Kinetics of crystallite size evolution by ball milling

One of the distinguishing features of mechanochemical treatment is crystallite size reduction. The crystallite size of the powder subjected to milling decreases to some minimal value characteristic for the given material. Two processes occur concurrently during milling: reduction of the crystallite size and grain growth, whereby the steady-state crystallite size is attained when the rates of these processes are in equilibrium. This study deals with the kinetics of crystallite size evolution by milling. Published experimental data were analyzed using three kinetic models: dr/dt=-k1r2 + k2r-1; dr/dt=-k1r3 + k2r and dr/dt=-nktn-1(r - rs), where r and rs are the crystallite radius and steady-state crystallite radius respectively, t time, k1 and k2 the rate constants of crystallite reduction and grain growth, respectively, k the rate constant and n the Avrami exponent. The applied kinetic relations describe the crystallite size dependence on milling time in a satisfactory manner

Mechanochemical treatment of Serbian kaolin clay to obtain high reactive pozzolana

Mechanochemical treatment of Serbian kaolin clay was carried out in a planetary ball mill using two different milling media, hardened steel or zirconia vials and balls. The samples obtained with various milling times were characterized by the particle size laser diffraction (PSLD), X-ray diffraction (XRD), differential scanning calorimetry/thermogravimetry (DTA/TGA) and Fourier-transform infrared (FTIR) analyses. Mechanochemical treatment induced amorphization of the kaolinite phase accompanied by dehydroxylation. It was found that for the given milling parameters, amorphization mainly took place in the milling period up to 15 min, and was completed after about 30 min of milling for both milling media used. The pozzolanic activities were determined by the Chapelle method. Milling in the hardened steel milling medium had no significant influence on pozzolanic activity, even though there was accumulated iron contamination. For both milling media, pozzolanic activity of 0.79 was obtained for the samples milled for 15 min and it remained almost unchanged with prolonged milling. The determined pozzolanic activity values are close to these of commercial metakaolinite or metakaolinite obtained by the calcination of the same clay, therefore, indicating possibility for obtaining high reactive pozzolana by mechanochemical treatment of Serbian kaoline clay. [Projekat Ministarstva nauke Republike Srbije, br. TR 36017 and 45001

Structural and thermal characteristics of mechanochemically treated metal powders

U ovom radu je ispitivano dobijanje neravnotežnin struktura u metalima mehanohemijskim tretmanom. Strukturne i termijske karakteristike prahova mehanohemijski tretiranih različito vreme mlevenja su ispitivane rendgenskom strukturnom analizom, diferencijalnom skanirajućom kalorimetrijom, diferencijalnom termijskom analizom, kao i skanirajućom i transmisionom elektronskom mikroskopijom. Eksperimentalni rezultati su diskutovani sa kinetičkog i termodinamičkog stanovišta.\ud Mehanohemijski tretmani, tj. mehaničko legiranje smeša čistih prahova aluminijuma i molibdena različitih početnih sastava (Al-0, 3, 10, 17, 20, 27, 50, 75 i 100 at.%Mo) su rađeni u horizontalnom kugličnom mlinu. U svim sličajevima mlevenje proizvodi nanokristalnu i/ili amorfnu strukturu. Tokom naknadnog termijskog tretmana, ovakvi metastabilni proizvodi lako reaguju obrazujući intermetalna jedinjenja: Al12Mo, Al5Mo, Al4Mo, AlgMo3 i AlMo3.\ud Smesa prahova nikla i molibdena (Ni-50 at.%Mo) je mlevena u različitim tipovima mlinova. Mehaničkim legiranjem u horizontalnom kuglicnom mlinu dobija se amorfna faza. U planetarnom i vibracionom mlinu, zbog veće energije mlevenja, dobija se neuređeno intermetalno jedinjenje.\ud Mehanohemijska reakcija amortizacije u oba ispitivana sistema je slična i odvija se u četiri stupnja: (i) obrazovanje veoma finog kompozitnog praha, (ii) obrazovanje čvrstog rastvora Al(Mo) ili Ni(Mo), (iii) transformacija presićenog čvrstog rastvora u amorfnu fazu i (iv) postepeno rastvaranje zaostalih kristalita molibdena u amorfnoj matrici.In this study the formation of non-equilibrium structures in metals by mechanochemical treatment has been investigated. The structural and thermal properties of powders mechanochemically treated for various milling times have been studied by X-ray diffraction, differential scanning calorimetry, differential thermal analysis, as well as scanning and transmission electron microscopy. The experimental results were discussed from kinetic and thermodynamic point of view.\ud Mechanochemical treatment, i.e. mechanical alloying of mixture of aluminium and molybdenum powders of various starting compositions (Al-O, 3, 10, 17, 20, 27, 50, 75 and 100 at.%Mo) was performed in a horizontal ball mill. In all cases milling produced nanocrystalline or/and amorphous structures. During subsequent heat treatment such metastable products easily react leading to the formation of intermetallic phases: Al12Mo, Al5Mo, Al4Mo, Al8Mo3 and AlMo3.\ud Mixture of nickel and molybdenum powders (Ni-50 at.%Mo) was treated in various types of mills. Mechanical alloying in the horizontal ball mill yields an amorphous phase while in planetary and vibrating ball mill a disordered intermetallic compound was produced as a result of greater milling energies.\ud The formation of amorphous phase in both systems is similar and occurs in four stages: (i) formation of very fine composite powders, (ii) formation of solid solution either Al(Mo) or Ni(Mo), (iii) collapse of supersaturated solid solution into the amorphous phase and (iv) gradual dissolution of residual molybdenum crystallites into the amorphous matrix

Idelana toplotna mašina Sadi Karnoa

Izlazak iz štampe kniige Razmišljanja o pokretačkoj snazi vatre i o mašinama za razvijanje ove snage (Reflexions sur la Puissance Motrice de Feu et sur les Machines Propres a Développer cette Puissance) Sadi Karnoa, 1824. godine, može se uzeti za datum rođenja termodinamike kao nauke. U svom besmrtnom delu Sadi Karno je dao nauci dva plodotvorna koncepta: reverzibilnost i ciklus. Pomoću ova dva nova koncepta on je, u jednom od najblistavijih imaginacija u istoriji nauke, pokazao da je od svih mašina koje rade između dva rezervoara toplote na različitim temperaturama, reverzibilna mašina najefikasnija. To je čuveni Karnoov princip - osnova Drugog zakona termodinamike

Idelana toplotna mašina Sadi Karnoa

Izlazak iz štampe kniige Razmišljanja o pokretačkoj snazi vatre i o mašinama za razvijanje ove snage (Reflexions sur la Puissance Motrice de Feu et sur les Machines Propres a Développer cette Puissance) Sadi Karnoa, 1824. godine, može se uzeti za datum rođenja termodinamike kao nauke. U svom besmrtnom delu Sadi Karno je dao nauci dva plodotvorna koncepta: reverzibilnost i ciklus. Pomoću ova dva nova koncepta on je, u jednom od najblistavijih imaginacija u istoriji nauke, pokazao da je od svih mašina koje rade između dva rezervoara toplote na različitim temperaturama, reverzibilna mašina najefikasnija. To je čuveni Karnoov princip - osnova Drugog zakona termodinamike

Mehanohemijski tretman neorganskih materijala

Pod mehanohemijskim tretmanom podrazumeva se mlevenje jednog ili smeše prahova u različitim tipovima mlinova. U radu je dat kratak pregled osnovnih aspekata ovakvog tretmana i to: (a) najčeše korišćenih mlinova – atricionog, vibracionog, horizontalnog kugličnog i planetarnog, kao i uticaja parametara mlevenja na brzinu i proizvode mehanohemijskog tretmana; (b) mehaničkog legiranja – procesa permanentnog lomljenja i slepljivanja čestica konstitutivnih prahova; (c) strukturnih promena koje dovode do obrazovanja neuređenih, tipično nanokristalnih i amorfnih metastabilnih struktura, i (d) mehanohemijskih reakcija – mehanički indukovanih reakcija koje se odvijaju u neravnotežnim uslovima

Phase composition of Bi2O3 specimens doped with Ti, Zr and Hf

Powder mixtures of α-Bi2O3 containing 2, 5 and 10 mole % of TiO2, ZrO2 or HfO2 were homogenized, heated at 820ºC for 24 h and quenched in air. X-ray powder diffraction technique was used to characterize the prepared samples. In all cases metastable Bi2O3 polymorphs, γ-Bi2O3 or β-Bi2O3, are found as single or major phases. Addition of Ti4+ ions stabilizes γ-Bi2O3 polymorph, while either Zr4+ or Hf4+ ions stabilize β-Bi2O3 polymorph. In the samples with 2 and 5 mole % of TiO2 the presence of even two γ-Bi2O3 phases (Bi12TiO20 compound and a very low Ti-doped γ-Bi2O3) was established. Similarly, in the sample with 2 mole % of HfO2 two β-Bi2O3 phases were found. Phase composition of prepared samples, values of unit cell parameters and the appearance of two polymorphs with identical crystal structure but different unit cell parameters are discussed and compared with known data

Mechanochemical synthesis and electrical conductivity of nanocrystalline δ-Bi2O3 stabilized by HfO2 and ZrO2

A powder mixture of α-Bi2O3 and HfO2, in the molar ratio 2:3, was mechanochemically treated in a planetary ball mill under air, using zirconium oxide vials and balls as the milling medium. After 50 h of milling, the mechanochemical reaction led to the formation of a nanocrystalline δ-Bi2O3 phase (fluorite-type solid solution Bi0.78Hf0.59Zr0.63O3.61), with a crystallite size of 20 nm. The mechanochemical reaction started at a very beginning of milling accompanied by an accumulation of ZrO2 arising from the milling tools. The samples prepared after various milling times were characterized by X-ray powder diffraction and DSC analysis. The electrical properties of the as-milled and pressed Bi0.78Hf0.59Zr0.63O3.61 powder were studied using impedance spectroscopy in the temperature range from 100 to 700°C under air. The electrical conductivity was determined to be 9.43×10-6 and 0.080 S cm-1 for the temperatures of 300 and 700°C, respectively.Smeša prahova α-Bi2O3 i HfO2 u molarnom odnosu 2:3 mehanohemijski je tretirana u planetarnom mlinu u atmosferi vazduha, koristeći cirkonijumske posude i kuglice kao medijum za mlevenje. Posle 50 h mlevenja, mehanohemijska reakcija dovodi do stvaranja nanokristalne δ-Bi2O3 faze (čvrsti rastvor fluoritske strukture Bi0,78Hf0,59Zr0,63O3,61), veličine kristalita 20 nm. Mehanohemijska reakcija otpočinje u samom početku mlevenja i praćena je akumulacijum ZrO2 koji potiče od medijuma za mlevenje. Uzorci dobijeni posle različitih vremena mlevenja karakterisani su rendgenskom strukturnom i termijskom analizom. Električna svojstva mlevenih i presovanih Bi0,78Hf0,59Zr0,63O3,61 prahova ispitivana su impedansnom spektroskopijom u temperaturnom opsegu od 100 do 700°C. Dobijena električna provodnost je 9,43·10-6 i 0,080 S cm-1 za temperaturu 300 i 700°C, redom

Synthesis of calcium oxide based catalysts for biodiesel production

In this work, synthesis of several types of calcium oxide based catalysts by mechanochemical treatment and subsequent calcination is presented. Prepared nanocomposite CaO·ZnO, calcium containing perovskites CaTiO3, CaMnO3, CaZrO3 and Ca2Fe2O5, a series of CaO·SiO2 mixed oxides and calcium diglyceroxide (CaDG) were characterized and tested in the methanolysis of sunflower oil under different working conditions: catalyst amount, agitation speed, temperature and methanol to oil molar ratio

Compressive strength and microstructure of ordinary cured and autoclaved cement-based composites with mechanically activated kaolins

The effects of two different mechanically activated kaolins, AKV (61% kaolinite, 14% quartz and 16% mica) and AKG (51.6% kaolinite and 40.6% quartz) on the compressive strength of cement composites and microstructure of pastes were investigated. Composite mixtures, in which 10, 20, 30, 40 and 50% of ordinary Portland cement (OPC) was replaced by AKV or AKG, were prepared with w/b of 0.5, and exposed to different curing conditions (ordinary curing for 28 days and autoclaving). Factors affecting microstructure were investigated on pastes by X-ray diffraction (XRD), Differential thermal analysis/thermal gravimetry (DTA/TG) analyses, Mercury intrusion porosimetry (MIP) and Scanning electron microscopy with Energy-dispersive spectroscopy (SEM-EDS). AKG composites exhibited higher compressive strengths under both curing conditions. Positive autoclaving effects on strengths were predominantly pronounced at the higher cement replacement levels. Comparison of the autoclaved and ordinary cured paste microstructure, revealed more intensive pozzolanic reaction during autoclaving conditions (CH content near zero) and higher total porosity. The negative effect of hydrogarnet on the strength was compensated by the formation of the crystalline tobermorite. Obtained results revealed that mechanically activated kaolin, with high content of quartz, could be a promising pozzolanic addition, even at high cement replacement levels (30–50%), especially when autoclaving curing conditions were applied. © 2018 Elsevier Lt