A Thesis Presented to Faculty of Alfred University PHOTOCHROMISM in RARE-EARTH OXIDE GLASSES by Charles H. Bellows in Partial Fu

A Thesis Presented to Faculty of Alfred University PHOTOCHROMISM IN RARE-EARTH OXIDE GLASSES by Charles H. Bellows In Partial Fulfillment of the Requirements for The Alfred University Honors Program May 2016 Under the Supervision of: Chair: Alexis G. Clare, Ph.D. Committee Members: Danielle D. Gagne, Ph.D. Matthew M. Hall, Ph.D. SUMMARY The following thesis was performed, in part, to provide glass artists with a succinct listing of colors that may be achieved by lighting rare-earth oxide glasses in a variety of sources. While examined through scientific experimentation, the hope is that the information enclosed will allow artists new opportunities for creative experimentation. Introduction Oxides of transition and rare-earth metals can produce a multitude of colors in glass through a process called doping. When doping, the powdered oxides are mixed with premade pieces of glass called frit, or with glass-forming raw materials. When melted together, ions from the oxides insert themselves into the glass, imparting a variety of properties including color. The color is produced when the electrons within the ions move between energy levels, releasing energy. The amount of energy released equates to a specific wavelength, which in turn determines the color emitted. Because the arrangement of electron energy levels is different for rare-earth ions compared to transition metal ions, some interesting color effects can arise. Some glasses doped with rare-earth oxides fluoresce under a UV “black light”, while others can express photochromic properties. Photochromism, simply put, is the apparent color change of an object as a function of light; similar to transition sunglasses. This thesis explores such photochromism in rare-earth oxide glass samples. I hypothesized that the reason for this is small ranges of color the glass absorbs, combined with gaps in the ranges of color certain light sources emit. [ii] Procedure The samples were made by mixing various rare-earth oxides with the glass frit used in the art department, allowing for the most compatible data for the artists, and then melting the mixture. Eight samples, each with different dopants, were produced. The samples were then observed under isolated lighting conditions to see what color they were. The light sources included: sunlight, incandescent, fluorescent, and LED. A photograph of each sample, under each light source was taken. As anyone skilled in photography knows, the colors you observe don’t always match the colors in the photo. Cameras register color and light differently than the human eye, adding discrepancies in color between the two devices. Additionally, a picture viewed on a computer screen can look completely different than the same picture after it has been printed out. To correct for these discrepancies, a color standard was used to normalize the observed pigments. The color standard assigns numbers and coordinates to colors to ensure exact replication. Determining the standard color of each sample under each light source was done by a RAL color standard wheel. RAL color scaling is a German color standard that assigns a 4-digit number to each color. The number can then be converted to other color standard systems, allowing one to choose the best system that fits their needs. The RAL color standard wheel itself resembles a paint swatch book; but the colors are guaranteed standards. It should be mentioned that, while standardized, the color assignments are still only approximations of the true color of the samples. At first glance, the RAL color standard containing colors seems like a lot; but it does not account for every possible color. For example, the praseodymium oxide sample appeared a yellow- green color. The RAL color scaling did not even have an adequate approximation, and therefore another color standard, Hex, had to be used for that particular sample. [iii] Through color leveling in Photoshop, colors were balanced similar to setting a white balance on a camera. This allowed the photographs to appear closer to what was observed with the naked eye. Correction was based on using the terbium oxide sample, which was clear under all lighting conditions, as the standard. Additionally, a white sheet of paper was used as a standard, neutral background. All photographs, raw and color corrected, can be seen in the Appendix. While the corrected images are more accurate to the observed colors, the raw images were included to showcase the colors and variations in “white light” emitted by the different light sources. Using a special type of film over the camera lens, spectra of the different light sources were photographed. The film, known as a diffraction grating, acts similarly to a crystal prism; separating the white light into the component rainbow colors. Thus, one could see what colors combined to make the white light of each source. Results Most of the samples did not exhibit photochromism, staying relatively the same color under every lighting condition. However, the samples containing samarium ions (Sm3+) and dysprosium ions (Dy3+) fluoresced under black light. The Sm3+ sample glowed a bright red orange and the Dy3+ sample glowed a pale ivory color. These, however, were not investigated further, because the focus was placed upon reasons why certain glasses changed color with variations in visible light. The only two samples to produce any photochromic qualities were the two samples containing neodymium ions (Nd3+) and holmium ions (Ho3+). In every light source except fluorescent, the Nd3+ sample appeared violet; while appearing blue under fluorescent lighting. The Ho3+ sample exhibited a similar trait; salmon pink under fluorescent, yellow under all others. [iv] Because one of the samples was doped with both Nd3+ and Ho3+, it was expected that that sample would also show photochromic qualities; however, the sample, for the most part, remained a neutral blue-grey without any drastic color changes. Spectroscopy was then conducted to determine what colors Nd3+ and Ho3+ absorbed, and what colors were transmitted. It turned out that the two ranges of wavelengths Nd3+ absorbs correspond to a blue and reddish-purple color. Similarly, Ho3+ possesses wavelength ranges that have complementary colors of yellows, reds, and oranges. When looking at the photographs of the light spectra, most showed the standard, continuous rainbow gradient from red to violet. Interestingly, fluorescent lighting did not. Instead, its spectra showed black bars separating the colors. This indicated that there were gaps of wavelength ranges where the fluorescent lighting did not emit that color of light. Conclusions As expected, the different arrangements of electron energy levels cause the rare-earth ions to absorb only very small ranges of wavelengths. When photochromism is exhibited, it is due to the fact that these narrow ranges coincide with small gaps in the emission spectra of various lights. If the light does not emit that wavelength range, it cannot be absorbed by the glass sample. If it is not absorbed, its complementary color is not observed. When under fluorescent lights, the light does not emit the wavelengths necessary for the reddish-purple of the Nd3+. Therefore only the blue can be seen. Likewise, the light does not emit the wavelengths corresponding to Ho3+’s yellow, leaving some red and orange that appears as salmon pink. Under other light sources, the reds and oranges are usually canceled out, leaving only the yellow. [v] Further work As the sample containing both Nd3+ and Ho3+ contained the same amount of 2g of dopant as the other samples, it only contained 1g of each of the individual oxides. Due to the lower dopant concentration, there may have not been enough rare-earth oxide to produce a noticeable photochromic effect. The two ion types may have also negated each other’s coloration, resulting in the greyer tones. It would be an interesting study to further explore the potential photochromism in samples containing both Nd3+ and Ho3+. [vi] ACKNOWLEDGEMENTS I would like to extend my thanks to: Dr. Alexis Clare, my thesis advisor, for her guidance and expertise in the subject matter, Drs. Danielle Gagne and Matthew Hall, my Honors committee members, for their support and suggestions, Professor Angus Powers and the Alfred University Hot Glass Studio for the System 96® glass frit used in the fabrication of the samples, The Kazuo Inamori School of Engineering, NYS College of Ceramics at Alfred University, for access to laboratory equipment and the rare-earth oxides, As well as everyone else who helped in this process. [vii] TABLE OF CONTENTS Page I. Introduction 1 II. Experimental Procedure 3 A. Fabrication 3 B. Photography 3 C. Color Analysis 5 D. Spectroscopy 6 III. Results 6 A. Tb3+, Sm3+ & Dy3+ 7 B. Pr3+ & Er3+ 8 C. Nd3+, Ho3+ & Nd3+-Ho3+ 9 D. Emission Spectra 11 IV. Discussion 12 A. Nd3+ 14 B. Ho3+ 16 V. Conclusion 17 VI. Future Work 18 VII. Appendix 19 VIII. References 37 [viii] LIST OF TABLES Page Table I: Spectral colors and their complements 2 Table II: Coloration of each sample under all lighting conditions 7 Table III: All colors observed with CIELab and RGB conversions 19 Table IV: Specifications of all light sources utilized 20 [ix] LIST OF FIGURES Page Figure 1: Visual representation of CIELab coordinates 2 Figure 2: Setup for photographing samples under various lighting 4 conditions Figure 3: Tb3+ sample under soft white incandescent a) raw and b) 8 corrected Figure 4: Nd3+ sample under a) soft white CFL and b) soft white 9 incandescent Figure 5: Absorption/Transmittance

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