synthesis of high purity power for solid state lasers
TRANSCRIPT
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Synthesis of High Purity Ho3+-Doped Lu2O3 Powder for High
Power Solid-State Lasers
Laura Bruce Scotch Plains- Fanwood High School
8.12.2011 Optical Science, Code 5621
Mentor: Woohong (Rick) Kim
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I. Abstract
This project regards the synthesis of Ho: Lu2O3 powder with the purpose of comparing
the transparency of the resulting hot-pressed ceramics with different doping concentrations.
These ceramics are used for solid-state lasers. High purity Ho3+ -doped Lu2O3 powder with three
different concentrations of Ho3+ (0.1%, 2%, and 5%) was synthesized through the coprecipitation
method. Each of the three powders samples was then analyzed using a Horiba particle size
instrument, an scanning electron microscope (SEM), and x-ray powder diffraction (XRD). Data
obtained from the x-ray diffraction was consistent with the characteristics of Lutetium and
Holmium from the database. The SEM images showed that the powders were fine and contained
mostly soft agglomerates composed of various size crystals after calcinations. The sample with
the smallest median particle size was the 2% Ho: Lu2O3 (4.97μm), followed by the 5% Ho:
Lu2O3 (10.25μm), and the 0.1% Ho: Lu2O3 (12.08μm). The sample that resulted in the most
transparent hot-pressed ceramic was also the 2% Ho: Lu2O3. Although a smaller average particle
size does not necessarily result in a more transparent ceramic, it is clear that Holmium doping
does affect the Lutetium Oxide powder. Thus, more research must be done to conclusively say
that a 2% doping concentration is the most effective and to verify and extend the results of this
research.
II. Introduction
Solid-state lasers refer to those that use a gain medium that is a solid as opposed to a
liquid or gas. Such a medium is composed of a crystalline or glass host material most commonly
doped with ions of a rare-earth element. Since the excited states of rare-earth elements do not
interfere with the crystalline lattice structure, they are most effective for lasing with a low optical
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pump brightness. 1 Solid state lasers are widely known for their reliability and consistency. they
are relatively low-maintenance, compact, and simple to use and produce once developed.
Further, there is a wide range of wavelength selection that can be obtained depending on the
materials used for fabrication. 2
Significant research has been focused on solid-state lasers since they were discovered and
developed beginning in the 1960s, as first published by E. Snitzer. This discovery led many more
researchers to focus their attention in the laser, optical, and fiber optics fields, and thus many
more related inventions were developed. 3 Today, solid state lasers have applications in
medicine, spectroscopy, optical industry and materials for communication, as well as research
for weapons and sensing materials, such as for the U.S. Department of Defense . 2
Ruby was the original lasing material, and although ruby lasers are still used to some
extent today, they have a very low efficiency and are thus not very effective or economical. 1
Although researchers have developed lasers with a variety of host materials and dopants, the
most common are neodymium-doped YAG (Yttrium Aluminum Garnet). Neodymium-doped and
Ytterbium-doped glasses are common for high-power lasers that can be utilized for welding and
marking metals and other materials, as well as spectroscopy. 1 Typical issues regarding the use of
solid-state lasers are related to thermal concerns, as power from the optical pump may produce
excess heat and thus reduce efficiency. 4
The goal of this research is to better understand how various doping concentrations of
Holmium affect the characteristics of Lutetium Oxide powders, and how that information can be
used to better develop ceramics for high power solid-state lasers. The transparency of ceramics is
affected by several factors, including the scattering of light and absorption related to chemical
purity. Thus, it is essential to maintain a high-purity powder before it is subject to the
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densification (hot-press) process. As previously demonstrated, the coprecipitation method is an
effective way to conveniently synthesize high-purity powders, and is also suitable for mass
production. 5
III. Materials and Equipment Used:
• Ho2O3 powder- Stanford Materials, Aliso Viejo, California
• Lu2O3 powder- Stanford Materials, Aliso Viejo, California
• nitric acid (99.999% HNO3)- Alfa Aesar, Ward Hill, Massachusetts
• ammonium hydroxide (99.99+% NH3)- Alfa Aesar, Ward Hill, Massachusetts
• acetone (electronic grade (CH3)2CO)- Alfa Aesar, Ward Hill, Massachusetts
• centrifuge
• peristaltic pump
• crucible
• hot plate
• mortar and pestle
• various glassware
• Horiba LA-950 particle size analyzer- Tokyo, Japan
• X-ray diffractometer Model XDS2000- Scintag Inc., Cupertino, California
• Scanning Electron Microscope (SEM)
IV. Procedures
In order to synthesize Ho:Lu2O3 powders of various doping concentrations (0.1%, 2%,
and 5%), the coprecipitation method was used. Although six samples were synthesized, due to
uncontrollable circumstances resulting in contamination during the process, not all of these
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samples were subject to further analysis. Due to a low yield from Samples 2 and 3, these two
were combined to serve as the 2% Ho sample. Sample 5 is the 0.1% Ho sample referred to in this
paper, and Sample 6 is the 5% Ho sample as discussed.
To begin, the appropriate amount of Lu2O3 and Ho2O3 powders for each concentration
were dissolved in nitric acid (400mL for Samples 2/3, 200mL for Samples 5/6), which was
typically heated to a temperature between 200°C and 215°C while subject to stirring. The nitric
acid (HNO3) was filtered through a 0.8μm filter media to eliminate any potential particle
impurities prior to use. Although it varied slightly between samples, after about an hour of
boiling, more nitric acid was added as needed and the hot plate temperature was increased to
about 225°C in order to boil off the solution until it was at the point of saturation. This was
followed by slow cooling resulting in a mixture of Lutetium and Holmium nitrates in crystalline
form. This mixture was then rinsed with deionized water to assist with purification, followed by
the addition of more deionized water to create a new solution (increased to 500mL for samples
2/3, and 350mL for samples 5/6) so that the recrystallization process could be repeated two more
times and a highly purified nitrates mixture achieved.
After the crystallization process was complete, deionized water was added to the
resulting solid to create a 500mL solution. This solution was slowly added to a heated 4L
containing a deionized water/ammonium hydroxide solution (~50-75°C) as 150mL of
ammonium hydroxide (NH3) was also being added at the same rate by a peristaltic pump
(~20mL/min) to keep the pH fairly constant. As the two solutions were being added to the large
beaker, they were subject to constant stirring by an electric stirring rod. During this reaction, a
white precipitate began forming near the surface of the solution, and stirring was continued for
approximately 1.5 hours. After this time, the solution was cooled to room temperature.
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The cooled mixture was next put into designated plastic jars with deionized water and
then placed in a centrifuge in order to separate the liquid in the solution from the wet solid
powder. This powder was washed with deionized water and the process was repeated four more
times. Two more of these washes were conducted with acetone to help further purify the powder
and remove any remaining water. Finally, the wet powder was dried in a crucible for
approximately 24 hours at about 115°C.
Once dried, the precipitate powder was subject to calcination for 6 hours in air at 600°C
in order to convert the precursor into an oxide. Following this process, each sample of powder
was ground up by hand using a mortar and pestle, and sieved through a plastic membrane in
order to prepare it for analysis. The median particle size and related data regarding distribution
was obtained by laser diffraction and scattering using a the Horiba LA-950 equipment. This
system sonicates the samples until the particle size distribution remains fairly constant for two
consecutive trials, and then performs the laser diffraction and scattering. Other characteristics of
the powder, including purity and polycrystalline structure, were determined using an X-ray
diffractometer (XRD) and compared to database information on Lutetium and Holmium. The
samples were also analyzed under a scanning electron microscope (SEM) at various levels of
magnification in order to further investigate the morphology of the powders and determine
whether mostly hard or soft agglomerates were present.
V. Results and Discussion
The XRD patterns are shown in Figure 1 parts (a), (b), and (c), each corresponding to a
different doping concentration. As visible in the images for all of the concentrations, the
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demonstrated peaks match up fairly well to the database information, considering the
introduction of the Holmium into the Lutetium crystal structure.
As determined by the Horiba particle size system analysis, the reported particle size for
each concentration is somewhat unexpected. Figure 2 parts (a), (b), and (c) report the median
particle size for the 0.1% Ho: Lu2O3 to be ~12.08μm , the median particle size for the 2%
Ho:Lu2O3 to be ~4.97μm, and the median particle size for the 5% Ho: Lu2O3 to be ~10.25μm.
According to this data, the median particle size does not directly correspond to the doping
concentration of Holmium as an increase in the concentration did not always result in an
increased or decreased particle size. Thus, it is difficult to determine if other variables affected
the results.
Figure 3 parts (a), (b), and (c) are the scanning electron microscope images at a 5000x
magnification. Part (a), CLHoLu-1, refers to the 0.1% Ho doping concentration (Sample 5). Part
(b), CLHoLu-2, refers to the 2% Ho doping concentration (Sample 2, 3). Part (c), CLHoLu-3,
refers to the 5% Ho doping concentration (Sample 6). As shown, the morphology of all of the
powders are quite similar and are composed of mainly soft agglomerates. This is most likely a
result of their synthesis by the coprecipitation method. However, none of the images demonstrate
an entirely uniform morphology, which has an effect on the transparency of the final polished
ceramic. When the hot pressed ceramics were compared, the 2% Ho doping concentration
resulted in ceramics that were the most was the most transparent, followed by the 5% Ho:Lu2O3
sample and the 0.1% Ho:Lu2O3 sample.
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VI. Conclusion
In summary, the analyzed data clearly demonstrates that the doping concentration of
Holmium has an effect on the crystalline characteristics of the Lutetium oxide powder. The
sample with the smallest median particle size was the 2% Ho: Lu2O3 (4.97μm), followed by the
5% Ho: Lu2O3 (10.25μm), and the 0.1% Ho: Lu2O3 (12.08μm). The sample that resulted in the
most transparent hot-pressed ceramic was also the 2% Ho: Lu2O3. However, it is known that a
smaller average particle size does not result in a more transparent ceramic. Thus, although it is
clear that Holmium doping does affect the Lutetium Oxide powder, more studies must be
conducted on this topic to conclusively say that a 2% doping concentration is the most effective
for producing a smaller particle size and to determine what exact characteristics lend to a more
transparent ceramic. Further research may also focus on the use of other rare-earth elements as
dopants with Lutetium Oxide powder, or the use higher doping concentrations.
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VII. Appendices
Fig. 1- X-ray Diffractomotry (XRD) Images:
(a): Sample 5, 0.1% Ho: Lu2O3
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(b): Samples 2/3, 2% Ho: Lu2O3
(c): Sample 6, 5% Ho: Lu2O3
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Fig. 2- Horiba Particle Size Analysis:
(a): Sample 5, 0.1% Ho: Lu2O3
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(b): Samples 2/3, 2% Ho: Lu2O3
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(c): Sample 6, 5% Ho: Lu2O3
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Fig. 3- Scanning Electron Microscope (SEM) Images:
(a): Sample 5, 0.1% Ho: Lu2O3
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(b): Samples 2/3, 2% Ho: Lu2O3
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(c): Sample 6, 5% Ho: Lu2O3
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VIII. Acknowledgements
I would like to thank my SEAP mentor Dr. Woohong (Rick) Kim, as well as Dr. Colin C.
Baker for their oversight and guidance with this research project. I also greatly appreciated the
companionship and advice from Andrew Miller and Bryan.
IX. Bibliography
1 "Schawlow and Townes invent the laser". Lucent Technologies. 1998. http://www.bell-
labs.com/about/history/laser/. Retrieved 2011-8-10.
2 Yehoshua Kalisky, The Physics and Engineering of Solid State Lasers,, Bellingham, WA
ISBN 9780819480460, DOI: 10.1117/3.660249 http://link.aip.org/link/doi/10.1117/3.660249
3 C. Stewen, M. Larionov, and A. Giesen, “Yb:YAG thin disk laser with 1 kW output power,”
in OSA Trends in Optics and Photonics, Advanced Solid-State Lasers, H. Injeyan, U. Keller,
and C. Marshall, ed. (Optical Society of America, Washington, DC., 2000) pp. 35-41.
4 Campbell, J. H., Hayden, J. S. and Marker, A. (2011), High-Power Solid-State Lasers: a
Laser Glass Perspective. International Journal of Applied Glass Science, 2: 3–29.
doi: 10.1111/j.2041-1294.2011.00044.x
5 Hongzhi Wang, Lian Gao, Koichi Niihara, Synthesis of nanoscaled yttrium aluminum garnet
powder by the co-precipitation method, Materials Science and Engineering A, Volume 288,
Issue 1, 31 August 2000, Pages 1-4, ISSN 0921-5093, DOI: 10.1016/S0921-5093(00)00904-7.
(http://www.sciencedirect.com/science/article/pii/S0921509300009047)