1 |
An Ising-like model to predict dielectric properties of the relaxor ferroelectric solid solution BaTiO₃ − Bi(Zn₁/₂Ti₁/₂)O₃ / An Ising-like model to predict dielectric properties of the relaxor ferroelectric solid solution BaTiO3 - Bi(Zn1/2Ti1/2)O3Jackson, Dennis L. 01 December 2011 (has links)
We developed a model to investigate the dielectric properties of the
BaTiO₃ − Bi(Zn₁/₂Ti₁/₂)O₃ (BT-BZT) solid solution, which is a relaxor ferroelectric and exhibits long range disorder. The model uses ab initio methods
to determine all polarization states for every atomic configuration of 2 x 2 x 2
supercells of BT-BZT. Each supercell is placed on a lattice with an Ising-like
interaction between neighboring cell polarizations. This method allows us to
consider long range disorder, which is not possible with ab initio methods
alone, and is required to properly understand relaxor ferroelectric materials.
We analyze the Monte Carlo data for a single lattice configuration using the
multiple histogram method, and develop a modified histogram technique to
combine data from multiple lattice configurations. Our calculated values of
dielectric constant, specific heat, and polarization agree reasonably well with
experiment. / Graduation date: 2012
|
2 |
An?lise de antenas utilizando substrato cer?mico, zpt, produzido por s?ntese auto propagante para aplica??es em sistemas de micro-ondasMartins, Jo?mia Leilane Gomes de Medeiros 07 June 2013 (has links)
Made available in DSpace on 2014-12-17T14:55:13Z (GMT). No. of bitstreams: 1
JoemiaLGMM_TESE.pdf: 3140610 bytes, checksum: 8bdf88d29c888d95e74fd24e403e60fc (MD5)
Previous issue date: 2013-06-07 / Coordena??o de Aperfei?oamento de Pessoal de N?vel Superior / Ceramic substrates have been investigated by researchers around the world and
has achieved a high interest in the scientific community, because they had high
dielectric constants and excellent performance in the structures employed.
Such ceramics result in miniaturized structures with dimensions well reduced and high
radiation efficiency.
In this work, we have used a new ceramic material called lead zinc titanate in the
form of Zn0,8Pb0,2TiO3, capable of being used as a dielectric substrate in the construction
of various structures of antennas.
The method used in constructing the ceramic combustion synthesis was Self-
Sustained High Temperature (SHS - "Self-Propagating High-Temperature Synthesis")
which is defined as a process that uses highly exothermic reactions to produce various
materials. Once initiated the reaction area in the reaction mixture, the heat generated is
sufficient to become self-sustaining combustion in the form of a wave that propagates
converting the reaction mixture into the product of interest.
Were analyzed aspects of the formation of the composite Zn0,8Pb0,2TiO3 by SHS
powders and characterized. The analysis consisted of determining the parameters of the
reaction for the formation of the composite, as the ignition temperature and reaction
mechanisms.
The production of composite Zn0,8Pb0,2TiO3 by SHS performed in the laboratory,
was the result of a total control of combustion temperature and after obtaining the
powder began the development of ceramics. The product was obtained in the form of
regular, alternating layers of porous ceramics and was obtained by uniaxial pressing.
10
The product was characterized by analysis of dilatometry, X-ray diffraction
analysis and scanning electron microscopy.
One of the contributions typically defined in this work is the development of a
new dielectric material, nevertheless presented previously in the literature. Therefore,
the structures of the antennas presented in this work consisted of new dielectric
ceramics based Zn0,8Pb0,2TiO3 usually used as dielectric substrate. The materials
produced were characterized in the microwave range. These are dielectrics with high
relative permittivity and low loss tangent. The Ansoft HFSS, commercial program
employee, using the finite element method, and was used for analysis of antennas
studied in this work / Substratos cer?micos t?m sido investigados por pesquisadores de todo o mundo
e tem despertado um grande interesse na comunidade cient?fica, pelo fato de
apresentarem altas constantes diel?tricas e um excelente desempenho nas estruturas
empregadas.
Tais cer?micas resultam em estruturas miniaturizadas, com dimens?es bem
reduzidas e de alta efici?ncia de radia??o.
Neste trabalho, utilizou-se um novo material cer?mico, chamado de titanato de
zinco chumbo, na forma de Zn0,8Pb0,2TiO3, capaz de ser utilizado como substrato
diel?trico na constru??o de diversas estruturas de antenas.
O m?todo utilizado na constru??o da cer?mica foi a S?ntese por Combust?o
Auto-Sustentada a Alta Temperatura (SHS Self-Propagating High-Temperature
Synthesis ) que ? definido como um processo que utiliza rea??es altamente exot?rmicas
para produzir os mais diversos materiais. Uma vez iniciada a rea??o numa ?rea da
mistura reagente, o calor gerado ? suficiente para tornar a combust?o auto-sustent?vel,
na forma de uma onda que se propaga convertendo a mistura reagente no produto de
interesse.
Foram analisados os aspectos da forma??o do comp?sito Zn0,8Pb0,2TiO3 por SHS
e os p?s obtidos, foram caracterizados. A an?lise consistiu na determina??o dos
par?metros da rea??o para a forma??o do comp?sito, como a temperatura de igni??o e
os mecanismos de rea??o.
A produ??o do comp?sito Zn0,8Pb0,2TiO3 por SHS, realizada em laborat?rio, foi
resultado de um controle total da temperatura de combust?o e ap?s a obten??o do p?
8
iniciou-se o desenvolvimento das cer?micas. O produto foi obtido na forma de camadas
regulares e altern?ncia de porosidades e as cer?micas foram obtidas por prensagem
uniaxial.
O produto foi caracterizado atrav?s de an?lises de dilatometria, difra??o de
raios-X e an?lise de microscopia de varredura eletr?nica.
Uma das contribui??es principais deste trabalho foi o desenvolvimento de um
novo material diel?trico, n?o obstante apresentado anteriormente na literatura. Portanto,
as estruturas das antenas apresentadas neste trabalho foram constitu?das por novas
cer?micas diel?tricas ? base de Zn0,8Pb0,2TiO3, geralmente utilizado como substrato
diel?trico. Os materiais desenvolvidos foram caracterizados na faixa de micro-ondas.
S?o substratos diel?tricos de alta permissividade relativa e baixa tangente de perda. O
software comercial empregado, Ansoft HFSS, baseado no m?todo dos elementos finitos,
foi utilizado na an?lise das antenas estudadas neste trabalho
|
3 |
Síntese e caracterização de materiais do sistema ternário SiO2-ZnO-TiO2 obtido por sol-gel/Pechini / Synthesis and characterization of materials from SiO2-ZnOTiO2 ternary system obtained by sol-gel/PechiniFógia, Michelly Patrícia Santana de Almeida 24 September 2014 (has links)
Submitted by Luciana Ferreira (lucgeral@gmail.com) on 2015-03-30T13:38:33Z
No. of bitstreams: 3
1_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (1).pdf: 19290836 bytes, checksum: 2968ddb5cb0f3df6082b9b4716172cff (MD5)
109_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (2).pdf: 5395221 bytes, checksum: 4db4ad13c96adee2667748bc5321c48f (MD5)
license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Approved for entry into archive by Luciana Ferreira (lucgeral@gmail.com) on 2015-03-30T15:36:48Z (GMT) No. of bitstreams: 3
1_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (1).pdf: 19290836 bytes, checksum: 2968ddb5cb0f3df6082b9b4716172cff (MD5)
109_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (2).pdf: 5395221 bytes, checksum: 4db4ad13c96adee2667748bc5321c48f (MD5)
license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5) / Made available in DSpace on 2015-03-30T15:36:48Z (GMT). No. of bitstreams: 3
1_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (1).pdf: 19290836 bytes, checksum: 2968ddb5cb0f3df6082b9b4716172cff (MD5)
109_pdfsam_Tese - Michelly Patrícia Santana de Almeida Fógia - 2014 - (2).pdf: 5395221 bytes, checksum: 4db4ad13c96adee2667748bc5321c48f (MD5)
license_rdf: 23148 bytes, checksum: 9da0b6dfac957114c6a7714714b86306 (MD5)
Previous issue date: 2014-09-24 / In this work the structural and optical properties of SiO2-ZnO-TiO2 system
doped with of Eu3+ (1% in mol) or Er3+ ions (0,2% to 2,0% in mol) were
investigated. The compositions xSiO2-([100-x]/2)ZnO-([100-x]/2)TiO2 with x
ranging from 40 to 80 were synthesized and studied. A mixed methodology
using the traditional sol-gel process and Pechini method was employed to
prepare stable and transparent sols, homogeneous gels, powders and
transparent thin films without cracks. The prepared powders were heat treated
from 100°C to 1100°C. The optical “band gap” values, the lifetime of Eu3+ 5D0
level, the R/O ratio of Eu3+ emissions and refractive index are dependent on the
composition and thermal treatment of the powders. Multilayered thin films were
deposited onto silica substrates using the “spin coating” technique. Afterwards,
the as-prepared thin films were calcined at 500°C followed by annealings from
700°C to 1000°C. The X-ray diffraction of powders showed that the
crystallization starts at 800°C with a mixture of zinc titanate phases (ZnTiO3,
Zn2TiO4) and titanium rutile (TiO2). In compositions with low silica content (x =
40 or 50) the formation of zinc silicate (Zn2SiO4) occurs at high temperatures.
The optical properties were evaluated for both powders and thin films.
Particularly, thin films exhibit transmittance above 80% in the visible region. The
refractive index at 632.8 nm and thickness of thin films were estimated using
the envelope method and m-Line spectrocopy. Furthermore, morphological
analysis of the thin films surface was performed by scanning electron
microscopy. The characteristic emission peaks of Er3+ or Eu3+ doped powders
and thin films were measured. The photoluminescent emission of Eu3+ ions,
obtained the highest intensities for the powders treated at 800°C, for films
treated at 700°C, under excitation at 394 nm, where the system remains
primarily amorphous. The photoluminescent emission of the Er3+ ions was more
intense for the powders treated at 1100°C under excitation at 980 nm, mainly
due to reduction of hydroxyl groups which act as photoluminescence
quenchers. In the film the highest intensity occurred at a temperature of 700°C
under excitation at 378 nm and showing only one guided mode. Thus, the
obtained thin films can be suitable for application as waveguides in integrated
optical systems. Moreover, the Eu and Er doped powders can be used as
phosphors in displays. / Este trabalho avalia as propriedades estruturais e ópticas do sistema ternário
SiO2-ZnO-TiO2 dopado com íons Eu3+ (1% em mol) ou Er3+ (0,2% a 2,0% em
mol). As composições xSiO2-([100-x]/2)ZnO-([100-x]/2)TiO2, com x = 40, 50, 60,
70 e 80 foram estudadas. Através de metodologia mista do processo sol-gel
com o método Pechini foi possível obter sóis estáveis e transparentes, géis
homogêneos, pós tratados termicamente de 100ºC a 1100°C e filmes finos
transparentes e sem trincas. Filmes finos multicamadas foram depositados
sobre substratos de sílica usando “spin coating” e calcinados a 500°C, seguido
de um tratamento térmico entre 700°C e 1000°C. Por fotoluminescência foram
observadas as transições de emissão características de íons Er3+ e Eu3+. A
caracterização por difração de raios X dos pós mostrou que a cristalização tem
início a partir de 800°C, com uma mistura de fases de titanatos de zinco
(ZnTiO3, Zn2TiO4) e segregação de titânio rutilo (TiO2). Em composições com
menores proporções de sílica (x= 40 ou 50) também ocorreu a formação de
silicato de zinco (Zn2SiO4) em temperaturas mais elevadas. Os espectros de
transmissão dos filmes apresentaram transmitância superior a 80%. Os valores
de “band gap” óptico, o tempo de vida, as razões R/O e o índice de refração
são dependentes do tratamento térmico e da composição dos pós. Usando o
método da envoltória, pôde-se calcular o índice de refração a 632,8 nm e a
espessura dos filmes finos, sendo que esta também foi estimada por
microscopia eletrônica de varredura (MEV) e espectroscopia m-Line. A emissão
fotoluminescente dos íons Eu3+ tem maior intensidade nos pós tratados à
800°C e nos filmes tratados à 700°C, sob excitação a 394 nm. Nestas
temperaturas o sistema permanece predominantemente amorfo. A emissão
fotoluminescente dos íons Er3+ foi mais intensa para os pós tratados a 1100°C,
sob excitação a 980 nm, devido à eliminação de grupos OH supressores das
emissões de Er3+. Nos filmes a maior intensidade se deu à temperatura de
700°C, sob excitação a 378 nm. Os filmes apresentaram apenas um modo
guiado, sendo o mais apropriado para uso como guia de onda e os pós são
indicados como fósforos podendo ser aplicados em “displays".
|
Page generated in 0.0453 seconds