The preparation of high-density aluminum-doped zinc oxide ceramics by cold sintering process

Journal Pre-proof The preparation of high-density aluminum-doped zinc oxide ceramics by cold sintering process X.Y. Hong, X.P. Jiang, G.S. Zhu, H.R. Xu, X.Y. Zhang, Y.Y. Zhao, D.L. Yan, J.W. Xu, S.C. Huang, A.B. Yu PII:

S0925-8388(19)34487-1

DOI:

https://doi.org/10.1016/j.jallcom.2019.153241

Reference:

JALCOM 153241

To appear in:

Journal of Alloys and Compounds

Received Date: 19 September 2019 Revised Date:

27 November 2019

Accepted Date: 1 December 2019

Please cite this article as: X.Y. Hong, X.P. Jiang, G.S. Zhu, H.R. Xu, X.Y. Zhang, Y.Y. Zhao, D.L. Yan, J.W. Xu, S.C. Huang, A.B. Yu, The preparation of high-density aluminum-doped zinc oxide ceramics by cold sintering process, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/ j.jallcom.2019.153241. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Author Contributions Section Hong XY: Conceptualization, Formal analysis, Data curation, Writing- Original draft preparation. Jiang XP: Methodology, Formal analysis, Investigation. Zhu GS: Resources, Formal analysis, Funding acquisition, Writing - Review & Editing. Xu HR: Resources, Supervision, Project administration, Funding acquisition. Zhao YY: Formal analysis, Funding acquisition. Zhang XY: Data Curation, Resources. Yan DL: Formal analysis, Resources. Xu JW: Formal analysis. Huang SC: Resources. Yu AB: Formal analysis, Project administration.

The Preparation of High-Density Aluminum-doped Zinc Oxide Ceramics by Cold Sintering Process a

a

a*

a

a

a

a

a

b

c

Hong XY, Jiang XP , Zhu GS , Xu HR , Zhang XY , Zhao YY , Yan DL , XU JW , Huang SC , and Yu AB a

Guangxi Key Laboratory of Information Materials, Engineering Research Center of Electronic

Information Materials and Devices，Ministry of Education，Guilin University of Electronic Science and Technology, Guilin 541004, China b

Guangxi Crystal Union Photoelectric Materials Co., Ltd., Liuzhou 545036, China

c

ARC Hub for Computational Particle Technology, Monash University, Clayton, Victoria 3800, Australia

* Guisheng Zhu (e-mail: [email protected])

Abstract High-density Aluminum-doped zinc oxide (AZO) ceramic was prepared via the densification process by ultralow-temperature Two-step Cold sintering processes (TS-CSP). Density, microstructure and electrical properties of the preparation AZO ceramics were investigated. The results show that Al-doping concentration has an important influence on the densification of AZO ceramics. When the doping amount of Al2O3 was 2.0 wt%, and an acetic acid solution was used as a co-solvent, a high-performance AZO ceramic was obtained. AZO ceramics has a uniform phase and containing no other impurities, with a relative density of up to 99.23%and a resistivity as low as 5.58 x 10-3 Ω·cm. The AZO ceramic prepared by this method has a significant application prospect. Keywords: Aluminum-doped zinc oxide; Two-step cold sintering; Resistivity; Densification process.

1 Introduction Indium tin oxid (ITO) is currently the main material for transparent conductive films for commercial use[1]. However, the global reserves of indium are limited and expensive, which limits the large-scale development of indium in transparent conductive films. Owing to the abundant source of AZO materials, low preparation cost and non-toxic. Therefore, AZO materials have great application potential in the solar photovoltaics, transparent electrode, transparent conductive film of LED chip and so on[2-5]. The density, microstructure, compositional uniformity and surface resistance of the AZO target directly determine the quality of the film[6]. Therefore, high-performance AZO ceramic is critical to obtaining a high-performance film[7, 8]. At present, the solid phase sintering method is the most common method for obtaining AZO ceramic[9]. Which requires the preparation of a ceramic target at more than 1000 ℃. However, there are many problems with the AZO ceramic target material prepared by this method, such as the AZO ceramic has too large grain size, low density, closed crystal pores and high energy consumption, which directly leads to AZO film cannot be applied on a large scale. Wang et al. obtained AZO ceramic targets with low ZnAl2O4 content and a relative density greater than 99% by hot pressing at 1150 ℃ and 35 MPa, which greatly improved the quality of the target[10]. Zhang et al. used a two-step sintering method to prepare AZO ceramic targets with a relative density greater than 99% at T1 = 1050 ℃ and T2 = 800 ℃, respectively[11]. However, the AZO ceramics prepared by these methods have large grain size and low density, which directly leads to a decrease in electrical properties of the AZO film. The high energy consumption also caused a serious waste of resources. According to reports, our group utilized Tow-Step Cold Sintering Process (TS-CSP) at an ultralow-temperature technology to prepare ZnO ceramics[12]. The method successfully produced a high density (relative density > 99%) ZnO ceramics by adding an acetic acid solution to the ZnO powder by a TS-CSP. Moreover, the crystal grains of the obtained ZnO ceramic are smaller than those of the conventionally prepared ZnO ceramic. The preparation of the AZO ceramic target is achieved by adding a co-solvent to the AZO powder under low temperature and high-pressure conditions. On this basis, we have improved the basis of previous research[13]. Through the combination of two-step cold sintering processes and annealing heat treatment, the AZO ceramic target prepared in the structure and performance is close to the AZO target obtained by the conventional method.

In this work, the synthesized AZO powder was used as the precursor, and a certain concentration of the acetic acid solution was added as a transient co-solvent to the AZO powder. A dense AZO ceramic target was prepared by ultra-low temperature conditions through TS-CSP. Compared with traditional methods, the crystallinity of AZO ceramics obtained by this method is higher, a relative density of 99.23%, and specific resistance of 5.58×10-3 Ω·cm. The relative density and resistivity of the AZO ceramic target prepared by the TS-CSP process are similar to those of the AZO ceramic target prepared by the conventional method. And this preparation process greatly reduces energy consumption and saves resources. 2 Experimental section 2.1 Characterization techniques Crystalline structure was examined using X-ray diffraction (XRD-D8 Advance, Bruker Inc, Germany) with Cu Kα source (λ=0.15405 nm). The scan step is 0.02° and the diffraction signal was collected for 150 s. The cross-sectional morphology of the bulk AZO ceramics was investigated using SEM (FE-SEM, FEI Tecnai-450, USA). The density of the bulk samples was measured by the mass/dimension ratio, as well as Archimedes’ Method using acetone as the liquid media. The electrical properties of the AZO ceramics were tested by a four-probe testing system, data collected was averaged over 3-4 runs. The chemical elements and valence of the AZO ceramics were shown by the X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250), which required the samples to be subjected to a vacuum of less than 5×10-8 Pa indoor. 2.2 Process AZO ceramic target AZO powders with different Al2O3 contents were prepared by hydrothermal method. To eliminate the influence of precursors on the preparation of AZO ceramics, we first performed XRD and SEM analysis on the prepared powders. As shown in Fig. 1a, the AZO powders with different Al doping amounts are ZnO phase with solely hexagonal wurtzite structure, and no other phase appears, such as Al2O3 and ZnAl2O4, and their diffraction peaks can correspond to the standard cards (PDF#36-1451), indicating that Al element may enter the lattice of ZnO. It indicates that the doping of Al does not destroy the crystal structure of No and the Al element has been successfully doped into the ZnO lattice to form a solid solution. At the same time, the Al element is incorporated into the ZnO lattice and can also be proved by the change of the ZnO grain size with the doping amount[14]. As shown in Fig. 1b, the effect of

doping different Al on the microstructure of AZO was characterized by SEM, the powder have a short rod shape and good dispersibility. The powder was characterized by XRD, SEM and Particle Size Distribution (PSD), it can be proved that the Al-doped powder with the average particle size distribution was 300 nm and the specific surface area was 9.57 m2/g, uniform morphology and good dispersibility was prepared by the hydrothermal method.

Figure 1 (a) XRD patterns of different Al doping amounts; (b) Microstructure of powders with different Al contents.

As shown in Fig. 2a, a thermocouple is placed directly between the heating coil and the sample to measure the change in heat of the sample during heating. The AZO target was prepared by a TS-CSP at a pressure of 300 MPa. First, precursor powders having different Al doping amounts (0.5 wt% - 4.0 wt%) were weighed, and then a certain amount of a 1.0 M acetic acid solution was slowly dropped into the powder and continuously ground in an agate mortar until the powder became viscous. In the first step, the slurry was transferred to a dedicated mold and heated to 180 ℃ at a heating rate of 5℃/min and held for 30 minutes. This process provides a hydrothermal environment for the mixture to allow the feed to react adequately to provide conditions for grain recrystallization. In the second part, the mold temperature was raised to 300 ℃ at a heating rate of 5℃/min and held for 30 min. Fig. 2b shows the temperature profile of the sample during the acquisition process. After cooling to room temperature, a complete AZO ceramic was obtained.

Figure 2 (a) the schematic diagram of CSP mold; (b) the sintering curve of TS-CSP AZO ceramics.

3. Results and discussion To study the phase of the AZO ceramic target after cold sintering, we prepared the AZO ceramic target by XRD. As shown in Fig. 3, the XRD patterns of the as-prepared sample. Diffraction peaks located at 2θ = 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, and 62.9° correspond to the (110), (002), (101), (102), (110), and (103) facets, respectively[15]. After comparison with the standard cards (PDF#36-1451), it was found that only pure AZO was present in the sample after cold sintering, and no other peaks such as phases of Al2O3 and ZnAl2O4 were found. It is suggested that the element Al may enter the crystal lattice of ZnO and form a solid solution. It shows that no phase change occurs after sintering, and it is still the wurtzite structure, but the diffraction peak intensity of the AZO ceramic samples higher than the AZO powder. The all shape diffraction patterns confirm good crystallinity of AZO.

Figure 3 XRD patterns of AZO powder, AZO ceramic target samples.

As shown in Fig. 4a, the resistivity of a two-step cold-sintered AZO ceramic sample at different Al doping concentrations. The density curve shows that the AZO

ceramic density decreases as the doping amount of Al increases. When the Al content is more than 3.0 wt%, the density of the AZO ceramics is greatly reduced. Since the presence of Al inhibits the growth of crystal grains, the density of AZO ceramics is greatly reduced. When the doping amount of aluminum is 2.0 wt%, the relative density of the AZO ceramic reaches 99.23%. As shown in Fig. 4b, the AZO resistivity exhibits an asymmetrical "smile" shape as a function of the amount of Al doping. That is, the conductivity gradually decreases first, and then rises rapidly after reaching the minimum value, and when the doping amount is 2.0 wt%, the conductivity is the smallest. When Al is added to ZnO, Al3+ enters the ZnO lattice and will displace the zinc ions on the unit cell. Moreover, because the valence state of the incorporated Al3+ is higher than that of Zn2+, the extra electrons under the action of lower activation energy are easily separated from the bond into free electrons, which increases the AZO carriers, thereby improving the conductivity of the sample. When the doping of aluminum in AZO is less than 2.0 wt%, as the number of aluminum doping increases, the valence electrons provided by Al3+ increased, which increases the free electrons in AZO, resulting in a decrease in sample resistivity. It can be seen from the experiment that when the doping amount of Al is 2.0 wt%, the density of AZO ceramic obtained by the two-step cold burning method can be as high as 99.23% and the conductivity is minimum 5.58×10-3 Ω·cm. This data is very close to the performance of AZO ceramics obtained by conventional sintering method. The cross-sectional morphology of AZO ceramics obtained by two-step cold sintering at different Al doping concentrations was observed by SEM. As shown in Fig. 4c, when the doping amount of Al is 1.0 wt%, the crystal grains of the sample are dense and the crystal grains have sharp edges. The grain growth is relatively complete, and the average particle size is about 235 nm. As shown in Fig. 4d, when the doping amount of aluminum in the sample is 2.0 wt%, the grain edge angle of the sample disappears, the particles exhibit an ellipsoidal shape, and the crystal grains become smaller, and the average size of AZO is about 91 nm. As shown in Fig. 4e, when the doping amount of aluminum in the sample is 3.0 wt%, the grain growth inside the sample is not obvious, and the crystallinity is poor, and the average size of AZO is about 85 nm. It can be seen from Fig. 4f that when the aluminum content reaches 4.0 wt%, the crystallinity of the sample is greatly affected, and the growth of the crystal grains is suppressed, resulting in an average grain size of only about 73 nm. Through observation of the microscopic morphology, it is found that when the Al doping

concentration is increased, the grain size of the AZO ceramic is significantly smaller, and the compactness of the sample is gradually deteriorated. The change of the microstructure of the AZO ceramic section shows that the growth of the AZO grain is inhibited by the doping of the Al element. When the doping amount of the Al element is too large, AZO crystallizing into porcelain cannot be realized. When the amount of aluminum doping exceeds 4.0 wt%, the rate of decrease in sample density increases, which corresponds to a gradual decrease in the properties of the ceramic. Because of Al doping, the growth of crystal grains is suppressed, and the size of crystal grains is greatly reduced[14, 16]. However, as the grain size gradually decreases, the gap between the crystal grains becomes smaller, making further densification of the ceramic more difficult. The physical properties of AZO ceramics prepared under different conditions are shown in Table 1, We can find that AZO ceramics prepared by TS-CSP have smaller grain sizes and excellent performance.

Figure 4 (a) the change curve of density with Al content; (b) the curve of resistivity change with Al content; SEM images of different Al content (c, d, e, and f) Table. 1 Comparison of physical and electrical properties of AZO ceramics prepared under

different conditions Sample

Temp. (℃)

Grain size

Density (%)

(μm)

Resistivity

Reference

(10-3 Ω·cm)

ZnO-Al2O3

1400

/

99.60

/

[17]

AZO

1350

3.60

99.80

/

[18]

AZO

1500

12.50

99.80

1.80

[9]

AZO

700

0.024

99.10

3.00

[2]

AZO

300

0.09

99.23

5.58

This work

Based on our experimental observation, the basic principle and the microstructure change of the sintering route are shown in Fig. 5. As shown in Fig. 5a, the AZO powder dissolves. The reaction is in an acidic liquid phase environment, the solution is weakly acidic, the surface of the powder is slightly soluble, increasing the area of contact between the particles, and the powder is in higher activation energy in an acidic environment. The regrowth of the granules provides conditions. As shown in Fig. 5b, the AZO powder are rearranged under the action of temperature and pressure. As the temperature and pressure increase, the powders gradually approach each other and are partially dissolved in an acidic liquid phase environment. As shown in Fig. 5c, the solution gradually evaporates completely under the action of temperature, and the crystal re-crystallize. When the temperature is raised to 180 ℃, the solution gradually evaporates, the liquid gradually becomes supersaturated, ions gradually precipitate from the solution, and crystal nuclei are formed, which are uniformly distributed on the gaps and surfaces of the powder. As shown in Fig. 5d, the solution is completely evaporated, the crystal nucleus gradually grows, and the ceramic is initially densified. After 30 min of incubation, the solution gradually evaporated completely, the crystal nucleus gradually grew, and most of the crystallization was completed inside the ceramic. As shown in Fig. 5e, AZO ceramics are completely densified. When the temperature is raised to 300 ℃, the crystal nucleus obtains a larger driving force, which promotes the progress of the crystal nucleus. The ceramic was sintered at a temperature of 300 ℃ for 30 min, and the final sintering of the ceramic was completed, at which the relative density reached 99.23%. The crystallization temperature of the

AZO ceramic obtained by the TS-CSP method was 180 ℃, which is different from the ZnO ceramic obtained by this method[12]. Since the doping of Al suppresses the rapid growth of crystal grains during the dissolution-recrystallization process, the AZO powder requires a higher crystallization temperature to promote recrystallization of the AZO powder dissolved in the solution. The recrystallized AZO grains fill the gaps of the grains and the grain surface. At this point, the grains require higher temperatures to promote the full growth of all grains.

Figure 5 Schematic diagrams of microstructure change during cold sintering of AZO ceramics.

To prove that the elements were evenly distributed in the AZO ceramic target, the EDS test of the AZO ceramic target section was performed. As can be seen from Fig. 6a, b, c, and d, three elements of Zn, O, and Al were found in the AZO ceramic target, and no other impurity phase was found. This is consistent with the results of the XRD test. The three elements were uniformly distributed in the AZO target, which indicates that the elements of the AZO ceramic did not segregate to the grain boundary during cold sintering. Because of the uniform distribution of elements, AZO ceramics have excellent electrical conductivity. At the same time, the AZO ceramic obtained by the TS-CSP method can avoid the uneven distribution of the internal and external elements of the ceramic caused by the diffusion unevenness of Al under the high-temperature sintering condition and the volatilization of the Zn element. As shown in Fig. 6e, the results of EDS elemental content indicate that the atomic percentages of Zn, Al, and O in AZO ceramics are 45.36, 3.31 and 51.33, respectively. The atomic percentage of the Al element was converted to a mass percentage of Al2O3 of 2.032%, which was consistent with the designed doping amount (2.0wt%). There are two mechanisms for Al-doped ZnO: First, the solid solubility of Al3+ in ZnO is limited. When the concentration of Al3+ exceeds its solid solubility, Al3+ enters the inside of the ZnO unit cell, causing lattice distortion, carrier movement is hindered, and the final conductivity decreases. Second, excessive Al3+ incorporation leads to the

presence of the second phase of ZnAl2O4 in the sample, which hinders the ion mass transfer process in grain growth, and the growth of crystal grains is hindered. No other phase appeared in the AZO ceramic, and the resistivity was low, which proved that the Al element all entered the lattice of ZnO, and did not destroy the lattice structure of ZnO, and formed a solid solution with it. Finally, low resistivity and high-density AZO ceramic were obtained by the TS-CSP method.

Figure 6 Corresponding mappings of Al, Zn, and O of AZO ceramic target.

To study the composition of the AZO samples after cold sintering and to determine the valence of each element, the samples were characterized by XPS energy spectrum system. Fig. 7 is the XPS test result of an AZO ceramic sample obtained by two-step cold sintering at 300 ℃ and 300 MPa when the doping amount of Al2O3 is 2.0 wt%. Fig. 7a is a full spectrum of the AZO sample. Only the Zn, O, Al, and C elements were found in the full spectrum, and there were no other impurities. After analysis, the C element in the AZO sample may be derived from the use of a graphite heater during the sintering process. The carbon standard value is 284.8 eV and the narrow spectrum of the other elements is calibrated using the standard values of carbon. As shown in Fig. 7b, the binding energy of the Zn element is 1023.52 eV and 1046.67 eV, corresponding to Zn 2p1/2 and Zn 2p3/2, respectively. It is indicated that the Zn element is in the form of +2 valence Zn2+ in the sample, and the zirconium hexagonal structure of ZnO is not destroyed by the doping of the Al element[19, 20]. As shown in Fig. 7c, the peak of O 1s at a binding energy of 530.94 eV is assigned to O2- in the Zn-O bond on the ZnO wurtzite structure[21, 22]; the peak at a binding energy of 531.98 eV is

assigned to O- and O2- in the oxygen-defective region of the sample lattice. It can be seen from the fig. 7d that the binding energy of the Al 2p peak is 74.64 eV, indicating that the Al element is successfully doped into the ZnO lattice in the form of +3 valences Al3+.

Figure 7 XPS spectra of the AZO ceramic target: (a) survey, (b) O 1s, (c) Zn 2p, and (d) Al 2p.

4. Conclusions In summary, the feasibility of preparation of high-density AZO ceramics at drastically reduced sintering temperatures under the TS-CSP scheme. A significantly enhanced densification process has been reported which makes this technique particularly prominent compared to other methods of preparing AZO ceramics. The densification

process

is

achieved

according

to

the

principle

of

dissolution-precipitation-recrystallization in the state of low temperature and high pressure. In this work, we successfully prepared AZO ceramic targets with a relative density of 99.23% and a resistivity of 5.58×10-3 Ω·cm. Our work provided a simple and effective route for preparing high-density ceramics at extraordinarily low temperatures. Acknowledgments

This work was financially supported by the Science and Technology Major Project of Guangxi (AA18118001), National Natural Science Foundation of China (No. 61540073), Guangxi Key Laboratory of Information Materials Foundation (No. 191027-Z).

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Highlights: > AZO powder with uniform particle size and good dispersibility was prepared by hydrothermal method. > AZO ceramics with good structure and performance are prepared while reducing energy consumption. > AZO ceramics has a uniform phase and containing no other impurities, with a relative density of up to 99.23%and a resistivity as low as 5.58 x 10-3 Ω·cm.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: