台灣中子束實驗設施興應用實例簡介. Taiwan neutron beam facility and beam applications

Transcription

1 National Central University 台灣中子束實驗設施興應用實例簡介 Taiwan neutron beam facility and beam applications Wen-Hsien Li ( 李文献 ) Center for Neutron Beam Applications ( 中子束應用研究中心 ) National Central University ( 中央大學 ) whli@phy.ncu.edu.tw

2 緣起 Official Arrangement bet. Taiwan & Australia Signed on June 8, 2005 by TECO & ACIO. NSC (1) Construct a high-performance cold TAS within 4 years. (2) 4 scientists to operate the cold TAS. (3) Obtain 70% of one instrument time for NSC related scientists. (~240 days neutron beam time) (4) Instrument time may be distributed over the full set of instruments at OPAL reactor.

3 The location Australian Nuclear Science and Technology Organisation * OPAL reactor Guide Hall Taiwan office

4 The OPAL Reactor Nuclear fission (1938), chain reactions Nuclear reactor, provides a continuous source. n U 36 Ba + 56 Kr + free n ( 平均 2.5 個 ) + betas D 2 O moderator: slowing fission neutrons (~1 MeV) down to ambient thermal energy (ev) to increase the chain reaction cross-section. OPAL: a 20 MW research reactor

5 Neutron flux of OPAL OPAL cold source Thermal source 20 K liquid H 2 cooled cold source Heat exchanger Cold head (liquid H 2 ) Α = λ E ( mev ) 1 Å = 82 mev 2 Å = 20.5 mev 4 Å = 5 mev

6 Neutron instruments at ANSTO TOF DCS 飛逝時間散射儀 HIPD 高強度中子繞射儀 HRPD 高解析度中子繞射儀 Quasi-Laue diffractometer 準勞厄繞射儀 散射儀 Thermal-TAS 熱三軸散射儀 Cold-TAS 冷三軸散射儀 Reflectometer 反射儀 Residual stress diffractometer 殘餘應力繞射儀 SANS 小角度散射儀 ANSTO 2nd phase projects: 3 He polarizer, USANS, 2 nd SANS, Back scattering, Neutron radiography

7 The Sika project 奈米物性實驗室

8 SIKA project Formosan sika deer Triple-axis spectrometer Inelastic scattering, analyzing k-space. Cold TAS : ΔE ~ 0.1 to 1 mev Spin-polarized Inelastic K-space Analyzer Ultra-high energy resolution resonance spin echo technique With RSET : ΔE ~ 1 to 100 μev (not in present project) Spin-echo Inelastic K-space Analyzer Spin-echo cold neutron triple-axis spectrometer

9 SIKA overview Analyzer & detectors Monochromator drum Sample stage

10 SIKA primary spectrometer specification Beam port CG4 Size of beam at reactor face 50 x 200 mm 2 (W x H) Take-off angular range 2Θ M Monochromator crystals Monochromator dimensions 30 o 2Θ M 135 o HOPG, η = 24, double focusing, Si(111) 220 x 340 mm 2 (Width x Height) Useful wavelength range 1.74 Å λ 5.8 Å (PG 002) Correspond. wave-vector range 3.6 Å -1 k i 1.0 Å -1 (PG 002) Correspond. incident energy range 2.4 mev E i 27 mev (PG 002) Virtual source aperture Distances - cold source monochromator - reactor face monochromator - virtual source monochromator - monochromator sample 0 W 65 mm ; H = 220 mm 6715 mm 2300 mm 2100 mm 2100 mm 設置雙向聚焦 PG 及 Si 單色晶體 2.4 mev E i 31 mev with (Si 111) Collimators - α 1 (pre-monochromator) α 2 (post-monochromator) Incident beam filters - Be cooled (10 cm) E i < 5 mev - PG (8 cm) E i = 13.7 & 14.7 mev - Sapphire (8 cm) E i > 15 mev 20, 40, 60, open 20, 40, 60, open Higher order suppression Higher order suppression Fast neutron suppression

11 SIKA monochromator HOPG(002) PG & Si Si(111) 法國 AZ-System 承製 mm 2 HOPG crystals, 雙向聚焦 已抵 ANSTO 瑞士 Swiss Neutronics 承製 mm 3 Si(111) 13 crystals Double focusing

12 SIKA secondary spectrometer 13 analyzers + 1 PSD + 1 single detector 次散射儀 : 能量解析與偵測系統 概念設計已完成 工程設計需再 1 個月 PSD SD Collimator 採購中 整體組裝工程預計 2010 年 3 月進行, 工時預估 3 月 13 組能量解析晶體

13 SIKA-Schedule Task Name SIKA Project Phase PRIMARY Spectrometer Conceptual Design McStas (Performance, Optimization) MCNP Source MCNP Shielding Conceptual Design Drawings Engineering Design Collimator change, Input optics Shielding Procurement Monochromator (PG & Si) Collimators, Input optics Sample Module Shielding Electronic Components Dance Floor SECONDARY Spectrometer Conceptual Design Engineering Design Procurement INSTALLATION Monochromator Shielding Mechanical Parts Electrical Parts IT Systems COMMISSIONING (Cold) OPERATION Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 t

14 奈米物性實驗室 核反應爐源 中子束特性

15 中子源 核反應爐中子源 : 穩態, 能量依 Maxwellian 分佈以 H 2 O/D 2 O/ 石墨降低能量至 mev 範圍 冷中子源 : 爐内增設致冷器 ( 液態氫 : 20 K) 加速器中子源 : 脈衝式, 瞬時通量較高 Average flux 1 MW spallation source 10 MW reactor source

16 中子散射作用力 X 光 - 電子 散射作用力 : 中子 原子核 : 強交互作用力 ( 見到原子核, 散射機率並非隨原子序增加而增大 ) 偵測原子核位置 晶體結構中子自旋 未成對電子自旋 : 磁偶矩交互作用力 偵測未成對電子自旋 磁結構 能量 :mev 範圍 ( 適合探討凝體系統動力現象 ) 超熱中子 : 200 mev, 0.64 Å 熱中子 : 20 mev, 2 Å 冷中子 : 2 mev, 6.4 Å

17 中子束基本特性 中子 : 能量範圍 mev 為電中性具磁耦矩質量遠大於電子 Scattering power: D O H Ca Si 穿透力高 : 除少數元素 ( 如 Gd Cd Sm B) 外, 基本上全然穿透 為一非破壞性探測的工具 能深入系統的內部 目前唯一能探測磁性結構的工具 磁性材料 在重原子間判定輕原子 生化分子裡氫原子的測定 動力組態探討 所具能量與動力激發子相當

18 奈米物性實驗室 常用中子散射儀 種類簡介

19 ANSTO Neutron Zoo Platypus (Reflectometer) Quokka (SANS) Wombat (Hi-Intensity Powder) Koala (Single Crystal) Sika (Cold-Triple Axis) Kowari (Residual Stress) Pelican (Polarization Spectrometer) Echidna (Hi-Resolution Powder) Taipan (Thermal TAS)

20 台灣中子散射實驗設施 高解析度中子繞射儀 (HRPD): 10% 晶體結構分析 核密度分佈 結構相轉變高強度中子繞射儀 (HIPD): 10% 磁相轉變 磁結構 磁相關 自旋密度冷 / 熱中子散射儀 (TAS): 20% 激發子 色散關係 形式因子小角度中子散射儀 (SANS): 10% 生物分子 高分子 外觀型態 vortices 中子反射儀 (NR): 10% 高分子組態 磁性薄膜結構勞厄繞射儀 (QLD): 5% 單晶樣品結構測定 10% = 35 days/year 應力繞射儀 (RSD): 5% 1 day 實驗運作時間 = 24 hr 材料應力殘留測定

21 Neutrons Neutron Powder Diffraction ( 粉末繞射 ) 高解析度粉末繞射譜圖 2θ Neutron counts 中子與原子核之強交互作用 Nuclear scattering Complex crystal structures - Magnetic structures Mn1 - Cation ordering Mn2 - High pressure studies O1 - Phase transitions (vs T, P, H) O2 O3 - Solid state chemical reactions - Location of light anions or oxygen vacancies La 0.7 Ce 0.15 Ca 0.15 MnO 3 P2 1 /c λ= Å Room temp Scattering angle 2θ (deg.) La/Ce/Ca

22 Neutron magnetic diffraction ( 磁繞射 ) 中子磁偶距與未成對電子交互作用 Magnetic scattering 高強度粉末繞射譜圖

23 High Resolution Powder Diffractometer (HRPD) Monochromator shielding Neutron guide 128 Position sensitive detectors SAMPLE AREA GRANITE FLOOR 高強度粉末繞射譜圖 Dr Klaus-Dieter Liss

24 High Intensity Powder Diffractometer (HIPD) Dr Andrew Studer 高強度粉末繞射譜圖 APPLICATIONS Real time: Single shot (kinetic) measurements Real time: Stroboscopic (cyclic, periodic) measurements (reversible) Small sample volumes Magnetic studies at long wavelengths (2.36 Å and 4 Å)

25 Reflectivity and grazing-incident scattering ( 反射 ) Layer thickness Interface roughness Mass density Spin arrangement Interference pattern

26 Time-of-Flight Reflectometer (TOFR) 掠角反射 ( 干涉 ) 譜圖 APPLICATIONS Complex fluids under flow Polymer coatings and interdiffusion Protein adsorption in biological membranes Magnetic multilayers

27 Small angle scattering ( 小角度散射 ) Intensity Size Shape Information may be obtained by neutrons Structure - exact arrangement of atoms. Overall shape - related to their function. Dynamic changes within a molecular structure - correct functioning Wave vector transfer Q(Å -1 )

28 Hydrogen/Deuterium contrast matching 0% Increasing %D 2 O in the solvent 100% H 與 D 對中子的散射方式 非常不同 (b H = fm b D = 6.67 fm) 以 D 取代部分 H 來調配背 景溶液或樣品的散射強度 判定 H 的位置 隱藏部份 分子

29 Small-Angle Neutron Scattering (SANS) Dr Elliot Gilbert APPLICATIONS Large scale structures (1-100 nm) Colloids/emulsions/micelles Nanomagnetic materials Polymers and polymer processing Biomineralisation & biomimetics Biomembrane physics Zeolites & mesoporous materials

30 Inelastic scattering ( 非彈性散射 ) Triple-axis spectrometer Beam shutter Collimator exchanger Filter selector Collimator Monochromator Beam stop Detector Beam aperture Monochromator drum Sample table Collimator Analyser Collimator Analyzer-detector system Beam stop 2nd axis ( 樣品 ) 3rd axis ( 分析晶體 ) 1st axis ( 單色晶體 )

31 Cold Three-Axis-Spectrometer (TAS) Sika Dr. Chun-Ming Wu

32 Thermal Three-Axis-Spectrometer (TAS) Dr Sergey Danilkin APPLICATIONS Inelastic scattering - excitations collective - phonons and magnons diffusive - spin fluctuations localised - crystal-field levels

33 奈米物性實驗室 中子散射 應用實例簡介

34 晶體結構分析 (PrBa 2 Cu 3 O ) 1500 Neutron counts PrBa 2 Cu 3 O λ = Å 15'-20'-7' P 4/m m m χ 2 = R wp = 8.47% R p = 6.66% Scattering angle (deg) O(2) Pr (7) (12) Å (7) (9) Å Cu(2) c b a Rietveld refining analysis General Structure Analysis System O(1) Ba O(4) Cu(1)

35 Nuclear density contours 巨磁阻系統 Bi 0.37 Ca 0.63 Mn 0.96 Cr 0.04 O 2.98 高溫超導系統 PrBa 2 Cu 3 O Cu(1) Center ( 1 / 2 1 / 2 0) Cu(2) a β=89.86º y=b/2 Formation of Mn-trimers b a Lattice distortion Center ( 1 / 2 1 / )

36 Basic physical properties of RBa 2 Cu 3 O 6+x (R123) 電洞庫層隔離層超導層稀土層 R Cu(2) R= 稀土元素 CuO x -BaO-CuO 2 -R-CuO 2 -BaO-CuO x 導電性及磁性均受控於 x RBa 2 Cu 3 O 7 : 98K 超導體 ( 除 R=Pr) R 反鐵磁有序 (T N <1K) O(2) 超導層隔離層 O(1) 電洞庫層 Ba Cu(1) O(4) 但 PrBa 2 Cu 3 O 7 : T C =0K ( 無超導電性 ) T N (Pr)=17K T N (Cu)=270K

37 輕元素含量分析 (PrBa 2 Cu 3 O 6+x ) Intensity (10 3 counts / min) PrBa 2 Cu 3 O 6+y λ = A 15'-20'-7' Scattering angle 2θ (deg.) o 4 組譜圖明顯不一樣 : 源於氧含量不同 PrBa 2 Cu 3 O PrBa 2 Cu 3 O PrBa 2 Cu 3 O PrBa 2 Cu 3 O 7.122

38 晶格對稱性精檢 (La 0.7 Ce 0.15 Ca 0.15 MnO 3 ) Neutron Counts (Arb.Unit) La 0.7 Ce 0.15 Ca 0.15 MnO 3 P2 1 /c a Rp=5.17% Rw=7.20% χ 2 = o b c La 0.7 Ce 0.15 Ca 0.15 MnO 3 Pnma Rp=5.21% b Rw=7.27% χ 2 =1.498 c a (a) (b) Jahn-Teller distortion 龐磁阻系統 Double exchange Jahn-Teller distortion Charge order Charge segregation Charge stripe Magnetic order Scattering angle 2θ (deg.)

39 Neutron counts ( arbitrary unit ) 結構相轉變 (LiMn 2 O 4 ) K 間結構相轉變 10 K 35 K 90 K 150 K 200 K 250 K 300 K Li 0.96 Mn 2 O 4 BT-1 o λ=1.5402a 15'-20'-7' Scattering angle 2θ ( deg. ) Reconfiguration of the band structure, results in a reduction of the carries density. Structural change induced capacity loss. Neutron counts ( arbitrary unit ) 10 K 35 K 90 K 150 K 200 K 250 K 300 K Fddd (9 9 5) Fd3m (3 3 5) Fddd (6 6 6) Fddd (6 18 2) Fddd (18 6 2) - - LiMn 2 O 4 Fd3m (2 2 6) Scattering angle 2θ ( deg. ) Li Li distortion discharge reconfiguration Electronic configuration

40 短程磁有序 (LiMn 2 O 4 ) Net intensity / min Li 0.96 Mn 2 O 4 BT-2 λ = A I 1.4K -I 80K 60'-40'-40'-analyzer {0 1 1/2} {0 1 1} {1 1 0} {1 1 1} I 1.4K -I 80K Magnetic unit cell Cluster structure of Mn 3+ and Mn Scattering angle 2θ ( deg. ) Separate ordering of Mn 3+ and Mn 4+. Two separate magnetic components. Ordering temperatures for the two components: at 90 K & 50 K. Mn 3+ (electron-rich region) 被 Mn 4+ (hole-rich region) 所環繞形成叢集的型態 並非 Mn4 + 和 Mn3 + 交替排列

41 雙層磁有序 (Polyaniline-intercalated FeOCl) 層塞導電高分子材料 Non-collinear spin arrangement Van-der-Waals gap a c Along b: antiparallel 1. Coupled bilayered quasi-2d magnetic scattering profile. 2. No magnetic correlation between the adjacent bilayers. O Fe Fe Magnetic modulation vector K=(1/3 1/2) Direct exchange (DE): Fe Fe Ferromagnetic Superexchange (SE): Mediated through O between the Fe ions Fe O Fe Antiferromagnetic Competition between FMDE & AFMSE: Non-collinear spin arrangement

42 磁有序團簇 (Bi 0.37 Ca 0.63 Mn 0.96 Cr 0.04 O 2.98 ) 高電荷有序溫度龐磁阻材料 Intensity ( 10 3 Counts / min. ) {1 1 0} + {0 1 1/2} Spin-charge coupling {2 0 0} + {1 1 1/2} + {0 0 1} {2 1 0} + {0 1 1} Bi 0.37 Ca 0.63 Mn 0.96 Cr 0.04 O 2.99 I 30 K - I 150 K {0 1 0} {1 1 3/2} + {3 1 1/2} {3 1 0} + {0 1 3/2} Scattering angle 2θ ( deg. ) Calculated pattern {1 1 1/2} b a φ=90 c θ= φ=90 [ 101] b a θ= 45 Mn 3+ (1), 2a Mn 4+ (3), 4f Mn 3+ (2), 2b Mn 4+ (3), 4f Mn 3+ (1), 2a Mn 4+ (3), 4f Mn 3+ (2), 2b Mn 4+ (3), 4f Mn 3+ (1), 2a c Ferromagnetic clusters created by Cr-doping.

43 T N Exchange integral ( arb. units ) Exchange integral calculation c a y=½b O(7) O(8) Mn(1) J O(8) Mn(1) O(7) O(6) J d -39 J b -42 J c -45 Jahn-Teller J (b) a distortion Temperature ( K ) O(5) O(6) Mn(2) O(8) O(6) O(7) O(5) a J d Mn(3) J O(7) b O(5) O(8) O(6) J c Mn(2) O(5) (a) a z=0c Exchange integral ( arb. units ) b b 24 J g J e J 15 f 12 Bi 0.37 Ca 0.63 Mn 0.96 Cr 0.04 O 2.98 Mn(1) Mn(2) J e J f 3 Bi 1 Mn(1) Mn(2) a z=½c Mn(3) 42 Bi 23 J g Mn 4+ (3)-Mn 4+ (3) Mn 3+ (1)-Mn 3+ (1) Mn 3+ (2)-Mn 3+ (2) Jahn-Teller distortion Temperature ( K ) J g Bi (a) Mn(3) Mn(3) Mn(3) Mn-O-Mn superexchange coupling strength (b)

44 磁相變 10 3 Counts / 3 min 量測磁繞射峰隨溫度的變化情形 Bi 0.4 Ca 0.6 Mn 0.95 Cr 0.05 O 3 80 K Bi 0.4 Ca 0.6 Mn 0.95 Cr 0.05 O 3 {1 0 1} {1/2 1/2 1} Temperature ( K ) Counts / 3 min Peak Intensity (10 3 /10 min.) (La 0.52 Ba 0.48 )(Ru 0.51 Mn 0.49 )O 3 (1 0 0) T C (Ru)= 80 K T C (Mn)= 195 K (La 0.52 Ba 0.48 )(Ru 0.51 Mn 0.49 )O Peak Intensity (10 3 /10 min.) Temperature (K) 25 T C (Ru)= 80 K (1 0 0) (1 1 0) Temperature (K) (1 1 0)

45 晶格空缺 (Tb 1-x Na x MnO 3-x ) O content 3-y x = y Structural analysis Closed-ampoule iodometry Na content x 多鐵材料 ( 鐵磁 + 鐵電 ) 氧空缺減弱次鄰近 Mn 離子間的超交換交互作用 b Mn(3) O(3) 5.00 Tb 1-x Na x MnO 3-x O(2) Separation ( A ) S 13 S Na content x Mn(2) S 13 S 24 O(4) O(1) Mn(1) a Mn(4)

46 短程擾動 (Tb 1-x Na x MnO 3-x ) 10 3 counts / min 10 3 counts / min Tb 0.85 Na 0.15 MnO 2.85 T = 3 K Long-range ordering Short-range ordering Tb 0.85 Na 0.15 MnO 2.85 T = 130 K Scattering 2θ ( deg. ) (a) (c) 10 3 counts / min 10 3 counts / min 2.0 Tb0.85 Na 0.15 MnO T = 45 K Short-range ordering Tb 0.85 Na 0.15 MnO Scattering 2θ ( deg. ) (b) (d) Peak Intensity ( 10 3 Counts ) 長程磁有序 long-range ordering Tb 0.85 Na 0.15 MnO 2.85 {1/2 1/2 1/2} short-range ordering T = 170 K Temperature ( K ) 短程磁有序 Formation of magnetic clusters Inter-cluster couplings

47 奈米顆粒自發與誘發磁性 (7 nm Sn) μ p Integrated intensity ( Arb. unit ) nm Sn 3 K Neutron data 300 K Magnetization ( emu/g ) M = 0 外加磁場將具自發磁性的 Sn 奈米顆粒排列整齊 Applied magnetic field H a (T) μ p Diffraction signals: extremely weak Incoherent scattering: degree of disorder H a M 0

48 焊接點殘餘應力 (stress) 焊接處 600 MPa -120 MPa Stress = force per unit area 1 MPa = 10 atm Longitudinal stress component map overlaying the cross section of a weld. ( 鋼板厚度 = 20 mm) Stress accuracy: 10 MPa; Spatial resolution: 2 mm

49 應變分佈 (Strain map) Pipe weld repairs 500 με Strain = Fractional change in dimension or volume The through-wall distribution of the strain from near mid-length to the stop-end of the ling circumferential weld repair in pipe με Map of axial residual elastic strains measured by neutron diffraction.

50 不均勻冷卻殘餘應力 ( 剎車鋼圈 ) 火車意外斷裂焊接點所受應力 New disc brake Used disc brake 謎題 : 使用後形變是源於製程的殘餘應力或正常使用的殘餘應力? 方法 : 比較新與使用後剎車圈的殘餘應力 結論 : 使用後剎車圈的殘餘應力大增 剎車後鋼圈承受非均勻冷卻

51 結構組態 & 受熱分解 ( 鄰苯二酚加氧酵素 ) R g ( Å ) log I o C R g = 290 Å Rg Activity Temperature ( o C ) S,A A D Activity (unit) 5 o C B C C23O(SH1) thermal cycle A D Enlargement of the enzyme size: due to partial unfolding of the tetrameric subunits of the enzyme. B C 52.5 o C 62.5 o C 72.5 o C 80 o C 5 o C

52 磁激發 (HoPd 2 Sn 的晶體場組態 ) Crystal field level scheme of HoPd 2 Sn and isostructure

53 低能量激發 (TlCuCl 3 的 Bose-Einstein 凝聚 ) 冷中子三軸散射儀之ㄧ典範型實驗, 在低溫高磁場環境下藉低能量激發探討 TlCuCl 3 中 triplet states 的 Bose-Einstein 凝聚 (Nature 423, 2003)

54 弱反鐵磁訊號 (La 2-x Sr x CuO 4 ) 在要求極低背影訊號情況下, 以彈性散射探討 La 2-X Sr x CuO 4 中 Cu 2+ 的反鐵磁有序現象, 確定出 Cu 2+ 具磁性並量測出其磁矩 (Nature 415, 2002)

55 分子動力 (molecular dynamics) C 60 的晶格震動 ( 聲子 ) 模 Phonon dispersion CH 3 I 的轉動穿隧位障

56 磁激發 (Magnetic excitation) La 0.85 Ca 0.15 MnO 3 的磁子激發 Spin wave dispersion ZnCr 2 O 4 的磁子激發 Local spin resonance

57 巨型分子動力激發 (Macromolecule dynamics) α-lactalbumin Protein dynamics Polycarbonate Polymer excitations

58 奈米物性實驗室 儀器使用時間 申請 Incoming deadline: November 27, 2009 Review: December, 2009 ~ January, 2010 Program Advisory Committee Meeting: Feb. 2~3y, 2010

59 ANSTO proposal - participants

60 ANSTO proposal - instrument

61 ANSTO proposal - sample

62 ANSTO proposal - science Description of Proposed Research: Scientific background Aim of the proposed experiment Detailed description of experiment Results of preliminary work carried out Reason for choice of requested instrument Indicate how you calculated requested beam time List relevant published literature 800 words or less

63 ANSTO proposal - background

64 ANSTO proposal - method

65 ANSTO proposal - known

66 Reviewer from PLEASE COMMENT ON 1. Quality of science (science interesting, relevance to the field) and rank numerically (100-0 with 100 best): 2. Quality of track record (publications, involvement of students, projects) and rank numerically (100-0 with 100 best): 3. Quality of planned experiment (use of technique adequate, efficient use of samples and sample environment) and rank numerically (100-0 with 100 best): Overall score in words: outstanding, excellent, very good, good, fair, poor (poor is < 50), [outstanding , excellent 89-80, very good 79-70, good 69-60, fair 59-50]

67 總結 1. 中子散射實驗適於整合性研究 2. 中子繞射與散射能提供多一層的資訊 3. 國科會已開始支助中子散射實驗差旅費 4.99 年可掌握自主性高的中子散射實驗時間 5. 國家級中子束應用實驗室需要大家共同參與 6. 可透過合作參與中子散射實驗 需要大家共同參與

68 歡迎使用 國科會為大家提供的 中子散射實驗設施 聯繫中央大學中子束應用研究中心網頁 李文献