Crystallization and Polymorphic Phase Transitions in Zirconia

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Crystallization and Polymorphic Phase Transitions in ZirconiaToughened Alumina Systems Induced by Dy3+/Gd3+ Cosubstitutions V. Ponnilavan, Rugmani Meenambal, and S. Kannan* Centre for Nanoscience and Technology, Pondicherry University, Puducherry-605 014, India

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ABSTRACT: The substitution of rare earth elements in zirconia-toughened alumina (ZTA) to expand the resultant material properties forms a crux of the investigation in recent years. In this work, the simultaneous substitutions of Gd3+ and Dy3+ in ZTA and the subsequent variation in the structural, mechanical, and magnetic imaging characteristics are demonstrated. A detailed structural analysis verified the combined occupancy of Gd3+/Dy3+ at the ZrO2 lattice, while the αAl2O3 component of the composite remains unperturbed. However, the free Gd3+ exceeding the substitution limit reacts with Al2O3 to yield GdAlO3. Further, the optimum level of Gd3+/Dy3+ in ZTA exhibited superior mechanical features with hardness and Young’s modulus of 18 and 251 GPa, respectively. Dy3+ presence persuaded emission in the visible region while Gd3+/Dy3+ combined in ZTA unveiled improved T1 and T2 magnetic resonance imaging features with relaxivity values of r1 = 22 and r2 = 39 mM−1 s−1. In addition, the strong resistance of ZTA against hydrothermal degradation influenced by Gd3+/Dy3+ combination still adds the material value for further applications. toughness in ZTA with CeO2 additions.20 The improved hydrothermal stability of ZTA with a maximum of 14 wt % CeO2 additions is also reported recently.21 The structural changes in ZTA systems induced by excess CeO2 additions and the resultant strong surge in the mechanical properties until a certain limit of CeO2, whereas the annihilation of ZTA grains beyond a critical amount of CeO2 is also presented.22 A recent report by the authors emphasized the improved structural stability of ZTA induced by Dy3+ occupancy at the ZrO2 lattice retaining either t-ZrO2 or c-ZrO2 at room temperature and further Dy3+ presence also shown to have persuaded optical emissions in ZTA systems.23 Gd3+ additions yield a structurally stable ZTA along with enhanced magnetic features, and moreover, the ability of Gd3+ substituted ZTA to induce T1 and T2 magnetic resonance imaging (MRI)24 properties is accomplished. This contrast features is expected to deliver a greater propensity to monitor the implant performance throughout its lifespan devoid of invasive surgical techniques. Thus, the prominence of rare earth additions to augment additional features in ZTA systems is well understood from the recent investigations. Nevertheless, individual additions are expected to impart specific features, while the combined inclusions are anticipated to offer multifunctional features such as mechanical strength, biocompatibility, resistance to in vivo degradation, and imaging contrast. For instance, the individual additions of Gd3+ and Dy3+ mark the enhanced structural and

1. INTRODUCTION Orthopedic implants based on the combination of zirconia (ZrO2) and alumina (Al2O3) are widely investigated in load bearing applications. Majority of the available studies are based on the two different combinations, namely, alumina-toughened zirconia (ATZ) and zirconia-toughened alumina (ZTA) in which the former is based on Al2O3 reinforcement in ZrO2 matrix while the latter is built with ZrO2 reinforcement in Al2O3 matrix.1−6 The major share of ZrO2 in ATZ exhibits higher mechanical properties than ZTA; nevertheless, the ZrO2 content beyond the percolation limit are prone to undergo aging phenomenon that leads to implant failure.7,8 In this context, ZTA is widely accepted and commercialized in orthopedic applications. However, the poor structural stability of ZrO2 in ZTA systems to experience tetragonal to monoclinic ZrO2 (t- → m-ZrO2) transitions under hydrothermal conditions are widely reported in recent years.9−14 To counter this shortcoming, many studies have been commenced to stabilize pristine ZrO2 with different additives (Y3+, Mg2+, Ca2+, Al3+, Fe3+, Ti4+, Si4+) that generally possess mismatch with Zr4+ valence.15−19 The presence of ZrO2 in Al2O3 matrix persuades to retain metastable t-ZrO2 at room temperature; however, with the negligence to restrict t- → m-ZrO 2 transitions under hydrothermal conditions. This drawback is rectified through Y2O3 additions during ZTA preparations in which Y3+ accommodate at the ZrO2 lattice and preserve metastable t-ZrO2 at room temperature. Recent studies emphasize the decisive role displayed by the rare earth element substitutions in ZTA to improve the resultant material properties. Rejab et al. observed an improved fracture © XXXX American Chemical Society

Received: March 23, 2018 Revised: June 9, 2018

A

DOI: 10.1021/acs.cgd.8b00435 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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described in earlier reports.24,28 T1 and T2 maps were acquired respectively through gradient echo pulse sequence using variable flip angle method and fast spin echo sequence. The Matlab software, (The Mathworks Inc., MA) were used to determine the relaxation rate maps. Nanoindentations were performed on specimen’s surface at room temperature using Nanoindenter machine (BRUKER, USA) and the testing was carried out at constant load of 100 mN with loading rate of 0.2 mm/sec. The bulk densities of the specimens were determined using Archimedes method. The specimen preparation for the indentation measurements were done in accordance with the previous report by the authors.29,30 2.3. Structure Refinement. The X-ray powder diffraction patterns were refined to gain quantitative information on ZTA structures due to simultaneous substitutions of Gd3+ and Dy3+. GSASEXPGUI software package was used to perform structure refinement. The standard source files for the refinement of t-ZrO2, m-ZrO2, cZrO2, α-Al2O3, and GdAlO3 structures were respectively obtained from Howard et al.,31 Smith et al.,32 Wyckoff,33 Newnham et al,.34 and Bouley et al.,35 from the American mineralogist crystal structure database. The sequential steps of scale factor, zero shift, background as Chebyshev polynomial of fifth grade, peak profile, and lattice parameters was used to perform the refinement. The fractional coordinates, isotropic temperature, and atomic parameters were also employed during refinement. 2.4. Aging Studies. Aging studies were performed to determine the phase stability of the Gd3+/Dy3+ cosubstituted ZTA under hydrothermal conditions. To this aim, selective specimens were subjected to hydrothermal tests, and the procedure for the sample preparation was done similar to the method adopted for the nanoindentation tests except the aging tests were performed on an unpolished surface. All the specimens were thermally treated at 1400 °C prior to aging tests. All the aging experiments were performed in a dedicated autoclave with the specimens subjected to aging tests were placed above the water level to avoid its immersion in aqueous solution. The conditions for hydrothermal tests were 134 °C and 2 bar pressure, which has been maintained in accordance with the literature protocols.8,36−38 The autoclaved specimens were removed at specific time intervals and were subjected to the XRD tests to monitor structural degradation.

mechanical stability of ZTA systems. Further, systems encompassing Gd3+ is well considered as T1 and T2 MRI contrast agent, while Dy3+ based systems could be used as a potential T2 MRI and computed tomography (CT) contrast agents.25−27 Thus, the dual substitutions of Gd3+ and Dy3+ in ZTA is projected to improve structural and mechanical features along with excellent multimodality imaging contrast characteristics. In this context, the present study aims at the simultaneous substitutions of Gd3+ and Dy3+ in ZTA systems with an aim to achieve multiple features of structural stability and multimodality MR imaging contrast abilities of the resultant material.

2. EXPERIMENTAL METHODS 2.1. Powder Synthesis. Citrate-assisted sol−gel process was employed for the Gd3+/Dy3+ cosubstitutions in ZTA systems. Analytical grade ZrOCl2·8H2O, Al(NO3)3·9H2O, Dy(NO3)3·6H2O, and Gd(NO3)3·6H2O were used as the precursors for the powder synthesis. Citric acid [C6H8O7] was used as a fuel to catalyze the reaction process. Table 1 presents the sample code and their

Table 1. Precursor Concentrations Used during the Synthesis molar concentrations of precursors sample code

Al(NO3)3

ZrOCl2

Dy(NO3)3

Gd(NO3)3

ZTA GDZTA10 GDZTA20 GDZTA30 GDZTA50 GDZTA100

0.750 0.750 0.750 0.750 0.750 0.750

0.250 0.250 0.250 0.250 0.250 0.250

0.0125 0.0250 0.0375 0.0625 0.1250

0.0125 0.0250 0.0375 0.0625 0.1250

respective precursor concentrations used for the powder synthesis. All the compositions were synthesized with Zr4+ to Al3+ in the ratio of 1:3, while the dopant (Dy3+/Gd3+) concentrations with respect to Zr4+ were varied to obtain five different combinations. In a brief description of the synthesis, the individual stock solutions of each metal ion were prepared in deionized water. Now, the stock solutions of Zr4+ and Al3+ were mixed under constant stirring (250 rpm) at 80 °C, and this was followed by the simultaneous additions of Dy(NO3)3 and Gd(NO3)3 solutions after 10 min. To this mixture, an appropriate amount citric acid solution was added, and the resultant mixtures were allowed for constant stirring until the gel formation. The resultant gel was dried at overnight at 120 °C. 2.2. Characterization Studies. The dried gels were ground well to fine powders and subjected to heat treatment at specific temperatures with a dwell time for 4 h followed by the characterization using analytical techniques. X-ray diffractometer (RIGAKU, ULTIMA IV, JAPAN) with Cu Kα radiation (λ= 1.5406 Å) produced at 40 kV and 30 mA to scan the diffraction angles (2θ) between 5 and 90° with a step size of 0.02° 2θ per second were performed to study the structural and phase behavior of the resultant powders. The characteristic vibrational modes of the powders were analyzed through back scattering confocal Raman microscope (RENISHAW, UNITED KINGDOM) with excitation at 780 nm semiconductor laser and data acquiring time of 30 s. Emission properties of the powders at selective temperatures were determined using spectrofluorometer (RENISHAW, UNITED KINGDOM). Vibrating sample magnetometer (VSM, LAKE SHORE 7404) measured at room temperature was used to study the magnetic properties of the powders with a maximum applied field of 10 K Gauss. Surface morphology of the sintered specimens was determined using Scanning Electron Microscopy (SEM, FEI QUANTA- FEG 200, USA). In vitro MRI tests were performed to determine T1 and T2 relaxivity of selective samples through Siemens Magnetom Avanto 1.5 T MRI scanner. The MRI tests were performed in accordance with the procedure

3. RESULTS 3.1. Phase Analysis. The phase analysis of ZTA systems with the assorted range of Gd3+/Dy3+ cosubstitutions after treatment at selective temperatures were determined through XRD analysis. The standard ICDD card nos. 01−079−1765 for t-ZrO2, 01−071−4810 for c-ZrO2, 01−080−0786 for αAl2O3, and 01−080−0786 for GdAlO3 were used for the analysis of diffraction patterns at different temperatures. The results ensure the concomitant effect of temperature and dopant concentrations to play a key role in the structural stability of ZTA systems due to Gd3+/Dy3+ cosubstitutions. The XRD patterns of pristine ZTA at 1100 °C (Figure 1) display diffraction peaks respective of t-ZrO2 and α-Al2O3. Nevertheless, the biphasic nature of ZTA turns into single phase accompanied by reduced crystallinity with simultaneous increments in the Gd3+/Dy3+ cosubstitutions. The absence of characteristic α-Al2O3 reflections accompanied by the gradual reduction in the X-ray intensity of ZrO2 reflections with simultaneous increment in Gd3+/Dy3+ content is apparent from the XRD patterns recorded at 1100 °C. Diffraction patterns distinctive of t-ZrO2 and c-ZrO2 polymorphs were, respectively, noticed in low (i.e., GDZTA10) and high (i.e., GDZTA20 and above) Gd3+/Dy3+ content in ZTA compositions at 1100 °C. Thus, the resilient nature of Gd3+/Dy3+ combine to delay the crystallization of α-Al2O3 alongside the t→ c-ZrO2 transition is ensured at 1100 °C. Further, a gradual shift of the t/c-ZrO2 reflections toward lower 2θ angle with the B



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Figure 3. XRD patterns of pristine ZTA wide range of Gd3+/Dy3+ cosubstitutions in ZTA recorded after heat treatment at 1300 °C.


simultaneous upsurge in the Gd3+/Dy3+ additions is noticed at 1100 °C. XRD patterns recorded at 1200 °C (Figure 2) witnessed the obvious t- → m-ZrO2 transition in pristine ZTA, while all the


that contradicts with the present results in which t-ZrO2 is preserved for the Gd3+/Dy3+ cosubstitutions.23,24 The presence of c-ZrO2 in ZTA systems with higher range of Gd3+ /Dy3+ cosubstitutions starting from GDZTA20 is confirmed from the single Raman active mode (F2g) at 620 cm−1.24,44,45 Further, this single active mode showed a steady shift toward lower wavenumber as a function of enhanced Gd3+/Dy3+ cosubstitutions. The peak positions of the F2g band of c-ZrO2 for GDZTA50 and GDZTA100 displayed good uniformity among each other at both 1200 and 1400 °C. Besides, the Raman active modes representative of α-Al2O3 is also determined at 380 and 410 cm−1 in the entire spectrum recorded at both 1200 and 1400 °C.46 Nevertheless, in addition to the bands typical of c-ZrO2 and α-Al2O3, the spectrum of GDZTA100 recorded at 1400 °C displayed additional bands typical of GdAlO3 that corroborates with the XRD results. Likewise, a minor band determined at 640 cm−1 in all the c-ZrO2 stabilized compositions at both 1200 and 1400 °C accounts for the presence of oxygen vacancies. The origination of oxygen vacancies is quite plausible by the substitution of trivalent Gd3+ and Dy3+ for tetravalent Zr4+ at the ZrO2 lattice. 3.3. Quantitative Structural Analysis. The quantitative influence of variable Gd3+/Dy3+ cosubstitutions in ZTA system were determined through the refinement of powder XRD patterns. The crystallization of various phases was realized from the refined powder XRD patterns through their


compositions with Gd3+/Dy3+ cosubstitutions retained t/cZrO2 phase stability until 1400 °C. The typical X-ray reflections of α-Al2O3 were noticed from 1200 °C onward, and its corresponding intensity indicated a strong surge as a function of temperature. Besides, GDZTA100 with high Gd3+/ Dy3+ content specified the typical GdAlO3 reflections at 1300 and 1400 °C (Figures 3 and 4). Nonetheless, structural changes of t/c-ZrO2 of GDZTA compositions perceived at 1100 °C remained identical until 1400 °C, however, with improved crystalline features. 3.2. Raman Spectra. Raman active modes of two different ZrO2 polymorphs alongside α-Al2O3 were detected at 1200 and 1400 °C (Figure 5). The six Raman active (A1g + 2B1g + 3Eg) modes at 147, 261, 318, 463, 611, and 641 cm−1 typical of t-ZrO2 were determined for GDZTA10.39−43 GDZTA20 also exhibited Raman active modes similar to GDZTA10, however, with dampened peaks that attribute to the initiation of t- → cZrO2 transition. Moreover, the active modes of GDZTA10 and GDZTA20 remained identical at both 1200 and 1400 °C. In the case of Dy3+-only substitutions in ZTA, a complete t- → cZrO2 transition was observed for DZTA20 in a previous study C



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Figure 5. Raman spectra of wide range of Gd3+/Dy3+ cosubstitutions in ZTA recorded after heat treatment at 1200 (a) and 1400 °C (b).

comparison with respective space groups. t-ZrO2 with P42/ nmc(137), c-ZrO2 with Fm3̅m(225), α-Al2O3 with R3̅c, and GdAlO3 with Pnma were the space groups determined from the refinement results. Figure 6 presents the refined diffraction patterns of selective compositions, and Table 2 displays the phase composition data derived from the refinement performed at selective temperatures. A good similarity in the phase composition was revealed at 1200 and 1400 °C with respect to the variable Gd3+/Dy3+ content in ZTA systems. GDZTA10 and GDZTA20 with low Gd3+/Dy3+ contents specify the mixture of t-ZrO2 and α-Al2O3 components at 1200 and 1400 °C. GDZTA30 and GDZTA50 verified the c-ZrO2 and α-Al2O3 combinations, while the mixture of c-ZrO2, αAl2O3, and GdAlO3 phases was determined for GDZTA100 at 1200 and 1400 °C. Moreover, all the compositions displayed an invariable upsurge in the phase fractions of α-Al2O3 at 1400 °C on comparison with their corresponding data gained at 1200 °C. A strong upsurge in phase fractions of t/c-ZrO2 component accompanied by a simultaneous decline in the αAl2O3 content as a function of increment in the Gd3+/Dy3+ cosubstitutions were obvious at both temperatures. Thus, a strong gain in the t/c-ZrO2 phase fractions symbolizes the combined influence of Gd3+/Dy3+ at the crystal structures of t/ c-ZrO2. A surge in the phase content of GdAlO3 with respect to the incremental heat treatments of GDZTA100 implies two postulates: (i) the reaction of amorphous Al2O3 with excess dopants and (ii) enhanced reaction kinetics between Gd3+ and Al2O3 influenced by a thermal drive. A considerable decline in the phase content of α-Al2O3 determined in the case of GDZTA100 attributes to the significant amount of Al3+ consumed by Gd3+ to yield GdAlO3. Tables 3 and 4 present the structural parameters of t/c-ZrO2 and α-Al2O3 derived from refinement at two different temperatures. The consequences of an assorted level of Gd3+/Dy3+ cosubstitutions are apparent as strong variations in the lattice data were noticed at two different temperatures. In the case of t-ZrO2 stabilized compositions, a sharp upsurge in the a-/b-axis with concurrent decline in the c-axis lattice parameters is noticed as a function of Gd3+/Dy3+ content, which implies the ordering of dopant along a-/b-axis. On the

contrary, a continuous expansion of the lattice parameters in case of c-ZrO2 stabilized compositions infers the steady level of Gd3+/Dy3+ occupancy at the ZrO2 lattice. The refined lattice data of α-Al2O3 remains uniform in all the investigated compositions. Attempt to refine Gd3+/Dy3+ occupancy at the α-Al2O3 lattice yielded negative values, which confirms the negligence of α-Al2O3 to host either Gd3+ or Dy3+. However, the refined occupancy values (Table 3) endorse the t/c-ZrO2 structures to host Gd3+/Dy3+ at its lattice. An incessant increment in the refined occupancy values is obvious as a function of enhanced Gd3+/Dy3+ cosubstitutions. 3.4. Optical Characterization. The strong absorbance peaks (Figure 7) noticed in 210−230 nm range centered at 215 nm accounts for the electronic transition of ZrO2 from valence [O(2p)] to conduction band [Zr4+(4d)]. Moreover, the absorbance intensity of ZrO2 showed a strong upsurge with respect to the enhanced Gd3+/Dy3+ cosubstitutions. Literatures also evince the presence of oxygen vacancies in ZrO2 crystal structure to enunciate a strong absorption in the UV region.47,48 The characteristic Dy3+ absorbance peaks are observed within 300−1400 nm range centered at 448, 749, 796, 899, 1085, 1265, and 1679 nm. These peaks account for the charge transition in Dy3+ that occurs from the ground state (6H15/2) to the excited states of 4I15/2, 6F3/2, 6F5/2, 6H5/2, 6F9/2, and 6H9/2.49 The absorption peak at 273 nm is assigned to the 8 S7/2 → 6I7/2 transitions of Gd3+.50 Assignments of the bands corresponding to transitions are in accordance with the energy level scheme of the Gd3+ and Dy3+ reported in the literature.51 The emission behavior (Figure 7b) of ZTA differs with respect to the Dy3+/Gd3+ contents. The emission band perceived at 480 nm of the blue region attributes to the magnetic dipole transition (4F9/2 → 6H15/2) of Dy3+, whereas at 580 nm of the yellow region it accounts for the electric dipole transition (4F9/2 → 6H13/2) of Dy3+.52−54 Further, the emission spectra display the characteristic nature of Dy3+, while the uniqueness of Gd3+ was not evident in the spectrum. 3.5. Magnetization. The M−H curves (Figure 8) recorded at room temperature under the applied field in range of −10000 to 10000 Oe confirmed the paramagnetic response of ZTA due to Gd3+/Dy3+ cosubstitutions. The D



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show linear correlation with incremental Gd3+/Dy3+ cosubstitutions and relaxivity values of r1 (22.09 mM−1 s−1) and r2 (39.11 mM−1 s−1) were determined from the slope. The attained relaxivity value is found higher than the clinically approved MR contrast agents (Gd-DTPA) that possessed a lower relaxivity (r1 = 4 mM−1 s−1 and r2 = 5 mM−1 s−1).55 This indicates that the developed GDZTA system has the potential features for its application as MRI contrast agent. 3.6. Morphological, Mechanical, and in Vitro Aging Studies. All the microstructures (Figure 9) depict the uniform distribution of grains and the existence of ZrO2 and α-Al2O3 components are, respectively, manifested from the light and dark grains. The nature and size of grain distributions were dependent on the concentrations of Gd3+/Dy3+ additions. GDZTA10 demonstrated spherical sized and loosely packed grains accompanied by minute voids. GDZTA30 displayed dense microstructures characterized by small sized grains and negligible voids, while GDZTA50 exhibited the grains typical of molten state complemented by void-free microstructures. The bulk density of the selective specimens (Table 5) envisaged the comparably low density of GDZTA10 than GDZTA30 and GDZTA50, and moreover, negligible difference is noticed among the values of GDZTA30 and GDZTA50. Figure 10 displays an indentation profile of selective GDZTA compositions after thermal treatment at 1400 °C. GDZTA10 demonstrated a nonuniform loading profile complemented by voluminous pop-ups (denoted by arrow marks in Figure 10a), which affirms the presence of voids in the sublayers. Due to these voids, the penetration depth of GDZTA10 also differed with every indent that varies from 0.86 to 1.08 μm. However, the uniform loading profiles with negligible pop-ups displayed by GDZTA30 and GDZTA50 (Figures 10b,c) infer the improved densification supplemented by enhanced Gd3+/Dy3+ cosubstitutions. Moreover, the differences in penetration depth are trivial for GDZTA30 and GDZTA50 as the former showed an average depth of ∼0.028 μm, while the latter envisaged ∼0.042 μm. The maximum hardness and Young’s modulus values were determined for GDZTA30 (Figures 10a−c) that are supplemented with uniform profile and least penetration depth. The mechanical data of GDZTA50 is also in the comparable level with GDZTA30 that exhibited almost similar indentation phenomena. The erratic indentation behavior in terms of irregular loading profile and inconsistent penetration depth exhibited by GDZTA10 is reflected with poor mechanical data (Table 5). Structural changes of GDZTA compositions after hydrothermal treatment at different time intervals were analyzed through XRD analysis. The absence of phase degradation in t/c-ZrO2 is confirmed from the XRD patterns recorded for the autoclaved samples even after 128 h (Figure 11).

Figure 6. Refined diffraction patterns of various GDZTA compositions: (a) GDZTA10 at 1200 °C, (b) GDZTA30 at 1200 °C, and (c) GDZTA100 at 1400 °C.

enhanced level of Gd3+/Dy3+ cosubstitutions induced a corresponding upsurge in the magnetization (M) values (Table 4). Further, T1 and T2 relaxation values of selective GDZTA compositions (GDZTA10, GDZTA30, and GDZTA50) were determined from in vitro MRI analysis. Figures 8b,c show the relaxation maps obtained for longitudinal and transverse relaxation rates with incremental Gd3+/Dy3+ cosubstitutions. The T1 MR images (Figure 8b inset) shifted from hyperintense to hypointense contrast tint, while T2 MR (Figure 8c inset) images showed a reverse trend with respect to Gd3+/Dy3+ cosubstitutions. Relaxation curves

4. DISCUSSION The structural stability of t/c-ZrO2 in ZTA systems in vivo directly correlates with their performance in hydrothermal conditions. ZTA systems fail to a large extent due to the gradual t- → m-ZrO2 phase transition associated with volume expansion, and as a consequence, the system lacks adequate ZrO2 to preserve the toughening of Al2O3. In this context, a wide range of stabilizers has been used to improve the stability of t/c-ZrO2 in ZTA. Here, Gd3+/Dy3+ cosubstitutions were used as stabilizers, and further attempts were also laid to E



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Table 2. Quantitative Phase Content Determined from the Refinement of Variable Gd3+/Dy3+ Cosubstitutions in ZTA Systems after Heat Treatment at 1200 and 1400 °C phase fractions (wt %) 1200 °C c-ZrO2

sample GDZTA10 GDZTA20 GDZTA30 GDZTA50 GDZTA100

1400 °C

t-ZrO2/*GdAlO3

α-Al2O3

50.10 51.90

49.90 48.10 42.90 36.60 26.40

57.10 63.40 72.30

1.30*

c-ZrO2

t-ZrO2/*GdAlO3

α-Al2O3

42.80 47.60

57.20 52.30 43.50 51.70 31.90

54.50 49.30 57.80

10.30*

Table 3. Refinement Agreement Factors and Lattice Parameters Derived from the Refinement of Different GDZTA Systems after Heat Treatment at 1200 °C lattice parameters refinement agreement factors

t-ZrO2

α-Al2O3 (Å)

c-ZrO2 (Å)

sample code

χ2

Rp

a/b-axis

c-axis

ZTA GDZTA10 GDZTA20 GDZTA30 GDZTA50 GDZTA100

1.47 1.86 1.39 1.73 1.36 1.63

7.11 7.10 6.18 7.80 7.14 8.25

3.5948(4) 3.6173(2) 3.6462(3)

5.1976(3) 5.1854(2) 5.1591(2)

a/b/c-axis

a/b-axis

c-axis

Gd3+/Dy3+ occupancy at t/c-ZrO2

5.1638(3) 5.1874(4) 5.2221(2)

4.7596(4) 4.7604(4) 4.7554(3) 4.7570(4) 4.7556(3)

12.9953(4) 12.9934(4) 12.9830(4) 12.9884(3) 12.9868(3)

0.38 0.49 0.61 0.73 0.86

Table 4. Refinement Agreement Factors and Lattice Parameters Derived from the Refinement and Magnetization Values after Heat Treatment at 1400 °C lattice parameters refinement agreement factors

t-ZrO2

c-ZrO2 (Å)

sample code

χ

Rp

a/b-axis

c-axis

GDZTA10 GDZTA20 GDZTA30 GDZTA50 GDZTA100

1.80 1.30 1.78 1.76 1.32

7.41 9.01 10.14 10.12 7.59

3.6082(3) 3.6477(3)

5.1873(4) 5.1515(3)

2

α-Al2O3 (Å)

a/b/c-axis

a/b-axis

c-axis

magnetization M (emu/g)

5.1682(3) 5.1758(2) 5.2008(2)

4.7586(2) 4.7599(3) 4.7595(3) 4.7598(2) 4.7566(3)

12.9919(4) 12.9971(2) 12.9948(2) 12.9962(3) 12.9911(1)

2.064 2.247 4.438 8.546 9.459

Figure 7. Optical properties of selective GDZTA powders recorded after heat treatment at 1400 °C: (a) absorption and (b) emission spectra with an excitation at 342 nm.

crystallization at 1200 °C in the ZTA systems followed by a gradual increment in the phase content as a function of temperature increments signifies the combined effect of Gd3+/ Dy3+ and ZrO2 on the crystallization kinetics of α-Al2O3. It is also explicit from the comparably high enthalpy of formation (ΔH) data of α-Al2O3 (1676 kJ/mol) than ZrO2 (1100 kJ/

explore the optical and magnetic potentials of the resultant material that is generally desired for in vivo imaging. Despite the wide range of Gd3+/Dy3+ additions to ZTA systems, the results affirm the unique crystallization of either tZrO2 or c-ZrO2 at 1100 °C that were dependent on Gd3+/Dy3+ contents. Notwithstanding the literature evidence on the crystallization of pristine α-Al2O3 at 1050 °C,56 its delayed F



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Figure 8. (a) Magnetization curves and (b,c) T1/T2 MRI contrast features of GDZTA compositions.

Figure 9. Morphological features of selective GDZTA specimens sintered at 1400 °C: (a) GDZTA10, (b) GDZTA30, and (c) GDZTA50.

mol) where the available thermal energy is consumed to host Gd3+/Dy3+ at the t/c-ZrO2 lattice. The stabilization of t/c-ZrO2 at room temperature is controlled by two important factors, namely, crystallite size and lattice defects. t-ZrO2 stabilization is reported to have retained a critical control on the particle size.57,58 Investigations also substantiate the accomplishment of t/c-ZrO2 at

Table 5. Bulk Density and Mechanical Properties of Various GDZTA Compositions sample

bulk density (g/cm3)

hardness, H (GPa)

Young’s modulus, E (GPa)

GDZTA10 GDZTA30 GDZTA50

3.60 ± 0.32 4.28 ± 0.26 4.25 ± 0.35

04.32 ± 0.43 18.18 ± 0.76 17.90 ± 1.50

103.77 ± 13.70 251.32 ± 11.30 215.26 ± 08.95

Figure 10. Load vs displacement curves of GDZTA specimen obtained from the nanoindentation tests. G



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Figure 11. XRD patterns of selective GDZTA compositions after various hours of hydrothermal treatments.

Figure 12. Variations in lattice parameters and unit cell density of ZrO2 with Dy3+*, Gd3+*, and Gd3+/Dy3+ substitutions in ZTA systems. The lattice and unit cell density data respective of Dy3+-only and Gd3+-only substitutions were taken from previous reports.15,16

infers the role of excess oxygen vacancies to augment c-ZrO2 stabilization, and further, it has been shown that a minimum of 4% oxygen vacancies are adequate to stabilize c-ZrO2.63 It is thus postulated from the present results that gradual accumulation of Gd3+/Dy3+ at ZrO2 lattice escalates the creation of more vacancies, and moreover, a gradual t→ c-ZrO2 transition is determined. A close examination of unit cell density and lattice parameters (Figures 12a,b) reveals the importance of Gd3+/Dy3+ size effect on ZTA systems. Up to 20 mol % of Gd3+/Dy3+ combinations, t-ZrO2 stabilization is achieved beyond which c-ZrO2 is accomplished. In the case of c-ZrO2 stabilization, a gradual upsurge in the overall a/b/c-axis parameters is observed with single and combined additions. It is notable that higher-sized Gd3+ induced comparably a much expanded ZrO2 lattice than the lower-sized Dy3+ substitution at identical conditions, while Gd3+/Dy3+ combines in the ZTA system experienced lattice data in the intermediate ranges23,24 (Figure 12a). The determined variations in the lattice data for Gd3+/Dy3+ substitutions are pretty convincing as the average ionic size for the Gd3+/Dy3+ combinations [(0.91 + 0.94)/2 = 0.925 Å] lies in the intermediate region between Dy3+(0.91 Å) and Gd3+ (0.94 Å). A similar trend is also witnessed in case of unit cell density wherein a gradual reduction in the density data is observed for Gd3+/Dy3+ cosubstitution (Figure 12b) that is also mainly due to the size effect of the dopants.

room temperature with the aid of stabilizers that creates oxygen vacancies at ZrO2 lattice.59−61 Usually, oxygen vacancies are generated to maintain net charge balance created by the occupancy of the dopant with different oxidation states. In the present study, the pristine ZTA system devoid of Gd3+/ Dy3+ inclusions attains t-ZrO2 stabilization after treatment at 1100 °C allied by the manifestation of Al2O3 in amorphous state. The refined data affirms the negligence of Al3+ occupancy at the t-ZrO2 lattice, and this fact also further infers the role of amorphous Al2O3 to engulf ZrO2 particles and subsequently restrict the grain size growth to accomplish t-ZrO 2 stabilization. The t- → m-ZrO2 transition at 1200 °C is evident from the crystallization of α-Al2O3, which is expected to have occurred from the uncontrolled growth of ZrO2 grain. Bartolome et al. has also made a similar observation; however, the reason for the t-ZrO2 stabilization is stated as the differences in residual stress of coefficient of thermal expansion between Al2O3 and ZrO2 while sintering.62 Further, the absence of discrete grains determined from the surface morphology of pure ZTA specimen (Supporting Information 1) at 1100 °C confirms their poor sintering ability. On the contrary, Gd3+/Dy3+ additions to the ZTA system retain structural stability until 1400 °C either as t-ZrO2 or c-ZrO2, while the α-Al2O3 component of the composites remains unperturbed. Zeng et al. proposed a mechanism for c-ZrO2 stabilization based on density functional theory (DFT), which H



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nevertheless, with GDZTA30 yielding better values among the investigated compositions. The mechanical data obtained in the present study exhibits good similarities with the data of commercial ZTA products.64−66 Further, long-term stability of these materials under hydrothermal conditions reveals their potential in orthopedic applications. Deville et al. reported a 15% t- → m-ZrO2 degradation in ZTA after 115 h of aging tests, and further stated, that such phase transition is observed for

Crystallization and Polymorphic Phase Transitions in Zirconia

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