Doping effect of divalent cations on sintering of polycrystalline yttria

The sintering behavior of Y₂O₃ doped with 1 mol% of Ca²⁺, Mg²⁺, Mn²⁺, Ni²⁺, Sr²⁺ or Zn²⁺ was investigated by pressureless sintering in air at a sintering temperature in the range 900–1600 °C. The sintering temperature required for full densification in Y₂O₃ was reduced by 100–400 °C by the cation doping, while undoped Y₂O₃ was densified at 1600 °C. The most effective dopant among the examined cations was Zn²⁺. The grain growth kinetics of undoped and cation-doped Y₂O₃ was described by the parabolic law. The grain boundary mobility of Y₂O₃ was accelerated by doping of the divalent cations. High-resolution transmission electron microscopy (HRTEM) observations and nano-probe X-ray energy dispersive spectroscopy (EDS) analyses confirmed that the dopant cations tended to segregate along the grain boundaries without forming amorphous layers. The improved sinterability of Y₂O₃ is probably related to the accelerated grain boundary diffusion owing to the grain boundary segregation of the dopant cations.     

Effect of dopants on microwave dielectric properties of Ba(Zn1/3Nb2/3)O3 ceramics

Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been prepared with various amounts of different dopants such as oxides of monovalent, divalent, trivalent, tetravalent, pentavalent and hexavalent elements. Effect of these dopants on microwave dielectric properties of BZN is investigated. Some of the dopants are found to increase quality factor Q × f and slightly alter the temperature coefficient of resonant frequency (τ_f). Annealing undoped BZN increased the quality factor. Small amounts of dopants such as oxides of Ni, In, Al, Ga, Zr, Ce, Sn, Ti, Sb, and W increased the quality factor. The doped ions substitute for the ordered B ions decreasing the order parameter. Annealing increased the quality factor for all doped BZN samples. Doping BZN with In₂O₃, Al₂O₃, WO₃ and SnO₂ decreased the order parameter but at the same time increased the quality factor indicating that order parameter alone is a poor indicator of quality factor. The quality factor is found to depend on the dopant ionic radii and its concentration. The quality factor increased when the ionic radius of the dopant is close to the ionic radius of the B site ions Zn or Nb. Microstructure studies using SEM showed that the doped high Q ceramics contained large grains.     

Ionic conductivity of tetragonal ZrO2 polycrystal doped with TiO2 and GeO2

The effects of the co-doping and the resultant co-segregation of 2 mol% TiO₂ and 2 mol% GeO₂ on the ionic conductivity and on the chemical bonding state in a tetragonal ZrO₂ polycrystal were investigated. The conductivity data and grain boundary microstructure showed that the doped Ti⁴⁺ and Ge⁴⁺ cations segregate along the grain boundary, and this segregation causes a reduction in the conductivity of both the grain interior and grain boundary and an increase in the activation energy of the grain boundary conductivity. Overall, the data indicate that the segregation retards the diffusion of oxygen anions. A first-principle molecular orbital calculation explains the retarded diffusion of the oxygen anion from a change in the covalent bonds around the dopant cations; an increase in the strength of the covalent bond between the oxygen and doped cation should work to suppress the diffusion of the oxygen anion.     

Direct measurement of interface energies of magnesium aluminate spinel and a brief sintering analysis

Surface and grain boundary energies are key parameters for understanding and controlling microstructural evolution. However, reliable thermodynamic data on interfaces of ceramics are relatively scarce, limiting the realization of their relevance in processes such as sintering and grain growth. In this work, the heat of sintering itself was used to quantify both surface and grain boundary energies in MgAl₂O₄ spinel. Nanoparticles were compacted and heated inside a Differential Scanning Calorimeter (DSC) when densification and grain growth were observed. The evolved heat signal was quantitatively attributed to the respective microstructural evolution in terms of interfacial area change, allowing determination of average surface and grain boundary energies for MgAl₂O₄ as 1.49 J m⁻² and 0.57 J m⁻², respectively. The data was then used to interpret the thermodynamics involved in density and grain growth during isothermal sintering of MgAl₂O₄.     

Influence of MnO2 and Co3O4 dopants on dielectric properties of Pb(Fe2/3W1/3)O3 ceramics

Dielectric properties of Pb(Fe_2/3W_1/3)O₃ ceramic doped with 0.05–1 mol% of MnO₂ or Co₃O₄ were investigated in a wide temperature range from −160 to 450 °C at frequencies 10 Hz–1 MHz. Besides the maxima corresponding to the ferroelectric–paraelectric transition, at higher temperatures other peaks in temperature dependencies of relative electrical permittivity and dissipation factor were observed, attributed to dielectric relaxation. The location and height of these peaks are strongly related to frequency and the dopant level. Both MnO₂ and Co₃O₄ addition caused a significant increase in the resistivity of PFW ceramic—from 10⁶ Ω cm for undoped samples to 10¹¹ Ω cm for those with 1 mol% of a dopant. The activation energies of relaxation calculated on the basis of dielectric measurements are very close to the conduction activation energies determined in similar temperature range.     

Effect of Fe2O3 doping on sintering of yttria-stabilized zirconia

This paper reports the effect of Fe₂O₃ doping on the densification and grain growth in yttria-stabilized zirconia (YSZ) during sintering at 1150 °C for 2 h. Fe₂O₃ doped 3 mol% YSZ (3YSZ) and 8 mol% YSZ (8YSZ) coatings were produced using electrophoretic deposition (EPD). For 0.5 mol% Fe₂O₃ doping, both 3YSZ and 8YSZ coatings during sintering at 1150 °C has similar densification. However, a significant grain growth occurred in 8YSZ during sintering, whereas grain size remains almost constant in 3YSZ. XRD results suggest that Fe₂O₃ addition substitutionally and interstitially dissolved into the lattice of 3YSZ and 8YSZ. In addition, colour of 3YSZ and 8YSZ changes differently with doping of Fe₂O₃. A Fe³⁺ ion interstitial diffusion mechanism is proposed to explain the densification and grain growth behaviour in the Fe₂O₃ doped 3YSZ and 8YSZ. A retard grain growth observed in the Fe₂O₃ doped 3YSZ is attributed to Fe³⁺ segregation at grain boundary.     

Spinel precipitation in Sn4+-doped Co1−xO polycrystals with dispersed Co2+xSn1−xO4 particles

Co_1?xO–SnO₂ powders in molar ratio of 92:8 were reactively sintered at 1400 °C to form Co_1?xO–Co_2+xSn_1?xO₄ composite and then cooled in furnace or air quenched for secondary Co_2+xSn_1?xO₄ spinel precipitation from the Sn⁴⁺ doped Co_1?xO grains. Electron microscope observations indicated the secondary spinel to precipitate at grain boundaries when slowly cooled, but as parallel-epitaxial platelets within the Sn⁴⁺ doped Co_1?xO grains with a precipitate free zone near the grain boundary when air quenched. A process of thermal-mismatch induced {1 1 0} cleaving, taking advantage of cobalt vacancies, and spontaneous healing by oxidation precipitation accounts for the platy spinel precipitation within the grains. The precipitate free zone can be attributed to cobalt vacancy depletion, i.e. site saturation, near the grain boundary during rapid cooling in air. The spinel nanocrystals nucleated from cobalt vacancies in association with Sn⁴⁺ dopant have well-developed {1 1 1} habit plane in order to minimize the coherency strain energy.     

(Al3+, Nb5+) co–doped CaCu3Ti4O12: An extended approach for acceptor–donor heteroatomic substitutions to achieve high–performance giant–dielectric permittivity

Substitution of (Al³⁺, Nb⁵⁺) co–dopants into TiO₆ octahedral sites of CaCu₃Ti₄O₁₂ ceramics, which were prepared by a solid state reaction method and sintered at 1090 °C for 18 h, can cause a great reduction in a low–frequency loss tangent (tanδ≈0.045–0.058) compared to those of Al³⁺ or Nb⁵⁺ single–doped CaCu₃Ti₄O₁₂. Notably, very high dielectric permittivities of 2.9 ? 4.1 × 10⁴ with good dielectric–temperature stability are achieved. The room–temperature grain boundary resistance (R_gb≈0.37–1.17 × 10⁹ Ω.cm) and related conduction activation energy (E_gb≈0.781–0.817 eV), as well as the non–Ohmic properties of the co–doped ceramics are greatly enhanced compared to single–doped ceramics (R_gb≈10⁴–10⁶ Ω cm and E_gb≈0.353–0.619 eV). The results show the importance of grain boundary properties for controlling the nonlinear–electrical and giant–dielectric properties of CaCu₃Ti₄O₁₂ ceramics, supporting the internal barrier layer capacitor model of Schottky barriers at grain boundaries.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

Rare‐Earth Dopant Effects on the Structural,Energetic, and Magnetic Properties of Alumina from First Principles

Density functional theory was used to study the effect of rare‐earth dopants on the structure, phase stability, and magnetic properties of α‐ and θ‐Al₂O₃. Lanthanide series rare‐earth dopants (Pr, Nd, Gd, Er, and Yb) were considered at a doping concentration of 0.83 at.%. Incorporation of rare‐earth dopants was found to increase the lattice parameters and exaggerate the local structural distortion around the dopant. The extent of local lattice distortion was correlated with the dopant ionic radii. The phase stability of rare‐earth‐doped Al₂O₃ was assessed by comparing cohesive and defect formation energies for doped and undoped α‐ and θ‐Al₂O₃. Rare‐earth dopants increased the relative stability of the metastable θ‐Al₂O₃, although doped α‐Al₂O₃ remained more stable. The total magnetic moment of the doped Al₂O₃ was shown to correlate with the number of unpaired electrons. The magnetic moment was also found to be strongly localized on the rare‐earth dopant for Er, Gd, Nd, and Pr‐doped Al₂O₃. In contrast, the Yb dopant induced a delocalized magnetic moment on ~80% of the oxygen atoms. These results further the understanding of dopant incorporation mechanisms, as well as the doping effect on phase stability and magnetic properties that may be applied to advanced field‐assisted material synthesis and processing for enhanced properties.     

Ionic conductivity and relaxations of In-doped GDC (gadolinium doped ceria) ceramics

The effect of In doping on the sintering behaviors and electrical properties of Gd_0.1Ce_0.9O_1.95 (Gd-doped ceria, or GDC) was investigated. The solubility limit of In in GDC was determined to be ~2 at%, and the lattice parameter of GDC was found to decrease from 5.417(7) Å to 5.416(5) Å with 2 at% In dopant. The mean grain size of the sintered body decreased with increasing In content. The concentration of In did not significantly affect the conductivity of the samples; however, undoped GDC showed the highest conductivity. Cole-Cole plots showed that the activation energies of the grain boundaries and grain interiors decreased and increased, respectively, as the In concentration increased to 1 at%. The decreased grain-boundary activation energy is attributed to the segregation of the negatively charged dopant at the grain boundaries, while the increased activation energy of the grain interiors is attributed to the decreases in both the lattice parameters and binding energies with In doping.     

Nucleation and growth of basal-plane inversion boundaries in ZnO

Grain growth studies of zinc oxide ceramics have indicated that inversion boundaries (IBs) are growth faults that control the growth of the zinc oxide (ZnO) grains. To substantiate this observation, we designed experiments to study the nucleation of IBs. Low-temperature experiments showed that in the ZnO–SnO₂ system, IBs form before the Zn₂SnO₄ spinel phase and grains with IBs grow exaggeratedly at the expense of the normal ZnO grains until they completely dominate the microstructure. Experiments using ZnO single crystals embedded into ZnO powder with the addition of SnO₂, Sb₂O₃ and In₂O₃ showed that depending on the oxidation state of the IB-forming dopant ions, there are two competing mechanisms of IB nucleation: (i) internal diffusion, and (ii) surface nucleation and growth. The first mechanism is typical for III+ dopants and is controlled by Zn-vacancy diffusion, whereas the second mechanism holds for all IB-forming dopants and is controlled by chemisorption of the dopants on Zn-deficient (0 0 0 1) surfaces. In both cases, the driving force for the inversion is the preservation of the local charge balance.     

Preparation,sintering and electrical properties of nano-grained multidoped ceria

Multiply doped ceria nanopowders were synthesized by applying MGNP (modified glycine/nitrate procedure). The overall concentration of dopants was kept constant (x = 0.2) whereby Gd ion as the main dopant was gradually substituted by Sm and by Sm + Y. The compositions of solid solutions were calculated by applying defect model introducing anion vacancy radius. Characterization of powders involved BET, TEM, XRD and chemical analyses. Densification was performed at 1500 °C, in an oxygen atmosphere for 1 h. The results showed that with increasing number of dopants, specific surface area of powders increased, followed by decrease of crystallite and grain sizes. Densification degree was also found to rise with increasing number of dopants. According to impedance measurements it was found that ionic conductivity was the highest 1.14 × 10^?3 S cm^?1 at 450 °C in sample doped with Gd, Sm and Y simultaneously.     

The influence of MgO,Y2O3 and ZrO2 additions on densification and grain growth of submicrometre alumina sintered by SPS and HIP

Spark plasma sintering followed by hot isostatic pressing was applied for preparation of polycrystalline alumina with submicron grain size. The effect of additives known to influence both densification and grain growth of alumina, such as MgO, ZrO₂ and Y₂O₃ on microstructure development was studied. In the reference undoped alumina the SPS resulted in some microstructure refinement in comparison to conventionally sintered materials. Relative density >99% was achieved at temperatures >1200 °C, but high temperatures led to rapid grain growth. Addition of 500 ppm of MgO, ZrO₂ and Y₂O₃ led, under the same sintering conditions, to microstructure refinement, but inhibited densification. Doped materials with mean grain size <400 nm were prepared, but the relative density did not exceed 97.9%. Subsequent hot isostatic pressing (HIP) at 1200 and 1250 °C led to quick attainment of full density followed by rapid grain growth. The temperature of 1250 °C was required for complete densification of Y₂O₃ and ZrO₂-doped polycrystalline alumina by HIP (relative density >99.8%), and resulted in fully dense opaque materials with mean grain size<500 nm.     

Screening and manipulation by segregation of dopants in grain boundary of Silicon carbide: First-principles calculations

The effect of segregation behavior of non-metallic dopants (H/He/O) and metallic dopants (Be/Al/Mg/Y) on the performance of grain boundary (GB) in SiC has been systematically investigated by first-principles calculations. Firstly, the GB energy and excess volume of different GBs have been studied to evaluate the stability of GB and the capacity to accommodate dopant atoms. The solution energies of dopant atoms greatly reduce in the GB region compared with those in the bulk, which makes the dopant atoms inside the grain tend to segregate and aggregate near the GB. The driving force of GB on dopant segregation generally decreases with the increase of distance from GB plane, and the preferential site of dopant is closely correlated with the atomic size of dopant. In addition, H and Y atom possesses the lowest segregation energy at the interstitial and substitutional site near the GB, respectively. Next, the segregation of single dopant induced the changes in the strength and stability of GB have been explored. It is found that non-metallic dopants have the significant embrittlement effects on GB strength. However, the segregation of most metallic dopants could strengthen the GB and Mg atom has the most significant strengthening effect on the GB. The stability of GB can be greatly improved by segregation of Al and Y dopants. Besides, the aggregation of H atoms has the obvious embrittlement effect on the GB. Furthermore, the co-segregation behavior of different dopants has also been explored. Be and Mg dopants have the most significant inhibition effect on the segregation of detrimental impurities H/He/O due to the repulsive interaction between dopant atoms. The present results provide a new insight into the effect of dopant segregation on GB properties and are expected to be a useful guidance for screening the chemical composition and manipulating the performance of SiC-based ceramics.     

Low loss alumina dielectrics by aqueous tape casting: The influence of composition on the loss tangent

Polycrystalline aluminas with various concentrations of oxide dopants CaO, MgO, and TiO₂, ranging from 0.05 wt.% to 5 wt.% (3 wt.% in case of MgO), as well as pure alumina references were prepared by tape casting of aqueous suspensions and sintered in air at 1600 °C for 4 h for applications as low dielectric loss electroceramics. Loss tangents were measured at room temperature in the frequency range between 1 and 100 kHz as the key parameter for the intended application. The values of loss tangents of doped materials were influenced by the concentration of dopants. The addition of 0.5 and 5 wt.% of TiO₂ and 3 wt.% of MgO decreased the value of loss tangent in the whole frequency range. The addition of these dopants eliminated abnormal grain growth and decreased the amount of residual porosity. By doing so, the dopants compensated the negative influence of process impurities and decreased the loss tangent values. The cations (Ti⁴⁺) with high solubility in the Al₂O₃ crystal lattice were preferably built into the grain boundary glass, thus efficiently reducing the concentration of polarizable defects in corundum matrix; the formation of vitreous phase had a positive effect on the value of loss tangent in TiO₂ doped samples. The increased values of loss tangent were related to lower density of prepared materials, and the presence of residual porosity. Other contributing factors were especially the formation of calcium-containing secondary crystalline phases, and the increased concentration of lattice defects due to incorporation of atoms with different valencies to alumina crystal lattice.     

Effect of grain growth on the thermal conductivity of liquid-phase sintered silicon carbide ceramics

The effect of grain growth on the thermal conductivity of SiC ceramics sintered with 3 vol% equimolar Gd₂O₃-Y₂O₃ was investigated. During prolonged sintering at 2000 °C in an argon or nitrogen atmosphere, the β → α phase transformation, grain growth, and reduction in lattice oxygen content occurs in the ceramics. The effects of these parameters on the thermal conductivity of liquid-phase sintered SiC ceramics were investigated. The results suggest that (1) grain growth achieved by prolonged sintering at 2000 °C accompanies the decrease of lattice oxygen content and the occurrence of the β → α phase transformation; (2) the reduction of lattice oxygen content plays the most important role in enhancing the thermal conductivity; and (3) the thermal conductivity of the SiC ceramic was insensitive to the occurrence of the β → α phase transformation. The highest thermal conductivity obtained was 225 W(m K)⁻¹ after 12 h sintering at 2000 °C under an applied pressure of 40 MPa in argon.     

CoAl2O4–mullite composites prepared by sol–gel processes

The formation of CoAl₂O₄–mullite composites from diphasic sol–gel precursors with 3:2 mullite composition doped with 1, 2 and 3 at.% Co²⁺ was studied by differential scanning calorimetry (DSC), X-ray diffraction and Rietveld structure refinement. The course of thermal reactions is dominated by the intermediate formation of two faint crystallized phases having different composition and activation energies. The former phase with smaller activation energy (822 kJ mol⁻¹) is attributed to cobalt-containing spinel structure and the latter with larger activation energy (about 1200 kJ mol⁻¹) to Al–Si spinel. With temperature increase Co-containing spinel transforms progressively in CoAl₂O₄, while Al–Si spinel forms mullite above 1100 °C. Mullite lattice parameters, Rietveld refinement data and the CoAl₂O₄/Co²⁺ ratio in annealed samples points out that the majority of cobalt is incorporated in CoAl₂O₄ and only about 0.6 at.% enters mullite structure or the glassy phase, or both.     

Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

Influence of α-Al2O3 addition on sintering and grain growth behaviour of 8 mol% Y2O3-stabilised cubic zirconia (c-ZrO2)

The effect of α-Al₂O₃ addition on sintering and grain growth behaviour of high purity 8 mol% yttria-stabilised cubic zirconia (c-ZrO₂) was investigated. For these purposes, 1 wt.% α-Al₂O₃ was selected as a dopant in c-ZrO₂. The slip-cast specimens were sintered at different temperatures between 1150 and 1400 °C. It was seen that doped c-ZrO₂ had a faster sintering rate and lower sintering temperature than undoped c-ZrO₂. In particular, doped c-ZrO₂ achieved a density of 95% of its theoretical value at 1275 °C, while undoped c-ZrO₂ reached the same value at 1325 °C. The different sinterability of doped c-ZrO₂ and undoped c-ZrO₂ can be attributed to their different behaviour of grain growth. For grain growth measurements, the specimens sintered at 1400 °C were annealed at 1400, 1500 and 1600 °C for 10, 30 and 66 h. It was seen that grain growth rate could be controlled by the deliberate addition of 1 wt.% grain boundary phase of α-Al₂O₃. A grain growth exponent of 2 and activation energy for grain growth of 298 kJ/mol were obtained for undoped c-ZrO₂. The α-Al₂O₃ containing specimens had a grain growth exponent of 3 and activation energy of 361 kJ/mol. The slow grain growth in doped c-ZrO₂ is attributed to solute ions segregation in grain boundary region. The addition of the grain boundary phase results in limiting matter transfer along the grain boundary resulting in slower grain growth.