ZnO with Additions of Fe2O3: Microstructure, Defects, and Fe Solubility

Zinc oxide (ZnO) crystals with additions of iron (III) oxide exhibit a characteristic inversion domain microstructure with domain boundaries on two different habit planes: parallel to {0001} basal planes and parallel to {2 1 1 5} pyramidal planes of ZnO. The structural inversion of the domains is proved by electron diffraction experiments. In the present transmission electron microscopy study, emphasis is placed on the early stages of domain formation in sintered polycrystalline material and in diffusion couples with single crystals of ZnO. For solute iron content >0.5 at.% of the cations, defects nucleate at the surface and in the interior of ZnO grains at >900°C. These primary defects propagate along the basal planes of ZnO and gradually widen in the positive c -axis direction of the ZnO host crystal. The widening along c is promoted by a second defect on {2 1 1 l } planes that moves away from the basal plane defect. The c -axis orientation in the ZnO region swept by the second defect is inverted, finally resulting in the inversion domain microstructure. A low iron content ≈0.1 at.% was measured in the inverted domains. Energy-filtered imaging and quantitative electron energy-loss spectroscopy show that the inversion domain boundaries (IDBs) parallel to the basal planes contain a full close-packed monolayer of iron whereas the pyramidal IDBs are occupied by iron with ≈2/3 of the content of the basal IDB. Based on experimental observations and arguments of structural chemistry, a mechanism is proposed explaining the nucleation and oriented growth of the inversion domains that are finally induced and driven by the trivalent iron ions at octahedral sites in the primary defects.     

Inversion domain boundaries in Mn and Al dual‐doped ZnO: Atomic structure and electronic properties

The atomic and electronic structures of inversion domain boundaries in Mn‐Al dual‐doped ZnO (Zn_0.89Mn_0.1Al_0.01O) have been investigated. Using atomic‐resolution scanning transmission electron microscopy, a head‐to‐head c‐axis configuration and cation stacking sequence of αβαβ|γ|αβαβ along the c‐axis were observed at the basal‐plane inversion domain boundary. Energy‐dispersive X‐ray spectroscopy and electron energy‐loss spectroscopy revealed significant localization of Mn and minor localization of Al at the basal‐plane inversion domain boundary. Based on experimental findings, a Mn‐doped basal‐plane inversion domain boundary slab model was constructed and refined by first principles calculations. The model is in agreement with atomic‐resolution images. The local electronic density of states of the slab model basal‐plane inversion domain boundary shows a hybridization of the Mn d and O p states within the valence band and localized Mn d states in the conduction band. The thermoelectric properties of Zn_0.99?xMn_xAl_0.01O ceramics have been reported in a previous work. In this work, the effects of inversion domain boundaries on the thermoelectric properties are discussed. In comparison to Zn_0.99?xMn_xAl_0.01O ceramics with x≤0.05, inversion domain boundaries in Zn_0.89Mn_0.1Al_0.01O caused thermal and electrical conductivity reduction due to interface scattering of phonons and electrons. The Seebeck coefficient increased, suggesting electron filtering at inversion domain boundaries.     

Structural features and thermoelectric properties of Al‐doped (ZnO)5In2O3 homologous phases

In this work, we investigated the influence of Al doping on the structure of the (ZnO)₅In₂O₃ homologous phase and the thermoelectric characteristics of (ZnO)₅(In_1?xAl_x)₂O₃ ceramics for x=0, 0.01, 0.03, 0.05, 0.1, and 0.2, prepared using a classic ceramic procedure and sintering at 1500°C for 2 hours. The Al substituted for In on both the primary sites in the Zn₅(In_1?xAl_x)₂O₈ homologous phase, the octahedral sites in the basal‐plane inversion boundaries and the trigonal bi‐pyramidal sites in the zig‐zag inversion boundaries, which resulted in a uniformly increased shrinkage of the unit cell with the additions of Al. The a and c parameters were reduced for x=0.2 by a maximum 0.8%. All the samples had similar microstructures, so the differences in the TE characteristics mainly resulted from the effects of the substitution of Al for In, decreasing the charge‐carrier concentration and affecting their mobility. Slightly improved TE characteristics were only observed for Al additions with x=0.01‐0.05, while larger additions of Al only resulted in a reduced electrical conductivity and decreased ZT values in comparison to the un‐doped composition.     

Inversion Domain Boundaries in Aluminum Nitride

Inversion domain boundaries (IDBs) have been identified in undoped, hot-pressed AIN. The microstructure and micro-chemistry of the IDBs have been studied using conventional transmission electron microscopy, convergent-beam electron diffraction, and analytical electron microscopy. Two distinct IDB morphologies are present: a planar variant which lies on the basal plane (0001), and a curved variant which does not possess a particular habit plane, but portions of the boundary are often seen lying on one of the {1011} planes. The boundaries exhibit α-like fringe contrast, indicating that a translation exists across the boundary. The displacement vectors R_F for the planar and curved boundaries have been investigated using both two-beam and multiple-beam techniques. Microchemical analysis has revealed oxygen segregation to the planar IDB; when present on the curved IDB, oxygen is at a lower concentration than in the planar case. Lattice fringe imaging and long-exposure selected area electron diffraction patterns have indicated the presence of thin, platelike precipitates at the planar IDBs. Sintering and annealing studies indicate that oxygen is necessary for the formation of the planar IDBs and that oxygen is not uniformly distributed along the curved IDBs.     

Metallocene‐Catalyzed Solution Polymerization of Ethene at Elevated Pressure

The kinetic of ethene polymerization catalyzed by (ⁿBuCp)₂ZrCl₂/AlⁱBu₃/[Me₂PhNH]⁺[B(C₆F₅)₄]^? has been investigated at 100 and 140 °C at pressures from 2 to 7 MPa. The initial polymerization rate R_p0 increases linearly with increasing catalyst concentration, whereas a second‐order dependence of R_p0 on the ethene concentration is found the number‐average molecular weight M _n increases with increasing ethene pressure (ethene concentration). The relation between M _n and ethene concentration can be explained by an equation based on a kinetic model involving a single center, two‐state catalyst system.

Influence of the Zr concentration on the initial polymerization rate R_p0. p_Ethene = 7 MPa, solvent (toluene/ethene) = 275 mL.     

In Situ Domain Structure Observation and Giant Magnetoelectric Coupling for PMN‐PT/Terfenol‐D Multiferroic Composites

In situ observations of ferroelectric domain structure evolution, and magnetoelectric (ME) coupling are investigated for PMN‐28PT/Terfenol‐D (abbreviation of Pb(Mg_1/3Nb_2/3)O₃‐28PbTiO₃/Tb_0.3Dy_0.7Fe₂) and PMN‐33PT/Terfenol‐D composites under the magnetic loadings. The composite of PMN‐33PT/Terfenol‐D shows stronger ME coupling than that in PMN‐28PT/Terfenol‐D. At a thickness of 0.10–0.12 mm for the single crystal plate, a giant magnetoelectric coefficient (α_ME) up to 2 V/cm·Oe is obtained for PMN‐33PT/Terfenol‐D at a static magnetic field of 200 Oe and 1 kHz of the alternating magnetic field. In situ domain structure observations reveal the domain morphology change during the applied magnetic loadings. In PMN‐28PT, the domains are of predominantly rhombohedral (R) phase and they change into monoclinic M_A phase upon the magnetic loading via the strain transferred between Terfenol‐D plate and PMN‐PT single crystal. In PMN‐33PT, domains of orthorhombic (O), R, and monoclinic M_C coexist and phase transitions from O to M_C and further to R phase occur upon the magnetic loading. The undulation and diversity of the domain structure makes the domains more susceptible to the magnetic loading via strain transferred between Terfenol‐D plate and PMN‐PT single crystal, and consequently, a strong ME coupling in the composites.     

MC Type Phase Structure and Temperature‐Induced MC‐C Transition in the As‐Grown PMN‐0.36PT Single Crystal

The phase structure in the as‐grown PMN‐0.36PT single crystal was studied and the temperature‐induced domain evolution was investigated using a polarized light microscope (PLM). The crystal is identified to be M_C type monoclinic based on the PLM results and tends to transform to cubic (C) phase directly through polarization rotation. The M_C phase is metastable below Curie temperature (T_C), but prone to tetragonal phase upon electric field poling or high‐temperature annealing. We suppose that the formation of the M_C phase structure is associated with the strain gradient originated from Ti⁴⁺ segregation and the special M_C‐C phase transformation style is mediated by the corresponding residual ferroelastic strain stored within the as‐formed domain structure.     

Investigations on Radiation Tolerance of Mn+1AXn Phases: Study of Ti3SiC2, Ti3AlC2, Cr2AlC,Cr2GeC,Ti2AlC,and Ti2AlN

Nanolaminated M_n+1AX_n phases as candidate materials for next generation nuclear reactor applications show great potential in tolerating radiation damage. However, different M_n+1AX_n materials behave very differently when exposed to energetic neutron and ion irradiations. Based on first‐principle calculations, the radiation tolerance of two M₃AX₂ and four M₂AX phases were studied in this work, covering all the M_n+1AX_n phases previously investigated with experiments. We have calculated the formation energies of Frenkel pairs and antisite pairs in these materials. The improved radiation tolerance from Ti₃AlC₂ to Ti₂AlC observed by experiments can be understood in terms of different Al/TiC layer ratio as the A atomic plane in the nanolaminated crystal M_n+1AX_n accommodates radiation‐induced point defects. The formation of M_A–A_M antisite pair in M_n+1AX_n materials would provide an alternative way to accommodate the defects resulted from radiation damage cascades, whereas this ideal substitution channel does not exist for Cr₂GeC due to its pronouncedly higher M_A–A_M antisite pair formation energy. To further elucidate their radiation damage tolerance mechanism, we have made a detailed analysis on their interatomic M–X, M–A, and X–A bonding characters. Criteria based on the bonding analysis are proposed to assess the radiation tolerance of the six M_n+1AX_n materials, which can be further applied to explore other M_n+1AX_n phases with respect to their performances under radiation environment.     

Vacancy- and doping-dependent electronic and magnetic properties of monolayer SnS2

We have performed first-principles calculations to investigate the formation and migration of vacancies and doping with M (M = Ti, V, Co, Mo, W, Re) at the Sn sites and X (X = O, Se, Te) at the S sites in monolayer SnS₂. We find that the formation energies for S vacancy under both Sn- and S-rich environments are lower than those for Sn vacancy, indicating that the vacancies at the S sites are likely to be formed during the synthesis. Reducing the possibility of vacancy cluster formation, both the vacancies at the Sn and S sites remain robust due to high migration barrier. Additionally, SnS₂ with the vacancies at the Sn sites induces magnetic ground states with a magnetic moment of 4.00 μ_B. Both the Sn- and S-sites vacancies preserve the semiconducting nature of pristine SnS₂ with band gaps of 2.47 eV and 0.30 eV, respectively. Furthermore, we find that the dopants Ti, V, Mo, W, Re can be easily incorporated at the Sn sites in monolayer SnS₂ due to the low formation energies under the S-rich environment. However, Co may not be easily incorporated into SnS₂. The doping with M at the Sn sites induces magnetic ground states in non-magnetic SnS₂. Additionally, a long-range magnetic ordering is observed in SnS₂ doped with V, Co, and Mo. In contrast, easy incorporation of O, Se, and Te at the S sites under the Sn-rich environment has been observed while the semiconducting nature of SnS₂ preserves with small band gaps.     

Self‐assembled structures in blends of disordered and lamellar block copolymers: SAXS,SANS and TEM study

BACKGROUND: The phase behaviour of copolymers and their blends is of great interest due to the phase transitions, self‐assembly and formation of ordered structures. Phenomena associated with the microdomain morphology of parent copolymers and phase behaviour in blends of deuterated block copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA), i.e. (dPS‐block‐dPMMA)₁/(dPS‐block‐PMMA)₂, were investigated using small‐angle X‐ray scattering, small‐angle neutron scattering and transmission electron microscopy as a function of molecular weight, concentration of added copolymers and temperature. RESULTS: Binary blends of the diblock copolymers having different molecular weights and different original micromorphology (one copolymer was in a disordered state and the others were of lamellar phase) were prepared by a solution‐cast process. The blends were found to be completely miscible on the molecular level at all compositions, if their molecular weight ratio was smaller than about 5. The domain spacing D of the blends can be scaled with M_n by D ～ M_n^2/3 as predicted by a previously published postulate (originally suggested and proved for blends of lamellar polystyrene‐block‐polyisoprene copolymers). CONCLUSIONS: The criterion for forming a single‐domain morphology (molecularly mixed blend) taking into account the different solubilization of copolymer blocks has been applied to explain the changes in microdomain morphology during the self‐assembling process in two copolymer blends. Evidently the criterion, suggested originally for blends of lamellar polystyrene‐block‐polyisoprene copolymers, can be employed to a much broader range of block copolymer blends. Copyright © 2008 Society of Chemical Industry     

Synthesis and properties of new novolacs based on heteroatom‐bridged phenol derivatives

We describe the synthesis and properties of new novolacs prepared by addition‐condensation of heteroatom‐bridged phenol derivatives and formaldehyde. The trifluoroacetic acid‐catalyzed polymerization of equimolar amounts of bis(4‐methoxyphenyl) ether ( 1a ) and formaldehyde proceeded homogeneously to afford the polymer ( 2a ) in 49% yield (M_n 2600, M_w/M_n 1.8). From the FTIR, ¹H‐NMR, and ¹³C‐NMR spectra of 2a , it was evident that the polymer had methylene moieties‐bridged repeating units in the polymer backbone. A higher molecular weight novolac ( 2a ′) (yield 99%, M_n 16,600, M_w/M_n 12.9) could be prepared by using an excess of formaldehyde. Bis(4‐methoxyphenyl) sulfone novolac ( 2b ) (M_n 1300, M_w/M_n 1.2) and bis(4‐methoxyphenyl) sulfide novolac ( 2d ) (M_n 1200, M_w/M_n 1.9) were also prepared. However, the polymerization of bis(4‐hydroxyphenyl) sulfone ( 1c ) did not proceed, even when it was attempted under various reaction conditions. From TGA, the temperatures at 10% loss in weight (T₁₀) for 2a , 2a ′, and 2b were found to be 413, 430 and 393°C, respectively. These results suggested that heteroatom‐bridged novolacs based on phenol derivatives have good thermal stability than other organosoluble polymers; moreover, these novolacs could be expected to function as processable materials, polymer blends for engineering plastics, etc. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Nonisothermal crystallization kinetics of linear metallocene polyethylenes

The effect of weight‐average molecular weight (M_w) on the nonisothermal crystallization kinetics of linear metallocene polyethylene (m‐PE) was studied with modulated differential scanning calorimetry. Six linear m‐PEs of molecular weights in the range 122–934 kg/mol were prepared by gas‐phase polymerization. The cooling rate (R) was varied in the range 2–20°C/min, and it significantly affected the crystallization behavior. M_w had a weak influence on both the peak crystallization temperature and the crystallization onset temperature. All m‐PEs showed primary and secondary crystallizations. At both low and high R's, the crystallinity showed a significant drop (～ 30%) when M_w was increased from 122 to 934 kg/mol. At low R's (< 10°C/min), the rate parameters in the modified Avrami method [primary rate constant (k_R)] and Mo method [F(T)] of analyses agreed in suggesting that an increased M_w slowed the rate of crystallization. The M_w dependency of k_R followed the Arrhenius type (k_R = k_Roe²⁸¹/M_w, where k_Ro is a rate‐dependent constant). However, at higher R's, k_R approached a constant value. The order parameters obtained by the different methods of analysis (n and α) were independent of M_w, which suggests that the crystal type remained the same. Hoffman–Lauritzen theory was used for data analysis, and activation energy per segment showed a significant decrease, from 225.0 to 11.8 kJ/mol, when M_w was increased from 152 to 934 kg/mol. Finally, all methods of analysis suggested a significant effect of M_w on slowing the overall crystallization process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Characterization of polyisobutylene-based model urethane networks

Polyisobutylene-based model urethane networks have been prepared by crosslinking liquid α, ω-di(hydroxyl)polyisobutylenes, i.e., PIB-diols carrying exactly two ? CH₂OH functions, F _n = 2.0 ± 0.1, and rather narrow molecular weight distributions, M _w/M _n = 1.5?1.6, with tritriphenylmethyl isocyanate \documentclass{article}\pagestyle{empty}\begin{document}$$\rm{HC}\ (\hskip-6pt\hbox{---}p\rm{C}_6\rm{H}_4\hbox{---}\rm{NCO})_3$$\end{document}. Networks prepared with M _n = 1400 and 7500 PIB-diols, and with 90/10 and 80/20 mixtures of these PIB-diols (bimodal networks), have been characterized by extraction, by the Flory–Rehner swelling method, and by the Mooney–Rivlin equilibrium modulus method, and tested by stress–strain measurements. M _c values of the M _n = 1400 PIB-diol network obtained by swelling (1550) and by equilibrium modulus studies (1500) were in excellent agreement with the M _n of the prepolymer. Also the C₂ parameter was negligible in comparison to C₁, suggesting the absence of interchain entanglements. This is the first hydrocarbon-based polyurethane network that exhibits a negligible C₂ value by stress–strain measurements of unswollen samples. The M _c values of the M _n = 7500 PIB-diol were also in good agreement with the M _n of the prepolymer; however, C₂ was larger than C₁, indicating interchain entanglements. Evidence for strain-induced toughening was observed with both networks prepared with the M _n = 1400 and 7500 PIB-diols. The ultimate properties of the two bimodal networks did not show improvement over those of the individual constituents; however, the M _n's of the constituents were not very different.     

Electrical characterization of the boron trifluoride doped poly(3‐aminoacetophenone)/p‐Si junction

Electrical and interface state properties of the borontrifluoride doped poly(3‐aminoacetophenone)/p‐Si junction have been investigated by current‐voltage and impedance spectroscopy methods. Al/p‐Si/P₃APBF₃/Aldiode indicates a nonideal behavior with electrical parameters (n = 3.53, ?_B = 0.82 eV, and R_s = 1.48 kΩ), which result from the interfacial layer, series resistance, and resistance of the organic semiconductor. The obtained barrier height value of the Al/p‐Si/P₃APBF₃/Aldiode is higher than that of the conventional Al/p‐Si (?_B = 0.58 eV) Schottky diode. The interface state density of the diode was of the order of 1.05× 10¹² eV^?1 cm^?2. It is evaluated that the barrier height and interface state density values of the diode are modified using the boron trifluoride doped poly (3‐aminoacetophenone) organic semiconductor. POLYM. ENG. SCI., 2010. © 2009 Society of Plastics Engineers     

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.     

Synthesis of Diorganotin Esters of 5-Bromonicotinic Acid: X-ray Crystal Structure of Polymeric 5-Bromonicotinatodiorganotin

Two types of diorganotins [R₂Sn(OCOC₅H₃N-3-Br-5)₂] _n and {[R₂Sn(OH₂)(OCOC₅H₃N-3-Br-5)₂]₂} _n (R= Me, n-Bu, Ph, n-Oc), are prepared from 5-Br-omonicotinic acid and diorganotin oxides. All the compounds, 1–8, are characterized by elemental analysis as well as IR and ¹H-NMR spectroscopy. The crystal structures of [R₂Sn(OCOC₅H₃N-3-Br-5)₂] _n (2) and {[R₂Sn(OH₂)(OCOC₅H₃N-3-Br-5)₂]₂} _n (8) were determined by single crystal X-ray diffraction. In compound 2, each carboxylate moiety of 5-Br-omonicotinic acid is involved in coordination to one Sn atom via two O-atoms, and the N-atom of one pyridine-ring coordinates to the neighboring Sn atom which leads to a polymeric chain. And the N-atom of the other pyridine-ring is dissociative. In compound 8, the compound proves to be dinuclear macrocyclic compounds with 5-Br-omonicotinic acid bridging the adjacent tin atoms with a 12-member ring. The hydrogen bonds ( ) are observed in the compound 8. These intermolecular hydrogen bonds form another ring, and lead to a polymeric chain in the lattice at the same time. An erratum to this article can be found at     

Oxidation Behavior of MAX Phase Ti2Al(1−x)SnxC Solid Solution

MAX phase Ti₂Al_(1?x)Sn_xC solid solution with x = 0, 0.32, 0.57, 0.82, and 1 was synthesized by pressureless sintering of uniaxially pressed Ti, Al, Sn, and TiC powder mixtures. Annealing in air atmosphere at 200°C–1000°C triggered a sequence of oxidation reactions which reveal a distinct influence of solid solution composition on the oxidation process. With decreasing Al/Sn ratio, the characteristic temperature of accelerated oxidation reaction of A‐element was reduced from 900°C (x = 0) to 460°C (x = 1). SnO₂ was formed at temperatures significantly lower than TiO₂ (rutile) and Al₂O₃. Substitution of A‐element in MAX phase solid solution by low‐melting elements such as Sn may offer potential for reducing oxidation‐induced crack healing temperatures.     

Multifunctional properties of Zn0·9Mn0.05M0.05O (M = Al,Bi, Sr,Ag) nanocrystals-structural and optical study: Enhance sunlight driven photocatalytic activity

Novel Zn_0·95Mn_0·05O and Zn_0·9Mn_0.05M_0.05O (M = Al, Bi, Sr, Ag) nanocrystals were prepared via the co-precipitation technique. The X-ray diffraction pattern confirmed the substitution of Mn, Al, Bi, Sr, and Ag dopants without altering the basic ZnO structure. The microstructural study was performed by employing the Scherrer plot, Williamson-Hall, and SSP methods. The energy bandgap calculated from UV–vis spectra using different methods observed red-shifted by co-doping. The other optical parameters were also studied and discussed in detail. The FTIR spectra confirmed the presence of Zn–O, and Zn-M–O vibrational modes. Raman spectra demonstrated the presence of ZnO phonon modes, and the Raman shift exhibited the structural defects induced by dopants. The PL spectra showed strong NBE and DLE in the UV and visible region due to extrinsic defects. The IV measurements exhibited the enhancement in the electrical conductivity of ZnO by co-doping. The photocatalytic activity was performed under direct sunlight for methyl orange and methylene blue dyes, and the enhanced degradation efficiency was achieved by co-doping. Furthermore, this article enhances the understanding of tuning the physical properties of ZnO by co-doping and introduces a new class of sunlight-driven photocatalysts.     

Better characterization of raw natural rubber by decreasing the rotor speed of Mooney viscometer: Role of macromolecular structure

The objective of our work was to characterize natural rubber (NR) samples with different macromolecular structures by measuring Mooney viscosities (V_R) at variable rotor speeds ≤2 rpm, called variable speed Mooney viscosity (M_VS). Model samples of technically specified rubbers of constant Mooney viscosity (TSR5CV) were prepared with chosen specific clones. The structures of the samples were characterized by size‐exclusion chromatography coupled with an online multi‐angle light‐scattering detector (SEC‐MALS). Rheological properties of the samples were also characterized by a dynamic moving die rheometer. Measuring monoclonal model samples by M_VS showed three types of V_R flow curves. The V_R at high rotor speed (2 rpm) was correlated with number‐average molar mass (M_n), whereas V_R at low rotor speed (0.05 rpm) was correlated with weight‐average molar mass (M_w). Measuring M_VS revealed the rheological behaviors of samples and enabled discrimination between samples with different macromolecular structures and should thus help in predicting processability. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

The Effect of Composition on the Optical Properties and Hardness of Transparent Al‐rich MgO·nAl2O3 Spinel Ceramics

The study investigates the transmittance and hardness of Al‐rich spinel ceramics (MgO·nAl₂O₃, 1 ≤ n ≤ 2.5) prepared by reaction air sintering (up to closed porosity) of different ratios of fine and coarse‐grained commercial Al₂O₃ and MgO raw powders completed by subsequent hot isostatic pressing (HiP). Different compositions give rise to a wide range of presintering temperatures. With starting compositions 1 ≤ n ≤ 1.5, presintering results in a formation of single‐phase spinel, in which the excess of Al is solved. With higher Al contents (n > 1.5), however, a biphasic ceramic of stoichiometric MgAl₂O₄ and residual alumina is formed first. This excess alumina is incorporated into the spinel lattice during the final HiP at a temperature of 1750°C. Single‐phase, highly transparent spinel is obtained by increasing the Al‐content up to n = 2.5, which gives about 85% in‐line transmittance in the visible range of light and about 63% at a UV wavelength of 200 nm. Whereas the optical properties can be improved, the hardness (HV1) slightly decreases with increasing Al content. Depending on the raw powders, the hardness of samples prepared by finer powders tend to higher values enabled by the development of a bimodal microstructure with a finer grain fraction (≤2 μm) between coarser grains (≤156 μm). In contrast, samples made of coarser powders need higher sintering temperatures and exhibit, then, a monomodal microstructure of very large grains (≤622 μm) only.