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Dilatometric study of the ZnTiO3 phase transition kinetic influenced by nano powder sintering
No full textSintering is often accompanied with phase transition. Phase transition kinetic is thushard to deconvolute due to the superposition of the sintering and phase transition dimensionalchange phenomena. Metastabile perovskite phase ZnTiO3 reforms to stabile spinel Zn2TiO4 at950°C with high kinetic rate. Dimensional change during heating of ZnTiO3 nano powdercompacts up to 1050°C was monitored using dilatometric thermo mechanical analyzer TMAmodel SETSYS Evolution. Shrinkage of simultaneous sintering and phase transition wererecorded. The obtained non-isothermally sintered specimens were then second run treatedwith same schedule. Sintering phenomenon of the ZnTiO3 nanopowder compact was alsorecorded up to 900°C with isothermal holding of 25 minutes. Here phase transition wasavoided with lower temperature and isothermal holding. Second run heating, of isothermallyobtained specimens at 900°C, was recorded with non-isothermal heating schedule to 1050°C.This has led to the dilatometric curve record of the ZnTiO3 phase transition in polycrystallinebulk specimen, now recorded without sintering. In such a manner when kinetic is complex,the separation of phenomena such as sintering, linear expansion and phase transition areleading to the knowledge of their mutual interconnected relations. Also application ofmathematical operations on dilatometric data leads to the established procedure for thesintering and phase transition data treatment.Sound recording of the presentation: Labus, Nebojša, Rosić, Milena, Marković, Smilja, Nikolić, Maria Vesna, "Dilatometric study of the ZnTiO3 phase transition kinetic influenced by nano powder sintering" in Program and the Book of abstracts / Serbian Ceramic Society Conference Advanced Ceramics and Application IX : New Frontiers in Multifunctional Material Science and Processing, Serbia, Belgrade, 20-21. September 2021 (2021):41-41, [https://hdl.handle.net/21.15107/rcub_dais_11894]Conference abstract: [https://hdl.handle.net/21.15107/rcub_dais_11894]Conference presentation: [https://hdl.handle.net/21.15107/rcub_dais_11892
Sinterovanje i svojstva sinterovanog ZnTiO3
Get PDFPhD defence presentation. The thesis "Sintering and Properties of Sintered ZnTiO3" was defended at the University of Belgrade, Faculty of Physical Chemistry, in 2011.Prezentacija prikazana na odbrani doktorske teze "Sinterovanje i svojstva sinterovanog ZnTiO3", odbranjene na Fakultetu za fizičku hemiju Univerziteta u Beogradu 2011. godine
Sintering and Properties of Sintered ZnTiO3
Get PDFUltrafini prah cink-metatitanata veličine čestica 40 do 80 nm podvrgnut je prilikom sintetskog dela tretmana uticajima pritiska i temperature korišćenjem procesa presovanja i reakcionog sinterovanja. S obzirom na nestabilnost praha posmatran je odgovor sistema na ova dva tretmana. Usled navedenih uticaja došlo je do specifičnih promena svojstava ispresaka i sinterovanih uzoraka. Što se presovanja tiče urađen je set ispresaka kompaktiranih na različitim pritiscima uniaksijalnim dvostranim načinom presovanja. Metode karakterizacije ispresaka koje su korišćene su: merenje gustine ispresaka, merenje specifične površine metodom adsorpcije azota na niskim temperaturama, određivanje poroznosti metodom živine porozimetrije. Za opisivanje fenomena presovanja upoređene su eksperimentalne vrednosti gustine ispresaka u zavisnosti od pritiska presovanja sa jednačinama kompresibilnosti. Pritisak od 98 MPa određen je kao najpogodniji pri kompaktiranju ispresaka za naredni proces sinterovanja. Urađena je rendgenostrukturna analiza kao i atomska mikroskopija ispreska cink-metatitanata ZnTiO3. Proces sinterovanja praćen je tehnikom dilatometrije, odnosno linearnog skupljanja uzoraka pri zagrevanju. Rađeno je u dva režima rada do 880°C i do 1200°C. Za režim rada pri zagrevanju do 880°C za sinterovanje određena je energija aktivacije sinterovanja. Urađena je rendgenostrukturna analiza sinterovanih uzoraka, i na osnovu različitog faznog sastava dokazano postojanje faznog prelaza u stabilniju formu spinelske strukture, cink-ortotitanat, Zn2TiO4. Dilatometrija kao osnovna tehnika za praćenje procesa sinterovanja je pokazala da pri grejanju na višoj temperaturi dolazi do faznog prelaza. Struktura sinterovanih uzoraka okarakterisana je takođe metodom atomske mikroskopije i uočena razlika u veličini zrna za različite režime grejanja.Zinc meta-titanate ZnTiO3 nano powder commercially available with particle size in a range from 40 nm to 80 nm was treated by conventional synthesis methods, namely, the compaction and the sintering. According to instability of the powder, response of the system to these two fundamental treatments was studied. Regarding used methods, changes in properties of the compacts and sintered specimens occurred. Uniaxial double sided compaction method produced set of green bodies obtained using different pressures. Employed characterization methods used were: density measurements, specific surface area measurements by nitrogen adsorption on low temperatures, porosity measurements with mercury porosimetry. For the compressibility phenomenon different compaction equations were used in order to mathematically describe compaction. Compacts obtained with 98 MPa pressure were determined as suitable for consequent sintering process. X-ray diffraction was conducted on green body of the commercial ZnTiO3 powder. Also, compact was characterized with atomic force microscopy. Sintering process was characterized with fundamental technique for following a linear change in dimensions, namely high temperature dilatometry. Experiments were conducted in two different isothermal temperatures: lower at 880°C and higher at 1200°C. For the lower temperature region of 880°C activation energy calculation was performed. Dilatometry as main technique for following of the sintering phenomenon registered at higher temperature region of 1200oC, phase transition to stabile zinc-orthotitanate spinel Zn2TiO4. Phase transition was justified with X-ray diffraction of specimens heated on 880°C showed characteristic perovskite ZnTiO3 structure diffractogram, while specimen heated at 1200°C gave spinel Zn2TiO4 characteristic diffraction patterns. Sintered specimens in two regimes were characterized with atomic force microscopy also, and significant difference between grain sizes was detected
The Influence of Mechanical Activation on the Synthesis of Zinc Metatitanate
Get PDFU ovom radu analiziran je uticaj mehaničke aktivacije na sintezu cink-meta titanata (ZnTiO3). Polazeći od toga da mehanička aktivacija utiče na strukturu praha, menjajući je za naknadnu sintezu procesom sinterovanja, mi smo definisali suštinsku zavisnost između strukture i sinteze. \ud
Vršena je aktivacija smeše prahova ZnO i TiO2 u ekvimolarnom odnosu u toku 0 do 160 minuta. Metodoma karakterizacije specifične površine, skanirajuće elektronske mikroskopije i rendgenske difrakcije određene su strukturne promene koje se odigravaju tokom mehaničke aktivacije. Proučen je i uticaj mehaničke aktivacije na kompresibilnost prahova. Uticaj mehaničke aktivacije na sinterovanje praćen je u izotermskom režimu rendgenskom difrakcijom i neizotermskom režimu, dilatometrijom i skanirajućom elaktronskom mikroskopijom. \ud
Proučene su karakteristike, mehanizmi formiranja i evolucija mikrostrukture tokom procesa aktivacije, kao i fazni sastav i kinetika mehanohemijske reakcije. Proces sinterovanja uslovljen je svojstvima prahova dobijenih mehaničkom aktivacijom i sa tog stanovišta svojstva su posmtrana u funkciji vremena mlevenja. Posmatran je razvoj faza na karakterističnoj temperaturi nastajanja cink-metatitanata (ZnTiO3) u funkciji vremena aktivacije. Dilatometrijskom analizom praćen je uticaj mehaničke aktivacije na energiju aktivacije procesa sinterovanja pri neizotermskom sinterovanju. \ud
Detaljno proučavanje kinetike i mehanizma mehaničke aktivacije smeše praha ZnO i TiO2 pokazuje da se proces aktivacije odigrava u tri stadijuma. Prvi karakteriše smanjenje veličine čestica, u drugom je dominantan proces sekundarne aglomeracije ili slepljivanje sitnih čestica, dok se u trećem razbijaju prethodno nastali skupovi čestica novih faza. \ud
Iz vrednosti specifične površine i rezultata rendgenostrukturne analize za karakteristična vremena mehaničkog tretmana utvrđeno je da proces mehaničke aktivacije u smislu unošenja defekata i smanjenja veličine čestica traje do 40 minuta za posmatrane parametre procesa mlevenja. Takođe, ovim rezultatima dokazana je kao moguća sinteza titanatnih jedinjenja mehanohemijskom rekacijom. Pokazano je, naime, da faze cink-ortotitanata i cink-metatitanata pri mehanohemijskoj sintezi nastaju odvojeno. Pri tom je ustanovljeno takođe da faza cink-ortotitanata (Zn2TiO4) nastaje prva i to u 80. minutu mehaničkog tretmana, da bi nakon toga u 160 minutu došlo do pojave faze cink-metatitanata (ZnTiO3) i smanjenja sadržaja ortotitanatne faze. Skanirajuća elektronska mikroskopija otkrila je da se u procesu mlevenja faze cink-oksida i titan-dioksida za mala vremena aktivacije ponašaju različito i da se prilikom mehaničke aktivacije veličina čestica i aglomeracija menjaju. \ud
Uticaj ovih veličina ogleda se u kompresibilnosti prahova. Kompresibilnost je posmatrana u odnosu na prah koji nije mehanički aktiviran i posmatrana je promena na početku i na kraju intervala vremena u kome je vršena mehanička aktivacija. Predložena su dva mehanizma odgovorna za povećanje gustine u posmatranom intervalu pritisaka. Proces povećanja gustine ispresaka usled rearanžiranja čestica i proces povećanja gustine pod većim pritiscima usled deformacije čestica. Na osnovu ove analize odlučeno je da se prahovi za izotermsko sintrovanje presuju na nižim pritscima, a da za neizotermsko sinterovanje presovanje bude na višim prtiscima u odnosu na posmatrani interval. Uticaj mehaničke aktivacije na promenu gustine doveo nas je do zaključka da promena oblika i veličine čestica kao i tekstura imaju dominantnu ulogu na proces kompaktiranja. Promena gustine govori o tome da se, za posmatrane uslove mlevenja, ovi proces odvijaju u najranijem periodu mlevenja i to do 20 minuta. \ud
U izotermskim uslovima grejanja na karakterističnoj temperaturi nastajanja cink-metatitanata posmatran je uticaj mehaničke aktivacije na proces reakcionog sinterovanja. Utvrđen je fazni sastav u kome težinski procenat titanatnih faza intenzivno raste i vreme mehaničke aktivacije, za date uslove mlevenja, nalazi se između 20-og i 40-og minuta mlevenja. Vremena unutar ovog intervala predstavljaju optimalna vremena aktivacije mlevenjem. To su vreme i uslovi mlevenja za koje se polazni prah ZnO i TiO2-rutil (1:1) maksimalno aktivira unošenjem defekata, a da pri tom ne dolazi do formiranja početnih klica titanatnih faza prilikom mlevenja. Određeni su, dakle, parametri mlevenja i grejanja i na taj način optimizovan je odnos mehaničke aktivacije i reakcionog siterovanja za sintezu faza produkata titanatnih jedinjenja. \ud
Proces neizotermskog sinterovanja je pokazao da je energija aktivacije sinterovanja, izračunata Dornovim metodom, najveća za prah aktiviran 10 minuta. Analiza dilatometrisjkih podataka u neizotermskom režimu pokazala je jasnu odvojenost krivih perioda mehaničke aktivacije i mehanohemijske reakcije pri naknadnom neizotermskom sinterovanju. Temperatura početka sinterovanja sve do vremena aktivacije od 40 minuta opada, da bi, usled nemogućnosti difuzije za prahove aktivarane 80 i 160 minuta ponovo porasla. Pri tome za prahove aktivirane 80 i 160 minuta krive brzine dilatacije ukazuju da je u pitanju reakciono sinterovanje, dok je kod prahova aktiviranih do 40 minuta reakcija prethodila sinterovanju, pa je u pitanju bilo sinterovanje reakcionog produkta. Takođe, ustanovljeno je da je pri neizotermskom sinterovanju skupljanje prahova aktiviranih 80 i 160 minuta najmanje u odnosu na neaktivarani prah i prahove aktivirane kraće vreme. \ud
U ovom radu pokušano je, na osnovu rezultata analize strukture i njenih promena u dva izrazito složena procesa sinteze kao što su mehanička aktivacija i sinterovanje, bliže upoznavanje sa svojstvima sistema ZnO-TiO2. Pri tome, posebna pažnja poklonjena je fazi cink-metatitanata (ZnTiO3), koja, po našim saznanjima, nije proučavana u ovakvim sintetskim uslovima
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
A phenomenological analysis of sintering kinetics of alumina
No full textIn this paper a phenomenological analysis of the sintering kinetics of alumina powder compacts with different proportions of small and large particles has been made. A phenomenological approach enables definition of functional connections between parameters characteristic of a certain material and the sintering time. A phenomenological equation is defined, which can be used to describe the densification process of alumina during isothermal sintering. Its parameters enable identification of the dominant diffusion mechanism
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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