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Spectroscopic Signatures of Sub-Second Laser-Calcined Dy Doped Oxide Precursors for Use in Ex-Situ Thermal Impulse Sensors Benjamin Richard Anderson, Ray Gunawidjaja, and Hergen Eilers J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06692 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 5, 2017

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The Journal of Physical Chemistry

Spectroscopic Signatures of Sub-Second Laser-Calcined Dy3+-doped Oxide Precursors for Use in Ex-situ Thermal Impulse Sensors Benjamin R Anderson,∗ Ray Gunawidjaja, and Hergen Eilers∗ Applied Sciences Laboratory, Institute for Shock Physics, Washington State University, Spokane, WA 99210-1495 E-mail: [email protected]; [email protected]

∗

To whom correspondence should be addressed

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ACS Paragon Plus Environment


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Abstract We measure the phase-dependent spectroscopic signatures – photoluminescence (PL) emission spectrum, PL excitation spectrum, and PL lifetime – of three different heat treated Dy3+ -doped oxide precursors (TiO2 , Y2 O3 , and ZrO2 ), which are calcined at temperatures ranging from 440 K to 1256 K for 100 ms. Based on these spectroscopic measurements we compute temperature calibration curves, which correlate a given spectroscopic signature to a specific calcination temperature. We find that the emission/excitation spectra-based calibration curves for a given material, in general, have the same transition temperature within uncertainty, but the lifetimebased calibration curve is found to transition at a lower temperature. This difference is due to the emission/excitation-based curves depending on crystallization of the oxide hosts, while the lifetime changes due to the decomposition of the precursor compound, which eliminates some PL quenchers, including high energy O-H, C-O, and C-O2 vibrations. The observation of two different kinetic phenomenon in a single oxide host has promising applications in ex-situ thermal impulse sensors, which have uses in explosive fireballs, combustion science, and arson investigations.

Introduction Currently, the primary method of combating chemical and biological weapons of mass destruction (WMD) in access denied areas is the use of explosive payloads, which detonate and incinerate the WMD agent. This incineration of the WMD agent requires that the explosive fire ball maintains a sufficient temperature over an adequate duration. This combination of temperature and duration is known as the explosive’s thermal impulse (TI). Despite many explosives being developed for this purpose, it is unclear whether these explosives posses the necessary TI to destroy these WMD agents. 1 To determine if such agents are exposed to the requisite TI for destruction in an explosion, it is necessary to have an accurate determination of the heating profiles of nanoscale particles within the explosive fireball. This 2


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task is nontrivial as current methods for in-situ temperature measurements – thermocouples and pyrometry – are ill-suited to measuring the thermal impulse of nanoscale particles in an explosion due to the fast time scales, high temperatures, opacity of the fireball, and the motion of particles within an explosion. 2,3 To address these difficulties we have developed an ex-situ TI sensor technique utilizing nanoscale lanthanide-doped oxide precursors. 4,5 These precursors undergo irreversible phase transformations when heated 4–12 resulting in the dopant lanthanide ions experiencing different crystal fields. These different crystal fields change the spectroscopic properties – photoluminescence (PL) emission spectrum, PL excitation spectrum, and PL lifetime – of the lanthanide ions, 13–25 allowing for the ions’ spectroscopic properties to be correlated to a given host phase and therefore a given TI. By performing an in-lab calibration – with isothermal heating profiles – these spectroscopic properties can be used to determine the isothermal-equivalent TI of a heating event with an unknown temperature profile. Thus far, we have primarily focused on temperature-only sensors (i.e. the duration remains unknown) using three different oxide precursors, including: TiO2 , 26 ZrO2 , 9–12 and Y2 O3 , 6–8 with the lanthanide dopant primarily being Eu3+ . While we have demonstrated the use of a single Eu-doped host as a temperature sensor, to determine both the temperature and duration of a heating event simultaneously, it requires a second sensor material consisting of both a different host and a different dopant lanthanide. The requirement of two different host materials is a consequence of the temperature and duration calculation requiring two unique kinetic equations 4 and the requirement of a different lanthanide is due to the need to keep the lanthanide probes’ emissions spectrally separated to minimize cross contamination of their spectra. As we have already demonstrated the usefulness of p-TiO2 , p-ZrO2 and p-Y2 O3 (where the p- denotes the precursor form of the material) for the purpose of ex-situ temperature sensors (using a Eu-dopant), we next consider how the spectroscopic signature of a different lanthanide behaves when doped into each of these host materials. To this end we choose

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Dy3+ as our second lanthanide ion, as it possesses strong PL in the visible range and its PL spectrum does not overlap with Eu3+ . 13 Therefore in this study, we measure its spectroscopic signature (PL excitation/emission spectra and lifetime) when doped into precursors of TiO2 , ZrO2 and Y2 O3 at different calcination temperatures for a fixed duration of 100 ms. Once the spectroscopic signatures are determined at different calcination temperatures we analyze these signatures using several intensity-ratio-based techniques to determine temperature calibration curves. These curves quantify the influence of irreversible phase changes on the spectroscopic properties, which can then be used to determine an unknown calcination temperature from the spectroscopic properties of a sample exposed to an unknown heating event. By combining these Dy-doped precursors with Eu-doped precursors (each of which displaying unique kinetics) we can also determine the duration of a heating event and thereby obtain its full TI. 4,27 We forsee these sensors being useful in explosive fireball studies, combustion research, arson forensics, and other applications requiring TI measurements of nano/micron scale particles.

Background Thermally-induced irreversible phase changes in TiO2, Y2 O3 and ZrO2 To provide context for the spectroscopic changes we observe in rapidly heated Dy3+ -doped oxide precursors, we first provide an overview of the relevant phase transformations of each host material. These transformations have been well studied in the context of lanthanidebased phosphors, where the three host materials (p-TiO2 , 26,28–30 p-Y2 O3 , 19,22–24,31–38 and p-ZrO2 39–45 ) are calcined for extended durations, typically on the order of an hour.

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TiO2 The precursor material for TiO2 is titanium hydroxide, Ti(OH)4 , which undergoes several different phase transformations when heated, with the first being a decomposition reaction of Ti(OH)4 → TiO2 +2H2 O. Once heated above ≈ 640 K nucleation occurs and anatase crystallites form. 26 Anatase TiO2 can then transform into the rutile phase when the material is heated to temperatures above ≈ 873 K. 46 Additionally, TiO2 has two other phases (TiO2 (B) and brookite), but neither are observed for our heating conditions. Both anatase TiO2 and rutile TiO2 have a tetragonal crystal structure with the Ti4+ ions having D2d symmetry for the anatase phase and D2h symmetry for the rutile phase. However, it has been shown that when substituting Eu3+ into the TiO2 lattice, the difference in oxidation state and ionic radius of Eu3+ and Ti4+ results in lattice distortions that cause the D2d symmetry to lower into: C1 , C2v , and D2 symmetries. 47 The C1 sites are believed to be located near or at the surface. ZrO2 To produce crystalline ZrO2 we begin with zirconium hydroxide Zr(OH)4 , as our precursor material.

When heated Zr(OH)4 undergoes a decomposition reaction (Zr(OH)4 →

ZrO2 +2H2 O) to produce ZrO2 and water. Once the precursor is heated above ZrO2 ’s crystallization temperature (≈673 K), nucleation and grain growth occurs with the ZrO2 forming the metastable tetragonal phase (t-ZrO2). 9 If the temperature is further increased (to above ≈ 1073 K) the metastable t-ZrO2 will transform into monoclinic ZrO2 (m-ZrO2). 10–12 While m-ZrO2 is often observed for long heating durations, it is almost always absent for subsecond to several seconds heating. 11,27 With regards to site symmetry, Zr4+ in t-ZrO2 has D4h symmetry and in m-ZrO2 it has C2h symmetry. 48

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Y2 O3 The final host material we use is precursor Y2 O3 which consists of Y2 (CO3 )2 . 49,50 Once again the first phase change experienced upon heating is a decomposition reaction: Y2 (CO3 )2 → Y2 O(CO3 )2 → Y2 O2 CO3 → Y2 O3 . As the p-Y2 O3 is heated to higher temperatures (above ≈ 973 K) the decomposition product undergoes nucleation and grain growth to form cubic Y2 O3 (c-Y2 O3 ), with the Y3+ ions occupying sites having either an S6(C3i ) or C2 symmetry.

Optical properties of Dy3+ With the three host materials described we now provide a brief overview of Dy3+ ’s optical properties. Dy3+ is a well characterized lanthanide ion used in many different applications for its optical properties, such as: lasers, 51–53 two-color thermometry, 54–63 photocatalysts, 64 and LEDs. 32,34,34–36,39,65–67 It is most readily excited using wavelengths in the UV or blue spectral regime with its main visible emission lines occurring at approximately 480 nm, 573 nm and 660 nm. Figure 1 shows an approximate Dy3+ energy level diagram with the primary transitions shown and labeled with their wavelengths. Note that the energies/wavelengths shown are approximate as the precise location of the energy levels depends on the host material. 13,68–70 While the transitions in Figure 1 are shown occurring between narrow energy levels with a single central wavelength, in reality – due to host material effects – these transitions can be broadened (both homogeneously and inhomogeneously) and/or split into several closely spaced transitions due to Stark splitting of the energy levels. Homogeneous broadening of the spectra occurs due to a number of mechanisms, including: size-dependent phonon interactions, surface charge distributions, dipole-dipole interactions between ions, and magnetic fluctuations due to spins in the crystal lattice, 71,72 while inhomogeneous broadening is due to ions residing in distribution of sites within the host material, such that there are different crystal fields experienced by the ions at the different sites. 13,71 This sensitivity of Dy3+ ’s optical transitions to the crystal field makes it a good optical probe of a host material’s 6


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Figure 1: Approximate energy levels of Dy3+ ion, with important optical transitions marked by their corresponding wavelength. symmetries and crystallinity, with the transitions being broad in amorphous materials and narrow in crystalline materials. While all of Dy3+ ’s electric dipole and quadrupole transitions are sensitive to the host material, Dy3+ also has two induced dipole transitions that are more sensitive to changes in the ion’s local crystal field. These transitions are the so called hypersensitive transitions and both can be observed in the excitation/absorption spectra with one transition located in the visible: 6 H15/2 → 4 I15/2 , 4 G11/2 (427 nm) and one located in the near infrared: 6 H15/2 → 6

F11/2 (1300 nm). 13

Method Sample Preparation For our study we prepare three different Dy-doped host materials: precursor Dy:TiO2 , precursor Dy:ZrO2 and precursor Dy:Y2 O3 . Both p-Dy:ZrO2 and p-Dy:TiO2 are prepared by coprecipitation 8,26 with a Dy concentration of 1 mol%. This method produces gels that are

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dried to give cakes of the precursor material which are subsequently crushed using a mortar and pestle and finally passed through a 100 mesh sieve to yield a powder with particles less than 149 µm. While p-Dy:ZrO2 and p-Dy:TiO2 are prepared by coprecipitation, the p-Dy:Y2 O3 samples are prepared by homogeneous precipitation, 6 which results in spherical nanoparticles with a diameter of 345 ± 30 nm. Once the precursor materials are prepared, we calcine samples of each precursor at different temperatures using a PID controlled indirect laser heating system, which consists of a graphite sample holder, K-Type thermocouple, PID loop controller and a Synrad Firestar Ti100 CO2 laser. 5,10 This heating method allows us to obtain temperatures up to 1273 K with an isothermal duration of 100 ms. For each calcination temperature we repeat the same heating profile on three different precursor samples to average our results over possible variations in the the heating profile and the material. Once the different precursor samples are calcined at different temperatures (for a duration of 100 ms), their spectroscopic signatures are measured and analyzed, allowing us to correlate specific spectroscopic changes to the calcination temperature.

Spectroscopy Spectroscopic measurements of the different samples are performed using a custom built PL spectrometer which consists of: a Continuum Panther OPO that is pumped by a frequency tripled Nd:YAG laser (Continuum Powerlite 8000, 355 nm, 10 ns), a SpectraPro 2750 monochromator with an attached PMT, and various optics for focusing the pump beam and collecting the emission from the samples. Additionally, for lifetime measurements, we connect the PMT to a Tektronix DP4010 oscilloscope. Using our PL spectrometer we measure PL emission/excitation spectra and lifetimes of the different samples to characterize the influence of the calcination temperature on the Dy ions spectroscopic properties in each host material. Once the spectra are obtained we quantify the phase transformation induced spectral changes using two different ratio-based 8


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techniques; with the first technique being the computation of the ratio of intensities at two different wavelengths, while the second technique involves taking the ratio of the integrated peak intensities of two different transitions. Table 1 lists the different excitation and emission electronic transitions of interest, with their approximate central wavelengths and the spectral integration ranges used in our analysis. Table 1: Tabulation of relavent Dy3+ excitation and emission electronic transitions used in this study, with the approximate amorphous peak locations and peak integration ranges used in this study. Transition H15/2 → 4 G11/2 6 H15/2 → 6 I15/2 6 H15/2 → 4 F9/2

6

Transition F9/2 → 6 H15/2 4 F9/2 → 6 H13/2

4

Excitation Approximate Center 425 nm 450 nm 470 nm Emission Approximate Center 480 nm 575 nm

Integration Range 410 nm – 435 nm 435 nm – 460 nm 460 nm – 490 nm Integration Range 460 nm – 500 nm 550 nm – 600 nm

Calculating the relevant ratios for each sample results in three different ratios for each Dy-doped host and calcination temperature (due to repeating the heating profiles three times). These three ratios are averaged to produce a single ratio for a given calcination temperature and host material. After averaging, this ratio is then plotted as a function of calcination temperature, allowing us to quantify the influence of sub-second laser heating on the spectroscopic properties of the different heated precursors. In general, we find that this ratio as a function of temperature behaves as a modified Arrhenius function, 4

( ) β T0 f (T ) = f0 + fA exp − T

(1)

where f0 is an offset, fA is an amplitude parameter, T0 is the characteristic temperature and β is a stretch parameter. 4,73–77 The offset f0 , represents the initial ratio of the precursor 9



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material, while the sum f0 + fA represents the asymptotic ratio as the temperature increases, where fA can be either positive or negative depending on the ratio used. Note that Equation 1 is phenomenological and therefore should not be understood as representing an underlying kinetic model.

Results and Discussion p-Dy:TiO2 The first host material we consider is p-TiO2 , which is heated to multiple isothermal temperatures ranging from 293 K to 1079 K for an isothermal time of 100 ms. Once heated, we measure each iteration’s PL spectrum (Figure 2a), excitation spectrum (Figure 2b) and PL lifetime (Figure 2c) at each isothermal temperature. For the PL spectra we use 355 nm excitation, to obtain the excitation spectra we measure the emission intensity at 576 nm, and for the lifetime measurements we measure time dependent luminescence decay at 576 nm.

(a)

(b)

(c)

Figure 2: (a) PL emission spectra (355 nm excitation), (b) PL excitation spectra (measured at 575 nm), and (c) PL lifetimes of heated and unheated p-Dy:TiO2 .

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Emission We begin our analysis of p-Dy:TiO2 spectroscopic properties by considering its PL emission at different calcination temperatures as shown in Figure 2a. We find, from Figure 2a, that the spectra consist of three broad peaks centered near 480 nm, 575 nm, and 650 nm. These peak locations correspond to the 4 F9/2 → 6 H15/2 ,4 F9/2 → 6 H13/2 and 4 F9/2 → 6 H11/2 transitions, respectively. Upon initial inspection of the emission spectra of p-Dy:TiO2 we find that for all temperatures tested the PL emission peaks remain broad implying that the Dy3+ ions occupy a wide distribution of sites within the host lattice. This broad distribution can be attributed to the charge and size mismatch between the Dy3+ and Ti4+ ions, which causes lattice distortions near the Dy3+ ions. 26,47 While less obvious than sharp peak formation, the relative area of the three peaks is found to change with increasing temperature with the largest effect found by comparing the peak near 480 nm (4 F9/2 → 6 H15/2 ) and the peak near 575 nm (4 F9/2 → 6 H13/2 ). This change in relative peak area physically represents a modification in the relative transition strengths. We compute the integrated intensity ratio by integrating the area under each peak, with the resulting ratio shown in Figure 3. From Figure 3 we find that the ratio as a function of temperature can be fit to Equation 1 with it’s characteristic temperature being 670 ± 14 K and β = 6.6 ± 1.0. This characteristic temperature is found to be higher than that observed for furnace heated p-Eu:TiO2 (≈ 600 K), 26 with this result anticipated given our shorter heating duration. 4 Given this characteristic temperature and our previous observations for p-Eu:TiO2 26 we conclude that the changing peak ratio is due to the amorphous-to-anatase phase transition. As a final observation concerning the emission spectra of p-Dy:TiO2 we note that the overall emission intensity drastically decreases for the samples heated above 800 K. This decrease in intensity also corresponds to a decrease in the luminescence lifetime of the heated p-Dy:TiO2 , as seen in Figure 2c. Both of these effects can be attributed to the anatase-to11



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Figure 3: Integrated peak ratio (575/480) as a function of temperature for heated p-Dy:TiO2 . The ratio is found to fit to Equation 1 with a characteristic temperature of 670 ± 14 K and offset and amplitude parameters of f0 = 1.693 ± 0.20 and fA = 1.67 ± 0.16, respectively. rutile phase transformation, as the lattice volume shrinks during this transformation and causes the Dy3+ ions to be ejected from lattice sites. These ejected Dy3+ ions can then result in the formation of a secondary phase, which most likely has poorer PL properties. 26,78–80 Excitation Next, we consider the excitation spectra of p-Dy:TiO2 for different isothermal temperatures (which are shown in Figure 2b. From Figure 2b we find that the excitation spectra of the as-prepared material (i.e. unheated) has three broad peaks centered at approximately 430 nm, 451 nm and 470 nm, which correspond to the 6 H15/2 → 4 G11/2 , 6 H15/2 → 4 I15/2 , and 6

H15/2 → 4 F9/2 transitions, respectively. The broad excitation spectra are characteristic of

the amorphous structure of p-TiO2 . As the p-Dy:TiO2 is heated we find, from Figure 2b, that the excitation spectral peaks remain broad overall, but that a narrower peak begins forming near 451 nm. Additionally, the relative peak heights of the excitation spectra are found to change as the isothermal temperature increases. We quantify this change by computing the intensity ratio of the 430 peak to the 451 peak as shown in Figure 4. The ratio is found to follow Equation 1 with a

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characteristic temperature of 708 ± 34 K and a stretch parameter of β = 7.1 ± 1.75, which are within uncertainty of the values obtained for the PL emission measurement. As with the characteristic temperature measured for the emission ratio, this temperature corresponds to the amorphous-to-anatase phase transition.

Figure 4: Excitation peak intensity ratio (430/451) as a function of isothermal temperature. The ratio is found to fit to Equation 1 with a characteristic temperature of 708 ± 34 K and offset and amplitude parameters of f0 = 0.700±0.0.22 and fA = −0.480±0.075, respectively.

Lifetime The final optical property of p-Dy:TiO2 we measure is the photoluminescence lifetime of the 4 F9/2 → 6 H13/2 transition as shown in Figure 2c. From Figure 2c we find that the lifetime of the precursor material is approximately 14 µs and then subsequently increases with temperature to ≈ 26 µs at 673 K, but then rapidly decreases as the material is heated further. As the lifetime is found to behave differently with temperature (as compared to the ratios) we can no longer use Equation 1 to determine the characteristic temperature. However, we can roughly estimate this temperature by noting that the transition occurs between 475 K and 675 K. Taking the midpoint between these two temperatures as our characteristic temperature estimate we determine a value of 575 ± 100 K. The mechanism behind the initial increase in p-TiO2 ’s lifetime is as follows. p-TiO2 begins 13



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in its hydroxide form, which contain O-H bonds that act as PL quenchers, due to O-H’s high energy vibrations. 81,82 As the material is calcined at higher temperatures the hydroxide decomposes into TiO2 and water vapor. With the loss of the attached O-H bonds, the PL is no longer quenched and the lifetime increases. However, as the temperature continues to increase the anatase TiO2 undergoes a second phase transformation to the rutile phase, which ejects the Dy3+ from the Ti4+ sites. Once ejected the Dy3+ ions can form a secondary phase with a shorter PL lifetime. 26,78–80

ZrO2 While the emission and excitation spectra of p-Dy:TiO2 remain relatively broad over the whole temperature range tested, we find that heated p-Dy:ZrO2 provides a rich peak structure in both excitation and emission. Figure 5a shows the PL emission spectra at different calcination temperatures, Figure 5b shows the PL excitation spectra, and Figure 5c shows the PL lifetime as a function of calcination temperature. From Figures 5a and 5b we find that the emission/excitation spectra of sub-second calcinated p-Dy:ZrO2 begins to form sharp peaks in between 690 K and 895 K. This temperature region is consistent with previous measurements of the precursor-to-metastable tetragonal phase transition. 9–11

(a)

(b)

(c)

Figure 5: (a) PL emission spectra (355 nm excitation), (b) PL excitation spectra (measured at 575 nm), and (c) PL lifetimes of heated and unheated p-Dy:ZrO2 . The lifetime is found to be fit by Equation 1 with an offset and amplitude parameters of f0 = 14.63 ± 0.56 µs and fA = 342 ± 19 µs, respectively.

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Emission The first optical property of p-Dy:ZrO2 we consider as it transforms from the amorphous phase to the metastable tetragonal phase is the emission spectra. Figure 5a shows the emission spectra at different temperatures with both the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 peaks splitting into multiple narrow peaks as the material transforms. To quantify this peak splitting we perform multi-peak fitting of the spectra and characterize the peak positions in each spectra, as tabulated in Table 2. Table 2: Emission Peak locations of the 4 F9/2 → 6 HJ transition determined using multipeak fitting. Temperature 494 K 690 K 885 K

1089 K

1257 K

J = 15/2 479.21 ± 0.30 489.84 ± 0.31 479.62 ± 0.49 489.72 ± 0.55 477.94 ± 0.19 484.17 ± 0.14 491.21 ± 0.14 497.47 ± 0.22 478.09 ± 0.15 484.09 ± 0.10 491.57 ± 0.14 498.00 ± 0.16 478.08 ± 0.12 484.18 ± 0.10 492.65 ± 0.30 498.13 ± 0.14

J = 13/2 569.79 ± 0.22 578.17 ± 0.20 570.88 ± 0.18 577.92 ± 0.10 579.23 ± 0.12 584.92 ± 0.12 584.31 ± 0.19 591.20 ± 0.10 579.00 ± 0.12 585.09 ± 0.15 584.59 ± 0.12 591.27 ± 0.12 577.73 ± 0.15 584.55 ± 0.15 586.28 ± 0.25 591.27 ± 0.12

J = 11/2 665.25 ± 0.79 693.86 ± 0.16

672.4 ± 2.8 680.35 ± 0.51

672.4 ± 3.0 680.65 ± 0.37

668.8 ± 2.7 680.65 ± 0.37

From Table 2 and Figure 5a we find that the emission from p-Dy:ZrO2 consists of three broad peaks for temperatures through 690 K. However, once the temperature is in the 800 K range the broad peaks begin to split into distinct peaks. Specifically, both the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions each split into four distinct peaks and the 4 F9/2 → 6 H11/2 remains a single peak which narrows with increasing temperature. The numbers of distinct peaks corresponding to each transition are consistent with group theory predictions of Stark level splitting. 13 15



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To use p-Dy:ZrO2 as a temperature sensor we first need to quantify the influence of calcination temperature on the emission spectra. For this purpose we calculate four different intensity ratios: 571/590.5, 473/469.5, 571/584, and 473/484, which characterize the peak splitting of the 4 F9/2 → 6 H15/2 and 4 F9/2 → 6 H13/2 transitions. Note that these wavelengths are determined by comparing the spectra at different temperatures and locating the wavelengths at which the intensities display the largest changes. They are not the same wavelengths as the peaks determined in Table 2. These ratios are are shown in Figure 6.

Figure 6: Intensity ratios as a function of temperature for heated p-Dy:ZrO2 . Note that the wavelengths are chosen such that the change in ratio with increasing temperature is maximized. From Figure 6 we find that each ratio decreases with temperature and can be fit by Equation 1. This behavior is similar to that observed for the intensity ratios of Eu:ZrO2 . 4 Table 3 lists the characteristic temperature and stretch parameter for each ratio. From Table 3 we find that all four ratios have characteristic-temperatures within uncertainty of each other with the weighted average characteristic temperature being 729.3 ± 6.6 K and β = 16.0 ± 1.1.

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Table 3: Stretched exponential fit parameters from different ratios for pDy:ZrO2 . The average fit parameters are T = 729.3 ± 6.6 K and β = 16.0 ± 1.1 T0 β

571/590.5 728 ± 10 17.8 ± 1.6

473/469.5 743 ± 47 16.5 ± 5.5

571/584 473/484 725 ± 11.4 737 ± 14 14.7 ± 1.9 13.2 ± 2.8

Excitation While the excitation peaks for Dy:TiO2 remain broad over the full range of temperatures, the peaks of Dy:ZrO2 sharpen and split as the temperature increases from 690 K to 895 K. This characteristic temperature is consistent with previous determinations of ZrO2’s crystallization temperature. 10–12 To characterize the PL excitation peaks’ splitting we perform multipeak fitting of the excitation spectra and determine the peak locations as tabulated in Table 4. From Table 4 we find that as the temperature increases the number of underlying peaks increases, with the maximum number of component peaks per transition being four. These numbers of peaks are consistent with group theory predictions. 13 In addition to determining the excitation peak locations for heated p-Dy:ZrO2, we also calculate the intensity ratio of the intensity at 444.7 nm and 454.6 nm as a function of temperature in Figure 7. From Figure 7 we find that the excitation intensity ratio behaves similar to the emission intensity ratios, with the ratio following Equation 1 with a characteristic temperature of 723 ± 23 K and β = 17.8 ± 4.1, which are consistent with the values observed for emission. Lifetime Next we consider the influence of the calcination temperature on the emission lifetime of p-Dy:ZrO2 ’s 4 F9/2 excited state by measuring the time resolved PL emission of the 4 F9/2 → 6

H13/2 transition. The PL emission for each calcination temperature is found to follow a

single exponential as a function of time, which is fit to a simple exponential to determine

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Table 4: Excitation Peak locations of the 6 H15/2 → X transition determined using multipeak fitting. Temperature 494 K

X = 4 G11/2 424.522 ± 0.044 430.6 ± 1.1

X = 6 I15/2 444.58 ± 0.14 451.27 ± 0.14

X = 4 F9/2 470.640 ± 0.059

690 K

424.314 ± 0.048 428.54 ± 0.55

444.29 ± 0.16 450.67 ± 0.13

470.147 ± 0.076

885 K

423.453 ± 0.086 444.810 ± 0.023 425.130 ± 0.044 445.53 ± 0.11 424.371 ± 0.072 450.861 ± 0.045 453.774 ± 0.052

463.826 ± 0.074 468.483 ± 0.058 473.360 ± 0.088 480.100 ± 0.070

1089 K

424.156 ± 0.038 444.732 ± 0.019 425.230 ± 0.016 445.710 ± 0.150 450.793 ± 0.060 453.760 ± 0.082

463.783 ± 0.094 468.400 ± 0.063 473.279 ± 0.085 480.130 ± 0.076

1257 K

422.64 ± 0.14 425.33 ± 0.43

444.472 ± 0.019 451.72 ± 0.150 451.724 ± 0.060 453.760 ± 0.082

463.783 ± 0.094 468.400 ± 0.063 473.279 ± 0.085 480.130 ± 0.076

Figure 7: Excitation spectrum intensity ratio (444.7/454.6) as a function of temperature for heated p-Dy:ZrO2 . The ratio is found to follow Equation 1 with a characteristic temperature of 723±23 K and offset and amplitude parameters of f0 = 1.219±0.035 and fA = 1.26±0.10, respectively. 18


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the lifetime. Figure 5c shows the measured lifetime as a function of calcination temperature, with the lifetime found to follow Equation 1, with a characteristic temperature of 550 ± 16 K and β = 5.4 ± 1.5. This characteristic temperature is smaller than observed for both the ratios used in excitation and emission. The mechanism behind this discrepancy in the characteristic temperature is due to the different effects influencing each quantity. For the excitation/emission ratios, the primary mechanism influencing their value is the strength and symmetry of the host crystal field, which mainly depends on the crystallization of the host. On the other hand the lifetime depends on both the crystallinity of the host and the nonradiative decay rate of the Dy3+ ions in the host. The observation of the lifetime transitioning at a lower temperature suggests that the nonradiative decay rate is modified at a lower calcination temperature than the temperature at which the material crystallizes. Based on previous TGA/DSC characterization of calcined p-Eu:ZrO2 , 10–12 we conclude that this lower temperature mechanism is the decomposition of the precursor host resulting in removal of hydroxyl groups, which are known photoluminescence quenchers. 81,82

Y 2 O3 The final Dy host material we test in this study is p-Y2 O3 , which differs from both ZrO2 and TiO2 in that it only has a single crystalline phase (cubic). 6–8,38 Figure 8 displays the emission spectra (a), excitation spectra (b), and lifetime (c) of heated p-Dy:Y2 O3 at different calcination temperatures for a heating duration of 100 ms. Examining Figure 8a and 8b we find that over the temperature range tested the PL emission and PL excitation peaks remain broad, which is surprising in light of previous measurements of p-Eu:Y2 O3 . 8 In these previous measurements the emission/excitation spectra of p-Eu:Y2 O3 was found to narrow for temperatures over 773 K, which is well below the peak temperature we test. While at first these results for p-Y2 O3 seem contradictory, we note that in the previous measurements (of p-Eu:Y2 O3 ) the isothermal durations were over 19



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(a)

(b)

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(c)

Figure 8: (a) PL emission spectra (355 nm excitation), (b) PL excitation spectra (measured at 575 nm), and (c) PL lifetimes of calcinated and unheated p-Dy:Y2 O3 . The lifetime is found to be fit by Equation 1 with offset and amplitude parameters of f0 = 16.04 ± 0.37 µs and f0 = 598 ± 22 µs, respectively. 1 s, whereas our current isothermal duration is only 100 ms. Recently we demonstrated this effect of isothermal duration on the emission spectra of a thermal impulse sensor cocktail (t-Ho:ZrO2 +p-Dy:Y2 O3 +p-Eu:ZrO2 ). 4 In that study we found that the emission from the p-Dy:Y2 O3 remains broad up to a temperature of ≈ 1100 K for the 100 ms isothermal time. Thus our current measurements of sub-second laser heating of p-Dy:Y2 O3 are consistent with our measurements of the material when combined in a thermal impulse sensor mixture. While p-Dy:Y2 O3 doesn’t produce sharp spectral features for our temperature range, with a 100 ms isothermal time, we note that given a long enough heating duration the material will crystallize and form sharp emission and excitation peaks. To demonstrate this we heat p-Dy:Y2 O3 to 1673 K for 30 min in a furnace and measure its spectra as shown in Figure 9. From Figure 9 we find that the fully crystalline Dy:Y2 O3 sample produces sharp emission/excitation peaks, with the strongest emission peak being located at 573 nm (4 F9/2 → 6 H11/2 ) and the strongest excitation peak being at 451 nm (6 H15/2 → 6 I9/2 )

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Figure 9: Emission (for 355 excitation) and excitation (for 575 nm emission) spectra for cubic crystalline Dy:Y2 O3 calcined at 1673 K for 30 min. Both the emission and excitation spectra are found to contain sharp well defined peaks, which differ drastically from the broad peaks of the amorphous precursor. Emission While the PL emission spectra of p-Dy:Y2 O3 remains broad for an isothermal duration of 100 ms – without any sharp peaks forming– we still observe obvious spectral changes in Figure 8a as the temperature is increased. Specifically we observe that the relative intensities between the 574 nm and 480 nm peaks change with calcination temperature, with the 575 nm peak becoming more intense than the 480 nm peak. To characterize this change we calculate the integrated intensity of both peaks and compute the ratio of the integrated intensities (480/575) with the resulting ratio shown in Figure 10. From Figure 10 we find that the ratio remains constant up to approximately 773 K, after which the 575 nm peak’s intensity increases causing the ratio to rise. While the upper asymptotic ratio is not obtained for our temperature range with a 100 ms duration, we know from Figure 9 that given sufficient heating this ratio will rise to a fixed asymptotic value. Therefore we fit the data in Figure 10 to Equation 1 and obtain a characteristic temperature of 973±45 K and β = 3.0±0.46. This temperature is consistent with previous measurements of sub-second heated p-Y2 O3 . 4,5 For longer duration heating the transition temperature is

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Figure 10: Integrated intensity ratio (575/480) as a function of temperature for heated pDy:Y2 O3 . The ratio is found to be fit by Equation 1 with offset and amplitude parameters of f0 = 1.320 ± 0.072 and fA = 3.36 ± 0.48, respectively. typically between 650 K and 750 K. 6–8,12 Excitation Having explored the influence of sub-second laser heating on the PL emission spectra of p-Dy:Y2 O3 , we now turn to examining its influence on p-Dy:Y2 O3 ’s PL excitation spectra (shown in Figure 8b). From Figure 8b we find that for all temperatures there are three broad peaks, but for temperatures above 782 K a narrow peak at 451 nm begins forming. This narrow peak can be attributed to the start of Y2 O3 ’s cubic crystalline phase. 6–8,37 To characterize the change in the excitation spectra we compute the integrated intensity ratio of the 6 H15/2 → 6 I15/2 and 6 H15/2 → 6 F9/2 transition peaks, with the resulting ratio shown in Figure 11 as a function of temperature. From Figure 11 we find that the integrated ratio follows Equation 1, with its characteristic temperature found to be 667 ± 31 K, which is significantly lower than determined from the emission spectrum and lifetime. While this result is unexpected, it is actually consistent with measurements of heated p-Eu:Y2 O3 , where the transition temperature for the excitation spectra is found to occur at a lower temperature than when using emission. 7 This difference is most likely due to the excited state being more

22


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sensitive to the crystal field than the ground state.

Figure 11: Integrated intensity ratio of the 6 H15/2 → 6 I15/2 and 6 H15/2 → 4 F9/2 transitions as a function of temperature. The ratio is found to be fit by Equation 1 with offset and amplitude parameters of f0 = 1.743 ± 0.063 and fA = 2.60 ± 0.40, respectively. While the integrated intensity ratio of the 451 nm and 470 nm excitation peaks is one method for quantifying the change in the excitation spectra, we can also consider the narrowing of the 451 nm peak by considering the ratio of intensities at 447.5 nm and 451 nm. For the precursor material both intensities are approximately equal, but as the temperature increases, the intensity at 447.5 nm decreases relative to the intensity at 451 nm as the peak narrows. Figure 12 shows the 451/447.5 intensity ratio as a function of temperature where the ratio is found to follow Equation 1, with a characteristic temperature of 1057 ± 93 K and and β = 2.71 ± 0.55. These fit parameters are within uncertainty of those determined from the emission spectra, but the characteristic temperature is significantly higher than observed for the integrated intensity ratio shown in Figure 11. The difference in transition temperatures for the two ratio techniques is due to the different mechanisms affecting each quantity. In the case of the integrated intensity ratios, the primary mechanism behind the transition is the crystallization of the host, while the peak narrowing of the 451 nm peak is due to a decrease in inhomogeneous broadening.

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Figure 12: Ratio of intensities at 451 nm and 447.5 nm as a function of heating temperature. The ratio is found to follow Equation 1, with a characteristic temperature of 1057±93 K with offset and amplitude parameters of f0 = 1.032 ± 0.048 and fA = 1.81 ± 0.20, respectively. Lifetime While the emission and excitation spectra for heated p-Dy:Y2 O3 (shown in Figures 8a and 8b) do not demonstrate crystallinity of the material in the temperature range tested (with an isothermal duration of 100 ms), we find, from Figure 8c, that the lifetime undergoes a sharp transition from ≈ 16 µs to ≈ 600 µs between 782 K and 875 K. Using Equation 1 to fit the data in Figure 8c, we find that the fit parameters are 804 ± 46 K and β = 50.4 ± 2.5, which corresponds to a lower characteristic temperature and larger β than observed for emission/excitation. The lower characteristic temperature can be attributed to the lifetime depending on both the crystallinity and nonradiative loss rate of the hosts. However, unlike p-Dy:TiO2 and pDy:ZrO2, the p-Dy:Y2 O3 is in a carbonate form (Y2 (CO3 )2 ), so the most likely sources of changes to the nonradiative decay channels are the C-O and C-O2 vibrations, whereas for the hydroxide precursors this effect is related to the O-H vibrations. In addition to the characteristic temperature being different between the lifetime and the emission/excitation calibration curves, the β is significantly higher, which correlates to a much steeper transition with temperature. This steepness is problematic for temperature 24


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sensing as the lifetime only displays changes over a narrow temperature range, which means that the lifetime can only be used to measure temperature in this range.

Summary In the section above we characterize the influence of sub-second laser heating on the spectral signatures (PL emission, PL excitation, and PL lifetime) of three Dy-doped oxide precursors. For all three materials the rapid laser heating results in irreversible phase transformations that modify the materials’ spectroscopic signatures. These spectral modifications are found to vary across the different hosts with Table 5 summarizing our observations for each host material. Using ratio-based spectral analysis techniques we quantify these modifications and find that, in general, the measured spectral ratios follow a modified Arrhenius function (Equation 1) as a function of temperature. By fitting these ratios to Equation 1 we determine the characteristic temperatures for each material and spectral signature, which are tabulated in Table 6. From Table 6 we observe that while the characteristic temperatures for all three spectral signatures are consistent for p-Dy:TiO2 , there are inconsistencies when comparing the different spectroscopic signatures for p-Dy:ZrO2 and p-Dy:Y2 O3 . Namely, for both materials, the lifetime curve has a lower characteristic temperature than for the emission/excitation ratio curve and for p-Dy:Y2 O3 there are two different characteristic temperatures for the excitation ratio curves, which depend on which ratio technique is used. These inconsistencies imply that there are other temperature-dependent transformations (aside from crystallization) occurring during calcination of p-ZrO2 and p-Y2 O3 . In the case of the lower characteristic temperature for the lifetime calibration curves, we can attribute this effect to decomposition reactions, which remove quenching sites from the precursor nanoparticles. While we can easily explain why the lifetime calibration curve transitions at a lower temperature than either the excitation or emission calibration curves, it is more difficult to 25



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Table 5: Summary of spectroscopic changes. Host Material p-Dy:TiO2

p-Dy:ZrO2

p-Dy:Y2 O3

Emission •All three emission peaks remain broad with calcination resulting in different peak centers and peak areas. This is due to size mismatch between Dy3+ and Ti4+ . • Heat changes ratio between peak areas. • Above ≈ 725 K the emission intensity decreases by a factor of ≈ 100×. • Calcination above ≈ 700 K results in the formation of multiple sharp narrow peaks for each transition. • All three emission peaks are broad for each calcination temperature.

Excitation • In general all three emission peaks remain broad with calcination resulting in different peak centers and peak areas. This is due to size mismatch between Dy3+ and Ti4+ .

Lifetime • Lifetime increases up to 26 µs near 725 K and then drops to 4 µs by 878 K.

• Above 475 K a narrow peak begins forming near 451 nm.

• Calcination above ≈ 700 K results in the formation of multiple sharp narrow peaks for each transition. • All three emission peaks are broad for each calcination temperature.

• Lifetime increases from 15 µs at room temperature up to 350 µs. • Lifetime displays a step increase from 16 us to 600 us near 800 K.

Table 6: Characteristic temperatures for each Dy-doped host material and spectral signature. Note that for p-Dy:Y2 O3 we find two different characteristic temperatures depending on which analysis method is used. Host Material

Characteristic T (K) Emission Excitation Lifetime p-TiO2 670 ± 14 708 ± 34 575 ± 100a p-ZrO2 733 ± 10 723 ± 23 550 ± 16 p-Y2 O3 973 ± 45 1057 ± 93 804 ± 46 667 ± 31b a Estimated characteristic temperature as the data cannot be fit by Equation 1 b Multiple characteristic temperatures are observed for excitation in pDy:Y2 O3 . 26


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explain the observation of two different temperatures for p-Dy:Y2 O3 ’s emission/excitation calibration curves. We hypothesize that these differences are the result of the crystal field having a stronger effect on the excited states of Dy3+ as compared to their effect on the ground state. To test this hypothesis we are looking into performing modeling the effects of the crystal fields on the different energy levels using Judd-Offlet theory. 13 Having now determined and summarized the spectroscopic changes of the different Dydoped hosts, we next make observations about the performance of the three materials with regards to the proposed application as the second component of a thermal impulse sensor cocktail for use in explosive fireballs: 1. p-Dy:TiO2 : Given the observation that p-TiO2 displays broad emission/excitation peaks and that the emission intensity significantly decreases above 725 K, we conclude that it is ill suited to our application as explosive fireballs can reach much higher temperatures. Spectroscopic changes are observed in a temperature range of 575 K to 1000 K. 2. p-Dy:ZrO2 : Since p-ZrO2 is the only host to display the formation of sharp peaks within the temperature/duration range tested in this study, we determine that it provides the most information-rich structure for analysis. The broad peaks of the other two materials provide less information than sharp peaks observed for p-ZrO2 . Spectroscopic changes are observed in a temperature range of 500 K to 900 K. 3. p-Dy:Y2 O3 : The observation of both a low and high temperature transition for p-Y2 O3 (at 667 K and 1000 K respectively) suggests that by using different analysis methods the host can be used to accurately determine temperatures over a wide range, which is beneficial to extending the functional range of the sensor cocktail. Spectroscopic changes are observed in a temperature range of 500 K to over > 1000 K. Based on these observations we conclude that the two most promising host materials for use in a TI sensor cocktail are p-ZrO2 and p-Y2 O3 . Additionally, we note that these results 27



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suggest that it may be possible to avoid using two different sensor materials for thermal impulse sensors as both p-ZrO2 and p-Y2 O3 display multiple kinetic phenomena, which can be probed using different spectroscopic technique.

Conclusion Previously, we measured the influence of calcination temperature on the spectroscopic signatures of Eu-doped oxide precursor hosts, with the overarching goal of developing ex-situ TI sensors for use in explosions. While we can use a single lanthanide-doped oxide host to estimate a heating event’s temperature, two materials are required to measure both the temperature and duration (i.e. the thermal impulse) of a heating event. Therefore it is necessary to determine a second lanthanide-doped oxide precursor host (in addition to the Eu-doped host) for use in a thermal impulse sensor cocktail. To achieve this we prepare three different Dy-doped oxide precursor hosts (p-Dy:TiO2 , p-Dy:ZrO2 , and p-Dy:Y2 O3 ), which are subsequently heated to different calcination temperatures ranging from 293 K to 1257 K for a 100 ms isothermal duration using CO2 laser heating. After calcination we measure the precursors’ spectroscopic signatures, including their: PL emission spectra (using 355 nm excitation), PL excitation spectra (measured using 575 nm emission) and PL lifetime (excited at 355 nm and emission measured at 575 nm). We analyze these signatures using several different analysis techniques and quantify the influence of the sub-second calcination-induced irreversible phase changes on the material’s spectroscopic properties. Based on these results we find that the measured phase transformation temperatures for the Dy-doped hosts are mostly consistent with those measured for the Eu-doped hosts. This implies that the use of a different lanthanide dopant has little to no influence on the phase transformations of the host material, which is to be expected as the lanthanide doping is only 1 mol%. These transformations are found to occur near 725 K for p-TiO2 , near 550 K and 730 K for p-ZrO2, and near 667 K, 804 K, and 1000 K for p-Y2 O3 .

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This observation (for p-ZrO2 and p-Y2 O3 ) of multiple characteristic temperatures for different spectroscopic signatures of the same host material is unexpected; as we initially hypothesized that the amorphous-to-crystalline phase transformation would be the primary mechanism of modifications to the spectroscopic signatures. 4 If this hypothesis were true it would produce the same transition temperature for all the calibration curves we measure. However, this is found not to be the case, implying that additional mechanisms occur during calcination (with unique transition temperatures). These mechanisms include the loss of high energy OH, CO and CO2 quenching channels due to decomposition, reductions in inhomogeneous broadening, and differences in sensitivity to crystal field effects between Dy3+ ’s ground and excited states. Since we find two different kinetic curves for a single host material when utilizing multiple spectroscopic signatures, it may be possible to perform TI determinations with only a single material. This ability would solve one of the primary difficulties facing our two-component ex-situ TI sensors, which is uncontrolled mixing of sensor particles in an explosion. 84,85 Further work is required to fully explore this possibility, with measurements needing to be performed over a wide range of durations to determine how heating duration effects these different temperature curves. Most likely this new technique will involve looking at both intensity ratios and spectral FWHMs 11,14 While further research is needed to determine if we can use a single material for TI determination, we can immediately apply our results to our current two material TI sensor approach. We conclude that p-TiO2 is a poor host for our application, with its emission intensity dropping for calcination temperatures above 725 K. However, both p-Dy:ZrO2 and p-Dy:Y2 O3 are found to be good materials for our application, with p-Dy:ZrO2 displaying spectroscopic changes in a temperature range of 500 K to 900 K and p-Dy:Y2 O3 displaying changes for temperatures from 500 K to > 1073 K.

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Acknowledgement This work was supported by the Defense Threat Reduction Agency, Award # HDTRA1-151-0044 to Washington State University.

References (1) Milby, C.; Stamatis, D.; Carney, J.; Horn, J.; Lightstone, J. Efficacy of Energetic Formulations in the Defeat of Bio Agents. Central States Section of the Combustion Institute Spring Technical Meeting 2012. Dayton, Ohio, 2012; pp 861–868. (2) Peuker, J. M.; Lynch, P.; Krier, H.; Glumac, N. Optical Depth Measurements of Fireballs from Aluminized High Explosives. Optics and Lasers in Engineering 2009, 47, 1009–1015. (3) Densmore, J. M.; Homan, B. E.; Biss, M. M.; McNesby, K. L. High-speed Two-camera Imaging Pyrometer for Mapping Fireball Temperatures. Appl. Opt. 2011, 50, 6267– 6271. (4) Anderson, B. R.; Gunawidjaja, R.; Price, P.; Eilers, H. Spectroscopic Determination of Thermal Impulse in Sub-second Heating Events Using Lanthanide-doped Oxide Precursors and Phenomenological Modeling. J. Appl. Phys. 2016, 120, 083102. (5) Gunawidjaja, R.; Anderson, B.; y Riega, H. D.; Eilers, H. Sub-Second Laser Heating of Thermal Impulse Sensors. AIP Conf. Proc. 2017, 1793, 060014. (6) Gunawidjaja, R.; Myint, T.; Eilers, H. Optical and Morphological Characterization of Tb0.01 Zr0.99 O2 /(Precursor Eu0.02 Y1.98 O3 ) Core/Shell Nanoparticles as Temperature Sensors in Fast Heating Events. The Journal of Physical Chemistry C 2014, 118, 5563– 5569. (7) Myint, T.; Gunawidjaja, R.; Eilers, H. Fast Pyroprobe-Heating-Induced Structural

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