1-inch Nordenfelt gun pounder gun 68-Pounder Lancaster gun B 4.7 inch Gun Mk I–IV
Canhões de Elétrons - CBPFemecbpf/Canhoes_Pinto.pdfCanhão de Elétrons ZrO/W(100) Schottky Type...
Transcript of Canhões de Elétrons - CBPFemecbpf/Canhoes_Pinto.pdfCanhão de Elétrons ZrO/W(100) Schottky Type...
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Canhões de Elétrons
André Luiz Pinto, DCChefe do Laboratório de
Microscopia Eletrônica do IME
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Jeol
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Canhão de ElétronsZrO/W(100) SchottkyType Electron Gun
Higher brightness, 100 times greater than LaB6 gun
Higher coherency
Higher energy resolution, 0.7 to 0.8eV
Higher stability over a cold FEG
Longer life time 1.5 to 3 years (guarantied time: 5,000 hours)
Jeol
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O que desejamos em um canhão?
Alto brilhoFonte de dimensões reduzidas EstabilidadeEnergia ControlávelCoerência
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Brilho
Definimos o brilho como a densidade de corrente (corrente, ie, por unidade de área) emitido em um ângulo sólido αo. Unidade: A/m2sr
( )22
2 oo
e
di
αππβ
⎟⎠⎞
⎜⎝⎛
=
Jeol
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Energia dos Elétrons
Através do Princípio da Dualidade Onda-Partícula de de Broglie podemos associar o momento da partícula ao seu comprimento de onda
Energia cinética do elétron a partir do seu potencial de aceleração
ph
=λ
eVmvmpvmeV ooo 22
2
==⇒=
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Energia dos Elétrons
Comprimento de onda
Correção relativísticaeVm
h
o2=λ
⎟⎟⎠
⎞⎜⎜⎝
⎛+
=
2212
cmeVeVm
h
oo
λ
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Fontes Termiônicas
Lei de Richardson para a densidade de corrente
kTeATJΦ
−= 2
A- Cte de RichardsonΦ - Função TrabalhoK – Cte de Boltzmann
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Filamento de W
Baixo custo (~ US$ 90)Baixa vida (~ 100 h)Baixo brilho
Williams e Carter
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Filamento de LaB6
Maior custo (~US$ 1-3k)Maior vida (~500 h)Maior brilhoMonocristal <100>Suporte resistivo de grafite ou rênioSujeito a choque térmicoAltamente reativoPodem ter efeito Schottky (ponta com r~1-10µm dobra o brilho)
Williams e Carter
Goldstein et all
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Saturação
Williams e CarterJeol
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Saturação
Williams e CarterJeol
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Canhão de Elétrons - WehneltJeol Williams e Carter
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Controle do Bias
Williams e Carter
Jeol
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Problemas das Fontes Termiônicas
Evaporação do catodo“Thermal drift”Baixo brilho
Brilho máximo
kTeVJ oc
Max πβ =
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Canhão de Emissão por Campo(Field Emission Gun - FEG)
Monocristal <310> com ponta afiada (r<100 ηm)Menor raio concentra o campo elétrico e diminui a necessidade de TV1 – voltagem de extração (3-5 kV)V2 – voltagem de aceleraçãoBrilho máximo
EeVJ oc
Max ∆=
πβ
Williams e Carter
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Canhão de Emissão por Campo(Field Emission Gun - FEG)
3 tipos básicosFrioTérmicoSchottky
CrossoverFrio - < 5 ηmTérmico - < 5 ηmSchottky - < 15-30 ηm
Variação da Energia (∆E)Frio – 0,3 eVTérmico – 1 eVSchottky – 0,3 - 1 eV
EstabilidadeFrio – 5%/hTérmico – 5%/hSchottky – 2%/h
Goldstein et all
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Goldstein et all
Williams e Carter
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Degradação da Fonte
Goldstein et all
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CoerênciaCoerência avalia quão “em fase” estão as ondas de elétronsCoerência temporal
Coerência Espacial α - ângulo de convergência
FEG possui grande coerência temporal e espacial
Evh
c ∆=λ
αλ2
<<cd
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Diâmetro do Feixe
O diâmetro final, dt, é resultado do diâmetro do “crossover” alargado pela aberração esférica da lente objetiva, ds, e a difração pela última abertura, dd.
222dsgt dddd ++=
αβπ12
2
⎟⎟⎠
⎞⎜⎜⎝
⎛=
idg35,0 αss Cd =
αλ22,1=dd
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Referências
Williams, D. B. e Carter, C. B., “Transmission Electron Microscopy” , Ed. Plenum, New York, 1996.Goldstein, J. et all, “Scanning ElectronMicroscopy and X–Ray Analysis”, Ed. Springer, 2003.Apostila da Jeol – METApresentação da Jeol – Jeol 2100F