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Abstract
In this study, three different types of ZrO2 films were prepared with different precursors and additives using the sol-gel method. High-temperature annealing was implemented to investigate the impact of temperature on optical properties, microstructure, surface morphologies and absorption of these films. According to the laser-induced damage threshold (LIDT) tests on films having experienced annealing and those implemented with in-situ high temperature, the ZrO2 film with ZrOCl2·8H2O as the precursor and copolymer of silicone and polyaldoxyl ether as the additive had the highest resistance to laser-induced damage. After annealing at 623 K, its LIDT was 21.4 J/cm2, while that at an in-situ high temperature of 523 K was 23.9 J/cm2. The strong high temperature resistance was likely attributed to the usage of carbon-free precursor and high temperature-resistant additive, which contributed to low carbon contents and less structural damage caused by organic matter evaporation. In this context, there were less high temperature-induced impurity and structural defects, leading to higher LIDT values. This study provided a novel method for preparing high temperature-resistant sol-gel films, which shed light upon wider potential application of sol-gel films at high-temperature conditions.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Due to its high transmittance in the visible and near-infrared light as well as its large laser-induced damage threshold (LIDT), ZrO2 film has been widely applied in the laser systems [1]. It is generally prepared by physical deposition methods, including electron beam evaporation deposition [2], radio-frequency sputtering deposition [3], and ion-assisted deposition [4]. Specifically, Yao et al. prepared a ZrO2 film using the electron beam evaporation method, which had the LIDT of 6.0 J/cm2 (1064 nm, 12 ns) [5]. Employing the radio-frequency sputtering deposition method, Clark et al. prepared a ZrO2 film that had the LIDT of 12.1 J/cm2 (1064 nm, 9.6 ns) [6]. Similarly, Xu et al. deposited the ZrO2 film on the BK7 substrate through the electron beam evaporation method, resulting in the LIDT of 11.1 J/cm2 (1064 nm, 12 ns) [7].
Physically prepared ZrO2 films are generally vulnerable to laser-induced damage in the high-power laser systems. In this context, the chemical sol-gel preparation method is an alternative. Specifically, high LIDT values were achieved in previous sol-gel preparation studies, such as Guo et al. that reported the LIDT of 26.98 J/cm2 (1064 nm, 9.3 ns) and Wang et al. that obtained the LIDT of 21 J/cm2 (1064 nm, 10 ns) [8,9]. In addition, organic zirconium alkoxides, such as zirconium propoxide (Zr(OPr)4) and zirconium butoxide (Zr(OBu)4), are the most commonly used precursor, while there are a few works using inorganic zirconium oxychloride (ZrOCl2·8H2O) as the precursor [9–11].
Though favored by their high resistance to laser-induced damage, sol-gel films are disfavored by their lower heat resistance than physically prepared counterparts. Specifically, the LIDT of physically prepared ZrO2 films can be enhanced by heat treatment techniques such as annealing, whereas that of sol-gel films suffer from significant LIDT decrease after annealing [12]. The dramatically decreasing LIDT of the sol-gel film after high temperature annealing has been attributed to organic matter evaporation, which results in destruction of the film network structure, carbonization of organic matter in the film, and defects of grain boundaries inside the film [13]. In this case, a smaller amount of organic matter (including the precursor and the additive) introduced in the sol-gel film can help prevent the film structure from destruction by organic matter evaporation at a high temperature, and alleviate the negative influences of high temperature carbonization and massive absorption. Consequently, the elevated high temperature resistance will contribute to much wider potential application of sol-gel films in the future, such as usage in the aerospace engineering or environments with severe temperature changes [14].
In this study, different precursors and additives were used to prepare three kinds of ZrO2 films using the sol-gel method, followed by annealing at different temperatures. Analyses were made to characterize viscosity evolution of the sols as well as transmittance, microstructure, absorption, and surface topography of the films. Furthermore, laser-induced damages to the films at in-situ high temperature were systematically investigated.
2. Experimental
Zirconium propoxide solution (Zr(OPr)4, 70 wt.%) and Zirconyl chloride octahydrate (ZrOCl2·8H2O, 99.9%) were purchased from Aladdin. Ethanol (99.8%) and acetylacetone (ACAC, 99%) were purchased from Sigma-Aldrich. Copolymer of silicone and polyaldoxyl ether (CSPE) was purchased from Dow Corning. Three groups of ZrO2 sol were prepared and respectively labelled as Z1, Z2 and Z3. Specifically, Z1 was prepared with Zr(OPr)4 as the precursor, and ACAC as the additive; Z2 was prepared with ZrOCl2·8H2O as the precursor and ACAC as the additive; and Z3 was prepared with ZrOCl2·8H2O as the precursor and CSPE as the additive. These three groups shared the same volumes of additives and the same molar ratios among precursor, ethanol and water (Table 1). The prepared sols were sealed and aged for 7 days at a temperature of 276 K.
Table 1. The contents of precursor and additives in the samples
After ultrasonic cleaning of the K9 substrate, above-mentioned aged sols were dip-coated at a speed of 60 mm/min to obtain the ZrO2 film, with the room temperature controlled within a range of 293 K to 298 K, and the humidity less than 50%. Then, 10-min annealing was implemented on each layer of film after it was prepared, followed by 0.5 h annealing at 353 K after two layers of films were obtained to completely evaporate the solvent ethanol. Such preparation method was employed on three groups of sols, resulting in two films for each group. The first films of three groups were labelled as Z1-353, Z2-353, Z3-353, respectively, while the second films were annealed for 0.5 h at 623 K, and labeled as Z1-623, Z2-623, Z3-623, respectively. The purpose was to explore the impacts of different precursors and additives on the optical properties, absorption and laser-induced damage resistance of these films as tested under normal (ambient) conditions after they had been annealed. Also, the prepared Z3 film was annealed at 423 K and 523 K for 0.5 h, and labeled as Z3-423 and Z3-523, respectively, and then a set of the Z3 films (Z3-353, Z3-423 and Z3-523) annealed at their respective high temperatures were tested for the LIDT under normal (ambient) environments; and another set of the Z3 films (Z3-353D, Z3-423D and Z3-523D) annealed at their respective high temperatures were test for the LIDT under in-situ environments at the same respective high temperatures (353 K, 423 K and 523 K) at which they were annealed. The purpose here was to compare the LIDT under ambient environments of the variously annealed Z3 films with the LIDT under environments at the same respective in-situ high temperatures at which the films were annealed.
Sol viscosity was measured by a glass capillary viscometer at a relative humidity of 40% and a temperature of 293 K. Transmittance of the films in the spectrum of 300-1200 nm was measured using a UV-3600 spectrophotometer. Microstructure of the films was determined by a D8 Advance X-ray diffractometer (XRD). The surface thermal lensing (STL) technique was applied to characterize the film absorption [15]. The surface topographies were measured using a Dimension V atomic force microscopy (AFM), followed by calculation of the root mean square (RMS) roughness. The LIDT of the films was tested following the “1-on-1” regime according to ISO standard 11254-1, using 1064 nm and 12 ns Nd:YAG laser in single longitudinal mode with up to a 5 Hz repetition rate [16]. The in-situ high temperature LIDT testing platform was set up according to previous study [17]. The Q-switched Nd:YAG was focused to provide a far-field circular Gaussian beam with a diameter of 0.306 mm at 1/e2 of the maximum intensity. The sample was placed inside the temperature-controlled chamber and was driven by a stepper motor. For normal temperature laser-induced damage tests, LIDT values were measured at the room temperature of 298 K and the relative humidity of less than 30%, while for in-situ high temperature laser-induced damage tests, they were measured at different temperatures of 353 K, 423 K and 523 K. The heating rate was about 3.7 °C/min and the holding time was 0.5 h. Ten sites on the sample were exposed at the same fluence, accompanied by recording of the fraction of the damaged sites. Then, this procedure was repeated for another fluence. The LIDT was defined as the incident pulse energy density when the damage occurs at 0% damage probability, and it was obtained by linear extrapolation of the damage probability data to zero damage probability. The total error was about 12% in the LIDT measurement. Damage morphologies after laser radiation were evaluated by a Sirion 200 field emission scanning electron microscope (FESEM). Carbon contents of the films were measured by an Elementar (Vario III) element analyzer.
3. Results
As shown in Fig. 1, Z1 sol has a relatively small and stable viscosity in the first 15 days, followed by a sharp increase after Day 15. Comparably, Z2 sol has the largest viscosity, while Z3 sol has an intermediate viscosity. Generally, Z2 and Z3 are of good stability as reflected by the extremely slow increase in viscosity during 19-day aging.
All the three groups of films present strong high temperature resistance after they are annealed at 353 K and 623 K, respectively, as is demonstrated by the stable and even slightly increasing transmittance (Fig. 2). This is unexpectedly promising, as previous research showed dramatic transmittance decline after annealing of the sol-gel TiO2 film, which was ascribed to carbonization/deoxidation at the high temperature [14]. An interesting phenomenon is that compared with Z1 and Z3, Z2 has the lowest transmittance. According to the transmission data of the precursors and the additives in Fig. 2, the fact that Z2 has the lowest transmission is attributable to the high absorption of ACAC [18]. The high transmittance of Z1 may be attributed to its thinner thickness, accompanied by lower sol viscosity, than Z2 and Z3.
Fig. 2. Transmittance curves of the ZrO2 films. The inset illustrates the transmittance curves of the precursors and additives.
All the three groups of films are indicated to be amorphous based on their XRD patterns after annealing at 353 K and 623 K, respectively, in which there are no obvious diffraction peaks (Fig. 3). However, weak diffraction peaks do occur after annealing of Z1 and Z2 films at 623 K, which suggests a tetragonal (101) crystal phase transition. In comparison, Z3 film is demonstrated to have the strongest high temperature resistance, as it presents no obvious diffraction peaks after annealing at a high temperature of 623 K.
Figure 4 shows the absorption of three groups of films, with 12 sampling points in each group. Specifically, average weak adsorption values of Z1, Z2 and Z3 films after annealing at 353 K are 35.2 ppm, 34.4 ppm, and 32.9 ppm, respectively. All the average weak absorption values of Z1, Z2 and Z3 films after annealing at 623 K are elevated, reaching 43.2 ppm, 37.3 ppm and 36.6 ppm, respectively. This is speculated to result from evaporation or carbonization of residual organic matter inside the film due to high temperature, leading to structural or impurity defects and consequent increased absorption.
Figure 5 illustrates AFM images of Z1, Z2 and Z3 films after annealing at 353 K and 623 K, respectively. Specifically, Z1 film presents overall flat surface after annealing at 623 K, with a few local protrusions, and its RMS roughness is slightly elevated from 0.52 nm to 0.58 nm (Figs. 5(a) and 5(b)). In comparison, the surface of the Z2 film is more convex and the film quality is slightly decreased after annealing at 623 K (Figs. 5(c) and 5(d)). RMS roughness of Z2-353 and Z2-623 reaches 0.56 nm and 0.71 nm, respectively, indicating slightly enlarged roughness after high temperature annealing. As for the Z3 film, no significant roughness changes are observed (Figs. 5(e) and 5(f)), and RMS roughness of Z3-353 and Z3-623 slightly drops to 0.59 nm and 0.56 nm, respectively, indicating strong high temperature resistance.
In terms of LIDT values (Fig. 6), those of Z1-353 and Z1-623 films are 24.1 J/cm2 and 13.6 J/cm2, respectively, which means a decrease of 43.6% after annealing at 623 K; those of Z2-353 and Z2-623 are 28.3 J/cm2 and 17.5 J/cm2, respectively, presenting a decrease of 38.2% after annealing at 623 K; and those of Z3-353 and Z3-623 are 33.1 J/cm2 and 21.4J/cm2, respectively, which suggests the least LIDT drop (35.4%) and thus the strongest resistance to laser-induced damage at the high temperature.
Due to the presence of a small amount of organic matter, fusion stress type damage was usually observed under the action of a nanosecond pulse laser. Taking Z2 film as an example, SEM images of films after laser-induced damage are presented (Fig. 7), including damage morphologies after annealing at 353 K and 623 K (Figs. 7(a) and 7(b)), as well as zoomed-in figures of the central parts (Figs. 7(c) and 7(d)) and edges (Figs. 7(e) and 7(f)) of the film. Induced damage points are observed in the center of the damage (Fig. 7(b)), which indicates them to be defect-induced [10,19]. At the same time, obvious spalling-triggered stress features are seen at the damage edge (e.g. Fig. 7(c)), which is ascribed to rapid organic matter evaporation at the high temperature condition due to the presence of a small amount of organic matter such as ACAC in the film after annealing at 353 K. Defect-induced damage mechanism remains after annealing at 623 K (Fig. 7(d)). Further decreased organic matter quantity due to high temperature annealing contributes to certain reduction of stress that resulted from organic matter evaporation, accompanied by slightly weakened spalling (Fig. 7(f)).
Fig. 7. Damage morphologies of the films. (a) Z2-353, (b) enlarged area A, (c) enlarged area B, (d) Z2-623, (e) enlarged area C, (f) enlarged area D.
Although the LIDT test under ambient environment after high temperature annealing to some extent reflects the film’s resistance to laser-induced damage at a high temperature, it is still different from the tests in the actual high temperature conditions. Correspondingly, normal temperature laser-induced damage tests and in-situ high temperature laser-induced damage tests were implemented on the Z3 film, which had the strongest high temperature resistance after annealing, to investigate the high temperature laser-induced damage characteristics of the film under real conditions. As shown in Fig. 8, Z3-353, Z3-423 and Z3-523 after annealing at different temperatures present LIDT values of 33.1 J/cm2, 28.5 J/cm2 and 24.6 J/cm2 at the room temperature, respectively, which indicates lowered LIDT as the annealing temperature rises. In addition, Z3-353D, Z3-423D and Z3-523D experience in-situ high temperature laser-induced damage at the same temperature, whose LIDT values respectively drop by 6.7% to 30.8 J/cm2, 4.6% to 27.2 J/cm2, and 3.2% to 23.9 J/cm2, respectively, compared to those of Z3-353, Z3-423 and Z3-523. However, it is noted that these reductions are less than the 12% test error margin. Therefore, the LIDT of the films is considered to be almost unchanged. Moreover, the LIDT value of the Z3 film experiencing in-situ high temperature laser-induced damage at 523 K is only 22.7% lower than that undergoing normal temperature laser-induced damage after annealing at 353 K, reaching 23.9 J/cm2. This demonstrates strong resistance of the Z3 film to high temperature. Interestingly, there are obvious differences between the slopes of linear fitting for damage tests on films having experienced annealing and those implemented with in-situ high temperature (Fig. 8). Specifically, the slopes of linear fitting for Z3-353, Z3-423, and Z3-523 are 4.5, 5.4, and 5.6, respectively, while those for Z3-353D, Z3-423D, and Z3-523D are much larger, reaching 9.4, 8.1, and 7.5, respectively. When sharing a similar laser energy density corresponding to 0% probability damage, films implemented with in-situ high temperature tests have a significantly lower laser energy density corresponding to 100% probability damage than those films having experienced annealing. In other words, the film would be more prone to 100% probability damage under high energy density laser irradiation due to the increase of ambient temperature, and to some extent, intrinsic damage would be more likely to occur. It is noted that the relative humidity is less than 30% when tested in a normal (ambient) environment. However, the chamber can be considered to be absolutely dry during the in-situ high temperature test, so that the change of the absorbed water content in the film may have some influences on the LIDT. In the previous research [20], the calculation showed that the lower relative humidity would decrease the LIDT. In this study, although the LIDT of the films that experience in-situ high temperature laser-induced damage is almost unchanged, compared to those that undergo the same temperature annealing (Fig. 8). However, from the 100% probability damage value, the intrinsic resistance to laser damage of these films is still slightly reduced. One possible contributor might be the disappearance of absorbed water, and the detailed mechanisms remain to be further studied.
Fig. 8. LIDT results of the Z3 films as tested under ambient environment (Z3-353, Z3-423 and Z3-523) and under in-situ environment at their respective annealing high temperatures (Z3-353D, Z3-423D and Z3-523D).
Figure 9 presents SEM morphologies of Z3-523 and Z3-523D films. As can be seen in the zoomed-in figures of the central parts of damages (Figs. 9(b) and 9(d)), the normal temperature laser-induced damage to the Z3-523 film is basically the same as that in Fig. 7, which is classified as obvious defect-induced damage. In addition, the in-situ high temperature laser-induced damage to the Z3-523D film is also defect-induced, which indicates no damage mechanism changes, and this is in line with previous studies [13,18]. Furthermore, the already ambiguous annular part inside the defect (Fig. 9(c)) is probably attributable to the rapid melting and massive evaporation of internal materials due to the extremely high temperature at the defect center during the occurrence of in-situ high temperature laser-induced damage.
Fig. 9. Damage morphologies of the films. (a) Z3-523, (b) enlarged area A, (c) Z3-523 and (d) enlarged area B.
4. Discussion
As a large amount of organic matter is involved in the preparation of the sol-gel film, it is generally inevitable to have some residual organic matter in the prepared film, which constrains its high temperature resistance. In general, it is believed that high temperature will result in the evaporation of organic matter inside the sol-gel film, causing damages to the film structure. In particular, if organic matter carbonization should be triggered by high temperature, the film absorption capacity would sharply reduce, accompanied by a consequent large drop of LIDT values.
In order to investigate specific effects of organic matter within the film, carbon contents of several prepared films were tested, with the results shown in Fig. 10. After annealing at 353 K, Z1 has the largest carbon content, which reaches 23.76%, followed by Z3 (13.59%) and Z2 (13.1%). In comparison, after annealing at 623 K, carbon contents of Z1, Z2 and Z3 are decreased to 6.17%, 8.96% and 4.96%, respectively. Such carbon content reduction is partially attributed to evaporation of organic matter, such as by-product alcohols produced by Zr(OPr)4 as the precursor, as well as evaporation of residual ACAC or CSPE. On the other hand, the hindered organic matter evaporation process at a high temperature will cause carbonization, further leading to accumulation of carbon that will not be discharged. In this case, there will be more absorption by impurity defects, which reduces the LIDT values.
It should be specially pointed out that the carbon content in the film after high temperature annealing does not present completely inverse relationship with LIDT values. For instance, Z2 has a higher carbon content after annealing at 623 K than Z1, but it also possesses the larger LIDT value. It has been well known that film damage under nanosecond lasers is usually caused by defects (Figs. 7 and 9). Under this kind of damage mechanism, there might be some other types of defects such as structural defects than carbon-induced ones [21]. These structural defects may be voids, microcracks or grain boundaries in the sol-gel films. At the same time, the special network structure of the sol-gel film should be also responsible for the high LIDT values. Because the sol viscosity of Z1 is lower than that of Z2, the thickness of the film prepared by the same process is thinner than that of the latter. This causes evaporation of organic matter more easily at high temperatures. It may be the reason why the carbon content in Z1 is lower than that in Z2 at 623 K. But at the same time, due to the thinner thickness of Z1 film, the more rapid evaporation of organic matter is consequently more likely to destroy the ordered structure or result in structural defects. The competition between the above two makes the absorption of Z1 (43.2 ppm) higher than that of Z2 (37.3 pm) (Fig. 4). As for Z3, the additive CSPE is featured by its -Si-O-Si-bond in the main molecular chain, which is hardly subjected to carbonization, and therefore the overall carbon content is low. Moreover, the higher boiling point of CSPE (448.5 K) than that of ACAC (413.5 K) also means less evaporation and thus smaller destruction to the film structure. In addition, CPSE is more capable of stabilizing the structure of ZrO2 or maintaining the ordered network structure of the film than ACAC, and it remains amorphous after 523 K. All the above mentioned effects enable Z3 to have the least defects, which consequently lead to the lowest amount of adsorption (36.6 ppm) and the highest LIDT value.
5. Conclusions
In this study, three different types of ZrO2 films are prepared with different precursors and additives using the sol-gel method. Specifically, the ZrO2 film prepared with ZrOCl2 as the precursor and CSPE as the additive has the largest LIDT value, reaching 33.1 J/cm2, which means the strongest resistance to laser-induced damage. After annealing at 623 K, the LIDT value of such film only drops by 35.4%, reaching 21.4 J/cm2. This film also presents strong high temperature resistance in the in-situ high temperature laser-induced damage test at 523 K, whose LIDT value is still as high as 23.9 J/cm2. This finding provides significant potentials for preparation of high temperature, high laser power-resistant films in the future. Achievement of such high temperature resistance is attributed to the replacement of a traditionally used organic precursor Zr(OPr)4 with a carbon-free precursor ZrOCl2 as well as the replacement of an ACAC additive with a CSPE additive that had Si-O backbone structures and consequent high boiling point. This improvement succeeds to reduce the carbon content in the film after high temperature treatment. Moreover, the more ordered film structure is less prone to destruction due to reduction of volatile matter, accompanied by less defects and consequent higher LIDT values.
Funding
Fundamental Research Funds for the Central Universities (2019ZDPY05).
Disclosures
The authors declare no conflicts of interest.
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