synthesis of high purity power for solid state lasers

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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:


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Fig. 3- Scanning Electron Microscope (SEM) Images:


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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)

synthesis of high purity power for solid state lasers

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