Synthesis of nanosized zirconium carbide powders by a combinational method of sol–gel and pulse current heating

Zirconium carbide nanopowders were synthesized by a novel method combining the advantages of sol–gel method and rapid synthesis using pulse current heating. The core-shelled structure of ZrO₂/C mixture was obtained during the sol–gel process, and further heat treatment in SPS led to the fast formation of ZrC. The particle size of ZrO₂ played an important role in the synthesis of nanosized ZrC powders. In addition, the coalescence and grain growth of ZrC particles could be also limited due to the fast heating rate. As a result, the reactions were thoroughly completed at a relatively low temperature and ZrC nanopowders of 60–100 nm were obtained. The corresponding powders also had low oxygen content (∼0.64 wt%) and residual carbon content (∼0.27 wt%). Additive-free ZrC powders could be sintered to ∼99% relative density with an average grain size of 0.8 μm at low temperature of 1750 °C.     

One-pot synthesis of monoclinic ZrO2 nanocrystals under subcritical hydrothermal conditions

This study reports a one-pot synthesis technique for the preparation of single-phase monoclinic zirconium oxide (ZrO₂) nanocrystals. The products were synthesized from only zirconium oxynitrate (ZrO(NO₃)₂) as the precursor under hydrothermal conditions using subcritical water. The precursor was heat-treated in a batch-type reactor at a reaction temperature of 250 °C for 24 h to obtain pure monoclinic-structured ZrO₂ nanocrystals. The crystallization temperature of the ZrO₂ phase was also greater than 200 °C. However, the products of reactions conducted at 200 °C for 24 h were mixtures of the tetragonal and monoclinic structures. At a reaction temperature of 250 °C, the volume fraction of the monoclinic phase increased; however, the reaction time was also important. The heat-treatment was performed for more than 12 h in order to obtain single-phase monoclinic ZrO₂ nanocrystals. The crystallite size of this product was approximately 20 nm, and water, hydroxide groups, and nitro groups were chemisorbed on its surface.     

Synthesis of zirconium diboride platelets from mechanically activated ZrCl4 and B powder mixture

ZrB₂ platelets were prepared by mechanochemical processing a zirconium (IV) chloride–boron mixture with subsequent annealing from 800 °C to 1200 °C. The phases present were identified by X-ray diffraction. The size and morphology of the synthesized ZrB₂ powders were characterized by scanning electron and transmission electron microscopy. At 800 °C, ZrO₂ was detected in absence of ZrB₂. At or above 1000 °C, ZrCl₄–B converted to ZrB₂. Moreover, at 1200 °C, ZrCl₄–B completely converted to ZrB₂ without trace quantities of residual ZrO₂. The synthesized ZrB₂ consisted of platelets with a diameter of 0.1–2.1 μm and a thickness of 40–200 nm.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.     

Combinatorial hydrothermal synthesis and characterisation of perovskites

In the present work we have applied combinatorial methodology to the hydrothermal synthesis and characterisation of perovskites. In a first series with 48 samples in the (Pb,Ba,Sr)ZrO₃ compositional field, synthesis conditions were optimised by variation of pH. In a second series of 96 samples the (Pb,Ba)(Zr,Ti)O₃ compositional field was investigated, with the aim to control crystallinity and particle size. Using powder X-ray diffraction (XRD), it was established that most compositions crystallised readily within 3–23 h at 200 °C in 2.0–3.3 M KOH. Using scanning electron microscopy (SEM), particle size was determined to be from 3 to 8 μm for the pure zirconates to 0.2–1 μm for the pure titanates. Microprobe wavelength dispersive spectroscopy (WDS) was used for chemical analysis. It was found that particle size was determined primarily by bulk chemistry whereas a polyacrylamide additive and pH had little effect. It has been shown that combinatorial hydrothermal synthesis and characterisation techniques are fully applicable to the synthesis and basic characterisation of perovskites.     

Spark plasma sintering of transparent Y2O3 ceramic using hydrothermal synthesized nanopowders

Y₂O₃ nanopowders were synthesized by the hydrothermal treatment of Y(NO₃)₃·6H₂O and citric acid (CA) as Y⁺³ and the capping agent, respectively. The effect of different CA:Y⁺³ mol ratios, heat treatment time, and calcination temperature was investigated in order to determine their influence on the morphology, particle size and phase of Y₂O₃ nanopowders. The narrow size distribution of particles was obtained with CA:Y⁺³ mol ratio=1.6, heat treatment time of 6 h, and a calcination temperature at 900 °C for 90 min. Then, the synthesized Y₂O₃ nanopowder was consolidated by the spark plasma sintering technique at 1500 °C with a heating rate of 100 °C/min and held for 8 min before turning off the power. As a result, the ceramic prepared with 3 mm thickness got the highest transmission of 80% at 2.5–6 µm wavelength. The highest density and the grain size of yttria ceramic were 99.58% and 1–1.2 µm at 1500 °C, respectively.     

Pressureless reactive sintering of ZrB2 ceramic

Pressureless reactive sintering was investigated to fabricate ZrB₂ from a mixture of elemental Zr and B powders. Both hand mixing and high-energy milling were used to blend the powders. The sintering experiments were carried out at 1800, 2000 and 2200 °C. The samples made from the high-energy milled powders had relative densities varying between 66 and 79%, while the relative densities of the hand mixed powders varied between 58 and 70%. The average grain size of the sintered ceramic was independent of the mixing procedure and increased with the sintering temperature (from 8.5 to 23.5 μm for the hand mixed samples and from 4.6 to 34.9 μm for the milled samples).     

Carbothermal production of ZrB2–ZrO2 ceramic powders from ZrO2–B2O3/B system by high-energy ball milling and annealing assisted process

Zirconium diboride (ZrB₂)-zirconium dioxide (ZrO₂) ceramic powders were prepared by comparing two different boron sources as boron oxide (B₂O₃) and elemental boron (B). The production method was high-energy ball milling and subsequent annealing of powder blends containing stoichiometric amounts of ZrO₂, B₂O₃/B powders in the presence of graphite as a reductant. The effects of milling duration (0, 2 and 6 h), annealing duration (6 and 12 h) and annealing temperature (1200–1400 °C) on the formation and microstructure of ceramic powders were investigated. Phase, thermal and microstructural characterizations of the milled and annealed powders were performed by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and transmission electron microscope (TEM). The formation of ZrB₂ starts after milling for 2 h and annealing at 1300 °C if B₂O₃ is used as boron source and after milling for 2 h and annealing at 1200 °C if B is used as boron source.     

Synthesis and pyrolysis evolution of glucose-derived hydrothermal precursor for nanosized zirconium carbide

Nanosized zirconium carbide (ZrC) was synthesized successfully by a novel hydrothermal precursor conversion method using chelation of polydentate glucose as the carbon source. During the pyrolysis, the core-matrix structure of intimate nanosized ZrO₂ and amorphous carbon mixture forms, resulting in short diffusion path and limit of grain growth. ZrC first appears at a much lower temperature of 1200 °C and completes conversion at 1400 °C in comparison with that of precursor without hydrothermal treatment. By raising the heating temperature to 1600 °C, oxygen content could be reduced (0.55 wt%) with a low residual carbon content (2.3 wt%), and the average size of the spherical crystallite increases from 100 nm to 200 nm. Based on above ZrC powders, the additive-free ceramic with 99.4% relative density by spark plasma sintering (SPS) at a low temperature of 1700 °C has been achieved.     

Sinter forging of rapidly quenched eutectic Al2O3–ZrO2(Y2O3)-glass powders

Spray dried agglomerates of Al₂O₃–ZrO₂ (1% Y₂O₃) with 4 wt.% borosilicate glass were arc plasma sprayed and rapidly quenched into water. Because of the rapid quenching the particles <25 μm were mostly amorphous. After annealing 1 h at 1200 °C the scale of the microstructure of the particles was on the order 30 nm. Hot forging of this powder yielded dense specimens with the width of the ZrO₂ phase still less than 100 nm. Since the particle size ranged from 5 to 25 μm and the scale of the particle microstructure was <100 nm, densification was controlled by creep of the particles rather than by the typical hot pressing mechanism of diffusion along the neck between particles to fill the pores. Thus, the scale of the microstructure controls densification rather than the particle size. These powders offer an alternate source for manufacturing nanostructured parts and should be more suitable for hot pressing or forging than nanoparticulate powders.     

Facile corrosion synthesis and sintering of disperse pure tetragonal zirconia nanoparticles

Disperse pure tetragonal zirconia (t-ZrO₂) nanoparticles smaller than 10 nm are essential for preparation of structural and functional zirconia materials, but syntheses of t-ZrO₂ nanoparticles using inorganic zirconium salts usually result in severe agglomeration. In this paper, we report a hydrothermal corrosion approach for improving the dispersity of t-ZrO₂ nanoparticles synthesized by precipitation using zirconium oxychloride without any surfactants. Disperse pure t-ZrO₂ nanoparticles with average sizes of 4.5 and 6 nm and size distributions of 2–11 and 3–12 nm were obtained by calcining precipitates at 400 °C for 2 h and 500 °C for 0.5 h followed by HCl corrosion at 120 °C for 75 h, respectively. Disperse t-ZrO₂ nanoparticles with an average size of 6 nm and a size distribution of 3–12 nm were pressed into green compacts at 500 MPa and sintered by two-step sintering (heating to 1150 °C without hold and decreasing to 1000 °C with a 10 h hold). The sintered bodies are dense pure monoclinic ZrO₂ nanocrystalline ceramic with a relative density of 99.9% and an average grain size of 110 nm.     

Structural and magnetic properties of the copper ferrite obtained by reactive milling and heat treatment

Copper ferrite (CuFe₂O₄) was synthesised from an equimolar mixture of copper and iron oxides by mechanosynthesis and subsequent heat treatment. After mechanosynthesis, depending on the milling time, the powder consists in a mixture of phases. The heat treatment at 600 °C did not lead to a complete reaction of the mechano-activated precursors. After the heat treatments at 800 and 1000 °C, the complete formation of copper ferrite for almost all the milling times was noticed. The crystal structure of the copper ferrite was found to be cubic for all the samples heat treated at 1000 °C and a mixture of tetragonal and cubic for the samples heat treated at 800 °C. The amount of copper ferrite with cubic structure predominates in the samples with prolonged milling duration and a decrease of the tetragonal distortion by increasing the milling time occurs. The crystallisation of CuFe₂O₄ in cubic structure for the samples milled for prolonged time is influenced by the powder contamination with iron. The magnetisations of the samples obtained after heat treatment at 1000 °C were found to be larger compared to the ones of the samples heat treated at 800 °C. The iron contamination, milling duration and heat treatment temperature influence the cations distribution, thus leading to the saturation magnetisation of the copper ferrite samples ranging from 11.9 μ_B/f.u. to 16.4 μ_B/f.u.     

Effects of tungsten and aluminium on the oxidation and phase formation in mechanically alloyed Ti(C,N)–W–Al systems

The effects of milling parameters and composition of the powder mixtures on the transformations of Ti(C,N)–W–Al powders processed by high energy ball milling were investigated by XRD, SEM and TEM. The strain energy and the fine particle size contributed to the high chemical reactivity with oxygen of the powders milled for 12–24 h. Powders milled for 48 h were chemically stable. The affinity with oxygen decreased after W dissolution in Ti(C,N), and the subsequent decrease in lattice strains. Aluminium lowered the lattice strains, and subsequently the strain energy stored in the deformed crystals of Ti(C_0.5N_0.05) and W milled above 25 °C. Fracturing of hard particles dominated in the early stage of milling in the absence of Al, whereas with Al, plastic deformation of particles and cold welding of Ti(C,N) and W particles by the softer Al prevailed at the same time.     

Influence of milling time on the synthesis,microstructure and mechanical properties of ZrB2SiCZrO2f ceramic composites

Zirconium diboride toughened by silicon carbide and zirconia fiber (ZrB₂SiCZrO_2f) was prepared by using planetary ball mill and the effect of milling time was investigated. The results showed that both the length of fiber and particle size of ZrB₂SiC-matrix were reduced as the ball milling time increased. When milling time varied from 8 h to 12 h, the accumulated fibers and agglomerated particles were observed. The production of a homogeneous ceramic could be successfully achieved by using a combination of 20 h milling time and hot-pressing at 1850 °C for 60 min under a uniaxial load of 30 MPa. The optimal flexural strength and fracture toughness of the hot-pressed ZrB₂SiCZrO_2f ceramics reached 1084 MPa and 6.8 MPa m^1/2, respectively. The main toughening mechanisms were fiber debonding, fiber pull-out and transformation toughening. The results indicated that the ball milling technique was proposed as a potential and simple method to obtain usable quantities of ZrB₂SiCZrO_2f ceramic.     

Synthesis of sub-micro sized high purity zirconium diboride powder through carbothermal and borothermal reduction method

Sub-micro sized zirconium diboride (ZrB₂) powders were successfully prepared via the boro/carbothermal reduction method using zirconium oxide and boron carbide as the primary raw materials. The prepared mixtures were thermally reacted at 1250 °C for 1 h. The optimized composition range containing the lowest oxide and carbide impurity, which was 0.14% of oxygen and 0.3% of carbon contents, was determined using crystallographic and elemental analysis. The particle size was reduced from 5 µm to 245 nm by the addition of B₄C as a reductant within a composition range that maintained the highest purity. The morphology changed from faceted to angular hexagonal bar-like with a simultaneous growth in particle size. Changes in the particle structure were a result of the existing liquid B₂O₃ phase during the reaction. The 245-nm particles contained 12.1% oxygen content and 16.2% oxygen content for the 5-μm particle in the circumstance in which limited oxides could be produced.     

Reaction mechanisms of synthesis and decomposition of fluorapatite–zirconia composite nanopowders

The reaction mechanisms of formation and decomposition of fluorapatite?zirconia composite nanopowders were investigated after the mechanochemical process and subsequent thermal treatment. Experimental results indicated that formation of fluorapatite?zirconia composite nanopowders proceeded in several steps. In the first stage, phosphoric acid formed immediately upon addition of phosphorous pentoxide to the reaction mixture. Afterwards, anhydrous dicalcium phosphate was generated as a result of reaction between reagents with phosphoric acid. The synthesis progressed by the formation of the stoichiometrically deficient hydroxyfluorapatite?zirconia composite at milling times between 5 and 15 min. Ultimately, the fluorapatite?zirconia composite nanopowder was obtained after 300 min of milling. Results revealed that the annealing process led to a decomposition of fluorapatite to tricalcium phosphate and calcium fluoride, and to the transformation of monoclinic zirconia to the tetragonal form. Field emission scanning electron microscope observations showed that the milled sample was composed of fine particles with a mean particle size of about 45 nm after 300 min of milling. Besides, the mean particle size increased progressively due to crystal growth in the temperature range above 900 °C. According to the gained data, reaction mechanism steps were proposed to clarify the reactions occurring during the above-mentioned solid state process.     

Fabrication of submicron alumina ceramics by pulse electric current sintering using M2+ (M = Mg,Ca, Ni)-doped alumina nanopowders

Dense submicron-grained alumina ceramics were fabricated by pulse electric current sintering (PECS) using M²⁺(M: Mg, Ca, Ni)-doped alumina nanopowders at 1250 °C under a uniaxial pressure of 80 MPa. The M²⁺-doped alumina nanopowders (0–0.10 mass%) were prepared through a new sol–gel route using high-purity polyhydroxoaluminum (PHA) and MCl₂ solutions as starting materials. The composite gels obtained were calcined at 900 °C and ground by planetary ball milling. The powders were re-calcined at 900 °C to increase the content of α-alumina particles, which act as seeding for low-temperature densification. Densification and microstructural development depend on the M²⁺ dopant species. Dense alumina ceramics (relative density ≥99.0%) thus obtained had a uniform microstructure composed of fine grains, where the average grain size developed for non-doped, Ni-doped, Mg-doped and Ca-doped samples was 0.67, 0.67, 0.47 and 0.30 μm, respectively, showing that Ca-doping is the most promising method for tailoring of nanocrystalline alumina ceramics.     

Effect of milling type on the microstructural and mechanical properties of W-Ni-ZrC-Y2O3 composites

This study reports the effect of milling type on the microstructural, physical and mechanical properties of the W-Ni-ZrC-Y₂O₃ composites. Powder blends having the composition of W-1 wt% Ni-2 wt% ZrC-1 wt% Y₂O₃ were milled at room temperature for 12 h using a Spex™ 8000D Mixer/Mill or cryomilled in the presence of externally circulated liquid nitrogen for 10 min using a Spex™ 6870 Freezer/Mill or sequentially milled at room temperature and cryogenic condition. Then, powders were compacted in a hydraulic press under a uniaxial pressure of 400 MPa and green bodies were sintered at 1400 °C for 1 h under Ar/H₂ atmosphere. Phase and microstructural characterization of the milled powders and sintered samples were performed using X-ray diffractometer (XRD), TOPAS software, scanning electron microscope/energy dispersive spectrometer (SEM/EDS), X-ray fluorescence (XRF) spectrometer and particle size analyzer (PSA). Archimedes density and Vickers microhardness measurements, and sliding wear tests were also conducted on the sintered samples. The results showed that sequential milling enables the lowest average particle size (214.90 nm) and it is effective in inhibiting W grain coarsening during sintering. The cryomilled and sintered composite yielded a lower hardness value (5.80±0.23 GPa) and higher wear volume loss value (149.42 µm³) than that of the sintered sample after room temperature milling (6.66±0.39 GPa; 102.50 µm³). However, the sequentially milled and sintered sample had the highest relative density and microhardness values of 95.09% and 7.16±0.59 GPa and the lowest wear volume loss value of 66.0 µm³.     

Preparation of Bi4Ti3O12 nanopower by azeotropic co-precipitation and dielectric properties of the sintered ceramic

Bi₄Ti₃O₁₂ nanopowders were prepared by an azeotropic co-precipitation method and the phase evolution process, microstructure and sintering behavior were investigated. The results indicate that well dispersed and agglomerate-free nanocrystalline Bi₄Ti₃O₁₂ with average particle size of 21 nm can be obtained by calcinating the precursor at 750 °C, which is 50 °C lower than traditional solid reaction. The relative density of the ceramic reaches 96% at 1000 °C and shows no evident decrease until 1100 °C. The broadened sintering temperature range and the lower loss tangent of the ceramic show good sintering activity of the nanopowders.     

Highly transparent AlON ceramics sintered from powder synthesized by carbothermal reduction nitridation

Aluminum oxynitride (AlON) powders were synthesized by the carbothermal reduction and nitridation process using commercial γ-Al₂O₃ and carbon black powders as starting materials. And AlON transparent ceramics were fabricated by pressureless sintering under nitrogen atmosphere. The effects of ball milling time on morphology and particle size distribution of the AlON powders, as well as the microstructure and optical property of AlON transparent ceramics were investigated. It is found that single-phase AlON powder was obtained by calcining the γ-Al₂O₃/C mixture at 1550 °C for 1 h and a following heat treatment at 1750 °C for 2 h. The AlON powder ball milled for 24 h showed smaller particles and narrower particle size distribution compared with the 12 h one, which was benefit for the improvement of optical property of AlON transparent ceramics. With the sintering aids of 0.25 wt% MgO and 0.04 wt% Y₂O₃, highly transparent AlON ceramics with in-line transmittance above 80% from visible to infrared range were obtained through pressureless sintering at 1850 °C for 6 h.