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Phase transformations in monoclinic zirconia caused by milling and subsequent annealing
Abstract
Monoclinic ZrO[sub 2] has been milled for various times down to a crystallite size of 145 [angstrom]. Using the time-differential perturbed-angular-correlation technique, it was possible to associate the resulting growing amount of disordered material with two different, very distributed and milling-time-dependent hyperfine interactions: one (Y) of quadrupole frequency similar to that of tetragonal zirconia after long milling, the other (X) one of a quadrupole frequency similar to that of monoclinic zirconia at shorter milling times. Upon annealing, all samples showed the recovery of the crystalline monoclinic phase at the expense of the disordered structures. In samples milled for shorter times, the (Y) interaction emerges as an ordered crystallization product, upon annealing at temperatures which depend nearly linearly on the crystallite size.
... Details of the atomic arrangements of materials of technological uses were studied using short range methods as extended X-ray absorption fine structure (EXAFS) [22] and the perturbed angular correlations (PAC) [23,24] techniques. In particular, PAC technique has proved to be an efficient tool in the investigation of the milling effect of the monoclinic zirconia by showing the quadrupole interactions of special atomic arrays through which the monoclinic phase transforms into the tetragonal form [25]. ...
... I 2 -Quadrupole interaction (m 0 -ZrO 2 ) similar but more assymetric and distributed than that of the crystalline monoclinic ZrO 2 . This interaction, associated to distorted monoclinic zirconia, was reported to exist in milled pure ZrO 2 [25]. I 3 -Already known quadrupole interaction depicting the metastable tetragonal t 0 -phase of the ZrO 2 . ...
... And estimated SS lattice parameters (XRD-Rietveld). c 0 100 0 --5 97 3 50,923 52,036 10 91 9 50,892 52,077 20 39 61 50,953 51,870 60 0 100 50, reported to appear as a consequence of the charge compensation when dopping zirconia with aliovalen oxides [32] as well as by milling pure zirconia [25]. ...
The mechanochemical activation process has proved to be an effective technique to enhance a solid-state reaction at relatively low temperatures. In such a process, the mechanical effects of milling, such as reduction of particle size and mixture homogenization, are accompanied by chemical effects. The actual chemical phases during the mechanical treatments are sometimes difficult to identify and quantify at an atomic level. In the present paper the XRD and PAC techniques are used to study the phase content evolution during the milling process to obtain the tetragonal stabilize (ZrO2)0.97(Y2O3)0.03 solid solution. It has been determined that 10 min of milling time are enough to start the solid solution formation. Both techniques allowed establishing that the obtained crystalline phase was tetragonal. PAC results showed that the stabilized solid solution occurs via the formation of distorted monoclinic ZrO2 form not distinguished by XRD. The time evolution of the phase contents could be modeled as a consecutive first order solid state reaction m-ZrO2→m′-ZrO2→t-SS. The values of the kinetic constants indicate that the process was controlled by the second reaction.
... The technological has led to of importance zirconia num investigations on the reversible erous tetragonal-monoclinic transition, which is of a martensitic shear type and normally appears at temperatures between 1100-1200 o C. The high temperature form, t-ZrO2, cannot be quenched, yet, surprisingly, it can exist at room temperature. It can be obtained by the thermal decomposition of zirconium salts, precipitation from aqueous solutions [1][2][3] or by mechanical processing of the monoclinic zirconia phase [4][5][6][7]. ...
... In spite of that the developed theory perfectly explains the stability of the tetragonal phase at room temperature, it still does not shed light on the phenomenon of the reverse transformation of the monoclinic phase to tetragonal, induced by mechanical treatment. The next step on clarifying the m → t transformation mechanism was made by a research group from Argentina, who introduced the time-deferential perturbed-angular correlation method in the investigation of the milling effect on m-ZrO2 powders [5]. The authors assumed, that with reduction of particle size, mechanical treatment also produces simultaneous structural changes which are associated with amorphisation of the crystalline network. ...
The phase transformation of zirconia from monoclinic to tetragonal polymorph at room temperature under mechanical processing has been a subject of a great interest due to technological importance of this material. Mechanism of this transformation has been widely investigated and plenty of explanation theories of zirconia stabilisation have been developed as well. This article critically reviews the systematic development regarding this transformation under mechanical processing and includes the summarised results of key-publications on this topic.
... Various proposed models, based on surface energy (nanocrystalline size), strain energy, internal and external hydrostatic pressure, structural similarities, foreign surface oxides, anionic impurities, water vapour and lattice defects (oxygen ion vacancies), were presented. [11][12][13][14] Gajović et al. 15 showed that the ball milling had little or no influence on the transition from the starting m-ZrO 2 to the high temperature t-ZrO 2 . It was concluded that a partial transition from monoclinic to tetragonal polymorph, observed in some earlier ball milling experiments, can be attributed to the stabilising influence of impurities introduced due to the wearing of the milling media. ...
Phase transformation of monoclinic ZrO2 to tetragonal and cubic ZrO2 was successfully performed during ball milling. X-ray diffraction and high temperature in situ X-ray diffraction were performed to study the progressing of this transformation. Quantity phase analysis, crystallite size and microstrain were calculated by the Rietveld refinement method. Morphology and microstructure were monitored by scanning and transmission election microscopy respectively. The results showed that this transformation was initiated after 30 h milling and progressed gradually at longer milling times. Longer milling led to the smaller crystallites with larger microstrain, which were proposed as the mechanism of stabilisation of tetragonal and cubic ZrO2 at room temperature. A nanostructure powder with a mean crystallite size of <10 nm was obtained at the end of milling on the basis of the Rietveld method, which was confirmed by transmission electron microscopic images.
... This peak could not be detected in the milled samples (Z5, Z10, Z20 and Z60), showing that this process produced the loss of crystallinity, together with the solid solution formation. Pure m-ZrO 2 submitted to the same treatment only presented a partial transformation with long milling treatments [5]. ...
The mechanochemical activation processing has proved to be an effective technique to enhance a solid-state reaction at relatively low temperatures. In such a process, the mechanical effects of milling, such as reduction of particle size and mixture homogenization, are accompanied by chemical effects, such as partial decomposition of salts or hydroxides resulting in very active reactants. The objective of the present work is to obtain (ZrO2)0.97(Y2O3)0.03 nanocrystalline tetragonal solid solution powders directly using a high energy milling on a mixture of the pure oxides. A second objective is to evaluate the efficiency of the processing proposed and to characterize both textural and structural evolution of the mixtures during the milling processes and throughout posterior low temperature treatments. The Textural and structural evolution were studied by XRD analysis, specific area measurements (BET) and SEM. Firstly a decrease of the crystallinity of the reactants was observed, followed by the disappearance of Y2O3 diffraction peaks and the partial appearance of the tetragonal phase at room temperature. The solid solution proportion was increased with the high energy milling time, obtaining complete stabilization of the tetragonal solid solution with long milling treatments (60 min).The obtained powders were uniaxially pressed and sintered at different temperatures (600-1400°C); the influence of the milling time was correlated with the sinterization degree and final crystalline composition of the materials. Finally, fully stabilized nanocrystalline zirconia materials were obtained satisfactorily by the proposed method.
... Le broyage des poudres A80ZY et A20ZY conduit à la transformation de la phase monoclinique de la zircone dont l'intensité des pics de diffraction des rayons X diminue fortement (figures 8 et 9). Elle peut correspondre à une transformation en la phase tétragonale [8] ou à une amorphisation comme observé dans le cas du broyage de zircone pure [9]. Simultanément à la transformation de la phase monoclinique de la zircone, une diminution de la taille des cristallites de la phase tétragonale de la zircone, d'environ 24 nm à 9 nm pour les poudres A20ZY et à 12 nm Taile des particules (µm) pour les poudres A80ZY est observée (figure 10). ...
By means of Perturbed Angular Correlation (PAC) analysis the resulting phases during mechano-chemical activation process on zircon (ZrSiO4) commercial fine powder (D50 ≅ 0.8μm) were accurately identified and characterized.A high energy planetary mill was employed with 850. rpm up to 120. min. The phenomenological macroscopic confirmation of the structural change and mechanical activation consisted in an important enhancement of the sintering behavior of the treated fine zircon powders.Three different well known zircon phases were identified and quantified as a function of the milling time: a fully crystalline phase, an aperiodic phase and a distorted phase. A decrease in the first two phases was accompanied by the appearance of the third one; finally, at long term treatments, a partial dissociation was observed. Particularly the resulting zirconium oxide is a highly distorted one. The results were discussed together with those obtained using XRD, SEM and laser scattering. The XRD only showed the partial dissociation of zircon and failed in the differentiation of its nanoconfigurations observed by PAC. The milling of this hard material can be optimized through the performed characterization strategy.
Compact pressure causes a martensitic transformation of partially stabilized tetragonal bismuth-doped ZrO2 powders to the monoclinic phase. A similar effect is not observed when pelletization, followed by firing, of amorphous bismuth/ZrO2 is performed. The stress-induced transition has been monitored via X-ray diffractometry, Raman spectroscopy, and Fourier transform infrared reflectivity. Surface bismuth segregation, which involves few ionic planes, has been probed by angle-resolved X-ray photoelectron spectroscopy. The bismuth segregation enthalpy has been evaluated.
Structural and microstructural developments of m-ZrO 2 during the ball-milling process strongly depend on the type of milling assembly used. Ball-milling with corundum milling assembly cause almost complete amorphization of the starting m-ZrO 2 , while ball-milling with agate or stainless steel assembly cause a decrease of the volume-averaged domain size up to ∼9 nm or ∼13 nm, respectively. Regardless of the type of milling assembly, a small amount of t-ZrO 2 appeared in the milled products. The onset of m-ZrO 2 → t-ZrO 2 transition occurred only in products ball-milled with stainless steel assembly and resulted in a complete transition after 20 h and probable further transition into c-ZrO 2 after prolonged milling. The stabilization of t-and c-ZrO 2 in this products resulted from the incorporation of aliovalent cations due to the wear and oxidation of the milling media. The small fraction of t-ZrO 2 in the product milled with corundum or agate milling assembly could not be clearly connected with the effect of crystallite size decrease.
In the field of refractory materials, ceramics containing mullite–zirconia are the basis of those most used in the industry
of glass and steel. It is known that the addition of zircon improves the behavior of the refractory used in service. Knowing
that some mullite–zirconia composites properties as fracture strength and the elastic modulus E are associated with the material
microstructure integrity, the eventual thermal decomposition of zircon into zirconia and silica could seriously alter the
material elastic properties. In this paper the phase content of a series of mullite–zirconia–zircon (3Al2O3.2SiO2–ZrO2–ZrSiO4) composites is determined at atomic level via perturbed angular correlations (PAC) and compared with that derived from the
long range X-ray diffraction technique. PAC results on the as-prepared materials indicate that all nominal zircon is present
and that it involves two types of nanoconfigurations, one of them describing aperiodic regions. The thermomechanical properties
already reported for these materials could be related to the crystalline to aperiodic zircon concentrations ratio they exhibit.
KeywordsCeramics–XRD techniques–Gamma ray–PAC
The short range technique of the Perturbed Angular Correlation (PAC) and x-ray diffraction (Rietveld) methods have been employed
to determine the phase content in commercial mullite–zirconia and zircon raw materials that are ordinarily used to produce
ceramic materials. The PAC technique, which probes zirconium-containing compounds at nanoscopic level, showed that zircon
contains crystalline ZrSiO4 and an important amount of a structurally distorted zircon, which is also observed accompanying monoclinic zirconia in mullite–zirconia.
This particular zircon phase was not detected by the long range x-ray diffraction–Rietveld technique. After an annealing treatment,
important changes in crystalline contents of the powders allow confirming, by the x-ray diffraction–Rietveld method, the preexistence
of this particular zircon phase. This fact must be taken into account when preparing multicomposites based on the present
raw materials.
KeywordsCeramics–XRD techniques–Gamma ray–PAC
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