100sqA3 is
the same as SM100sqA2 only this sample has not been annealed.
This sample was created to determine if the absorption peak in
SM100sqA2 was due to doping or if it was from unintentional
contamination during thermal processing. SsqA8 is a
replacement for SsqA3 which was contaminated in a thermal
etching operation and could no longer be used for further testing.
The sample labeled ST1 was obtained from TA & T. The
thermal history of the sample is not known, however the cloudy
appearance of the sample is similar to that of otherTA & T
samples immediately after hot-pressing and before annealing or
hot-isostatic-pressing. It should be noted that this sample is not
indicative of the performance of TA & T’s commercial grade
spinel products and is of a lower quality.
The maximum transmission of the TA & T spinel sample
was found to be 68.86% at a wavelength of 4.14 µm. The spinel
samples produced for this study all have a maximum
transmission that is superior to the TA & T sample. Sample
SM100sqA2 has a maximum transmission of 75.37% at a
wavelength of 4.11 µm. Sample SsqA8 has a maximum
transmission of 74.75% at a wavelength of 4.16 µm. Sample
SM100sqA3 has a maximum transmission of 76.76% at a
wavelength of 4.03 µm. Sample SsqA4 has a maximum
transmission of 79.05% at a wavelength of 4.23 µm.
The UV-Vis range was also tested,however; the results
were not reportable due to the incident beam being diffracted.
The diffraction of the beam caused the detector to produce
values for transmission that were either extremely low or
extremely high. Although these values could not be used to
make any conclusions the results are still available in appendix *
3.3 - Mechanical Testing
The results for the samples that underwent mechanical
testing are displayed in table 1.
Table 1: Hardness by Vickers indentation and fracture
toughness by the Vickers Indentation Method for four samples.
Sample Hardness (kgf/mm2)
Fracture Toughness
(MPa√m)

8. 8
George J. Ferko V
SsqA8 1625.3 ± 27.8 1.15
SsqA4 1493.7 ± 28.8 1.15
SM100sqA3 1544.3 ± 20.5 1.16
ST1 1423.7 ± 44.9 1.05
The data shows that the undoped and unannealed sample,
SsqA8, has the highest hardness. The MgO doped and
unannealed sample, SM100sqA3, has a lower hardness than
SsqA8 indicating that doping spinel with MgO may be lowering
the hardness. The sample obtained from TA & T, ST1, was
found to have the lowest hardness,however, ST1s hardness is
not significantly lower than the hardness of sample SsqA4.
Aside from the nominal empirical data it was also observed that
as the average grain size of the sample became larger and the
grains became abnormal the ideal crack geometry needed to
make the measurements in table 1 became more difficult to
obtain. Larger and more abnormal grained samples would
exhibit crack geometries similar to the geometry shown in figure
9 obtained from a study done by G.R. Anstis,et al. [20].
Figure 9: Micrograph obtained from a study on grain size
dependence on the affectivity of the Vickers indentation method
on measuring fracture toughness authored by G.R. Antis, et al.,
width of field 200 μm [20].
4 - Discussion
4.1 - Grain Growth
The increased hot-pressing temperature has resulted in a
larger grain size and greater densification in undoped spinel due
to an increase in grain boundary mobility. The difference in
grain size between sample SsqA7, shown in figure 1, and sample
SsqA3, shown in figure 2, appears consistent with the theoretical
temperature dependence of grain growth rate and densification
rate, shown below in equations 3 and 4.
𝐷𝑒𝑛𝑠𝑖𝑓𝑖𝑐𝑎𝑡𝑖𝑜𝑛 𝑅𝑎𝑡𝑒 =
(𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)×𝐷 𝐿×𝛾×Ω
𝐿3 ×𝑘×𝑇
(equ. 3)
𝑑𝐺
𝑑𝑡
=
(𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡)×𝐷 𝑏
⊥
×𝛾
𝑘 ×𝑇×𝐺
(equ. 4)
In both equations 3 and 4 the effect that temperature has on the
defect concentration,diffusivity, and interfacial energy is great
and so the grain growth and densification increase with
temperature, both across the grain boundary and through the
lattice. The difference between sample SsqA3 in figure 2 and
SsqA4 in figure 3 shows a continued grain growth during the
annealing process similar to that which is described in studies
done by Chiang and Kingery [33,34]. Chaing and Kingery
observed further grain growth during annealing with an increase
in grain size distribution, but did not report any abnormally large
grains. In this study the same effect is observed.
Sample SsqA6, shown in figure 4, exhibited grain growth
behavior inconsistent with previous studies []. Spinel has been
found to be an impurity tolerant system, however; this undoped
sample has undergone abnormal grain growth. Throughout the
sample there are abnormally large grains. There are also grains
of the expected grain size in clusters. The abnormal grain
growth must be contributed to contamination during hot-pressing
or annealing. This sample has shown that greater care in
preventing contamination must be observed when working with
the spinel system.
Doping with MgO has yielded even larger abnormal grains
than in the contaminated sample. The same bimodal distribution
of grain size is observed in sample SM100sqA2 as in the
contaminated sample with even larger grains. The larger grains
indicate a greater amount of grain boundary mobility in the MgO
doped sample than in the undoped samples. This suggests that
MgO doping is increasing the defect concentration in the spinel
sample and thus altering the diffusivity. Doping with MgO
cannot be held responsible for the abnormal grain growth with
certainty. The contaminated undoped sample has proven that
abnormal grain growth can be observed without any intentional
doping. Despite this it is suspected that the MgOis, in part,
responsible for the abnormal grains.

9. 9
George J. Ferko V
The bi-crystal in figures 6 and 7 indicates that spinel shows
differing grain growth rates dependant on crystallographic
orientation. The grains that nucleated near the <111> oriented
single crystal and those nearthe <100> oriented single crystal
are of differing average grain size. This part of the experiment
has been repeated in unpublished work by A. Kundu yielding
similar results. These results suggest that spinelmay exhibit
some anisotropic grain growth that has been previously
unobserved. However, it is far more likely that the difference in
grain growth between the <111> and <100> directions is due to
differing levels of contamination between the two single crystal
surfaces.
4.2 - Transmission
The IR transmission spectra for samples SsqA8 and SsqA4,
shown in figure 8, indicate that annealing spinel increases the
percent transmission. This increase in transmission occurs for
two reasons. First, impurities that act as scattering sites have
been removed during the annealing process. Second,the grain
boundary surface area has been reduced by grain growth
resulting in less scattering on the grain boundaries.
This effect has not been observed among the MgO doped
samples. SM100sqA2 has been annealed and SM100sqA3 has
not been annealed yet SM100sqA3 has a higher transmission.
This discrepancy is believed to be due to contamination in the
annealed sample. The annealed sample also exhibits a large
absorption peak at a wavelength of 2.96 μm. This absorption
peak must be due to contamination of the sample and cannot be
a result of doping with MgO because the unannealed sample has
no suckabsorption peak. The peak must therefore be caused by
contamination from annealing in the box furnace, surface
finishing, or cleaning. The location in the spectra of the
absorption peak is characteristic of O-H bonds commonly found
in alcohols, however the depth and width of the peak are
uncharacteristically large [**]. This sample must be re-polished
and re-cleaned to isolate the cause of the peak. It is possible that
future work may find a relationship between different dopants
and the absorption peaks they cause. This would be valuable to
the application of spinel as a dome cover for multi-mode
sensors. The transmission spectra for the unannealed MgO
doped sample lies in close proximity to that of the undoped
unannealed sample. This suggeststhat doping with 100ppm
MgO does not cause any significant additional scattering in the
IR spectrum. This result is promising as it indicates that the
spinel structure has not been significantly altered enough to
reduce its transmission. All of the samples tested for
transmission in this study have been found to have greater
transmission then the sample obtained from TA & T, label ST1
in figure 8. This is largely due to the fact that the TA & T
sample was much thicker than the other samples tested. It was
also observed in a light optical microscope that the TA & T
sample had undergone some differential polishing. The
differential polishing allowed for the grains to be resolved and it
was noted that they were extremely large compared to those
produced for this study. The differential polishing that occurred
on the TA & T sample indicates that a greater surface roughness
was present in ST1 than in the other samples, which may be
contributing to the low percent transmission in sample ST1.
Sample ST1 also contained some visible opaque scattering sites
the composition of which is unknown. These sites are also a
likely candidate for the reduced transmission in the TA & T
sample.
4.3 - Mechanical Properties
One of the observations made during indentation was that as
the grain size of the spinel sample got larger the cracks
propagating away from the indentation got more irregular. This
is believed to occur because the indentation is only hitting a
small number or only one grain at a time. Hitting a small
number of grains with the indentation is resulting in the cracks
only propagating along the preferred cleavage planes of one
grain rather than many grains. The result is cracks that don’t
necessarily propagate straight out of the indentation corners. A
schematic of irregular crack propagation is shown in figure 10.
The irregular nature of the cracks does not mean that samples
with large grains cannot be tested. If the test is repeated many
times than the indentation corners eventually will roughly line
up with the preferred cleavage planes in the sample and by luck
the cracks will propagate straight out from the indentation
corners. It is assumed that the energy dissipated by a crack
moving through a spinel grain is roughly the same as the energy
dissipated by cracking along the grain boundaries at room
temperature because of the mixed mode cracking ( inter- and
trans-granular fracture) that has been observed at room
temperature. This mixed mode fracture might showthat the
fracture toughness is not biased dependant on how many grains
the cracks are propagating through,of course this cannot be
known for certain.
The decrease in hardness observed during mechanical
testing in some samples can be explained by the relationship
between hardness and grain size. Both doping with MgO and
annealing have caused further grain growth in the spinel samples
and thus decreased hardness. This data shows that spinel
hardness exhibits a normal inverse relationship with grain size.

10. 10
George J. Ferko V
Figure 10: Schematic of the cracks propagating irregularly
away from the indentation in a large grained spinel sample.
The last result worth noting from the indentation testing is
the increase in fracture toughness found to occurwith MgO
doping. The theoretical effect of such small amounts of dopant
on fracture toughness is not well explained in the literature. It
cannot be determined from this data the exact cause of the
increase in fracture toughness. One theory is that the MgO
dopant has not completely segregated to the grain boundaries
and is in the lattice effecting the bond energy between ions. This
change in bond energy or bond type may be causing more
energy to be dissipated during fracture. Ting and Lu have
provided the possible defect reactions that may be occurring for
MgO in spinel, they are shown below in equations 5, 6, and 7.
4𝑀𝑔𝑂 → 2𝑀𝑔 𝐴𝑙
′
+ 𝑀𝑔 𝑀𝑔
𝑥
+ 4𝑂 𝑂
𝑥
+ 𝑀𝑔𝑖
…
(equ. 5)
4𝑀𝑔𝑂 + 𝐴𝑙 𝐴𝑙
𝑥
→ 3𝑀𝑔 𝐴 𝑙
′
+ 𝑀𝑔 𝑀𝑔
𝑥
+ 4𝑂 𝑂
𝑥
+ 𝐴𝑙 𝑖
…
(equ. 6)
3𝑀𝑔𝑂 → 2𝑀𝑔 𝐴𝑙
′
+ 𝑀𝑔 𝑀𝑔
𝑥
+ 3𝑂 𝑂
𝑥
+ 𝑉𝑂
..
(equ. 7)
These equations have been used to describe the rate
controlling mechanism during sintering in previous papers. The
change in energy caused by defects that results in a change in
sintering kinetics may also be effecting the kinetics of fracture.
5 - Conclusions
The objective of this study was to analyze the affect of
doping magnesium aluminate spinel with MgO on mechanical
properties and transmission. This study has led to the following
conclusions:
1. The doping of spinel with 100ppm MgO does not cause a
significant reduction in transmission throughout the IR
range.
2. The sensitivity of the spinel systemto contamination is
greater than was previously thought.
3. Annealing results in higher transmission in the IR range due
to grain growth and decreased concentration of impurities.
4. Doping with 100 ppm MgO produces a sample with lower
hardness,but higher fracture toughness.
5. Doping with 100 ppm MgO causes abnormal large grains as
well as a larger grain size throughout all grains.
6 - Future Work
7 - Acknowledgements
It is my pleasure to acknowledge Animesh Kundu PhD for
the abundance oftechnical advice throughout this research
endeavor. His contribution to what I’ve learned about the spinel
systemand research in general has been almost entirely
responsible for my growth as a student of ceramics and has led
me to be capable enough to perform research in the ceramics
field.
I would also like to thank Eva Campo PhD for bringing me
this opportunity to work on spinel and for her aid in keeping my
work focused. Her ability to inspire my persistence with this
project was vital to its progress. It is because of her direction
that the breadth of my knowledge has increased so greatly in
only a few short months.
I would like to thankProfessor Martin Harmer for his class
room teachings and the example that he sets for me and his other
students. His success has enabled my research and the research
of many others in the ceramics field.
I am grateful to ShuaiLei Ma for her instruction in previous
work, her appreciation of my work, and her advice throughout
this study.
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Cleavage Plane
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Appendix