Understanding the nature of "superhard graphite"

Numerous experiments showed that on cold compression graphite transforms into a new superhard and transparent allotrope. Several structures with different topologies have been proposed for this phase. While experimental data are consistent with these models, the only way to solve this puzzle is to find which structure is kinetically easiest to form. Using state-of-the-art molecular-dynamics transition path sampling simulations, we investigate kinetic pathways of the pressure-induced transformation of graphite to various superhard candidate structures. Unlike hitherto applied methods for elucidating nature of superhard graphite, transition path sampling realistically models nucleation events necessary for physically meaningful transformation kinetics. We demonstrate that nucleation mechanism and kinetics lead to $M$-carbon as the final product. $W$-carbon, initially competitor to $M$-carbon, is ruled out by phase growth. Bct-C$_4$ structure is not expected to be produced by cold compression due to less probable nucleation and higher barrier of formation

Large amplitude fluxional behaviour of elemental calcium under high pressure

Experimental evidences are presented showing unusually large and highly anisotropic vibrations in the “simple cubic” (SC) unit cell adopted by calcium over a broad pressure ranging from 30–90 GPa and at temperature as low as 40 K. X-ray diffraction patterns show a preferential broadening of the (110) Bragg reflection indicating that the atomic displacements are not isotropic but restricted to the [110] plane. The unusual observation can be rationalized invoking a simple chemical perspective. As the result of pressure-induced s → d transition, Ca atoms situated in the octahedral environment of the simple cubic structure are subjected to Jahn-Teller distortions. First-principles molecular dynamics calculations confirm this suggestion and show that the distortion is of dynamical nature as the cubic unit cell undergoes large amplitude tetragonal fluctuations. The present results show that, even under extreme compression, the atomic configuration is highly fluxional as it constantly changes

Na12Ge17: A compound with the zintl anions [Ge-4](4-) and [Ge-9](4-) - Synthesis, crystal structure, and Raman spectrum

Na12Ge17 is prepared from the elements at 1025 K in sealed niobium ampoules. The crystal structure reinvestigation reveals a doubling of the unit cell (space group:P2(1)/c; a = 22.117(3) Angstrom, b = 12.803(3) Angstrom, c = 41.557(6) Angstrom, beta = 91.31(2)degrees, Z = 16; Pearson code: mP464), furthermore, weak superstructure reflections indicate an even larger C-centred monoclinic cell. The characteristic structural units are the isolated cluster anions [Ge-9](4-) and [Ge-4](4-) in ratio 1:2, respectively The crystal structure represents a hierarchical cluster replacement structure of the hexagonal Laves phase MgZn2 in which the Mg and Zn atoms are replaced by the Ge-9 and Ge-4 units, respectively The Raman spectrum of Na12Ge17 exhibits the characteristic breathing modes of the constituent cluster anions at v = 274 cm(-1) ([Ge-9](4-)) and v = 222 cm(-1) ([Ge-4](4-)) which may be used for identification of these clusters in solid phases and in solutions. Raman spectra further prove that Na12Ge17 is partial soluble both in ethylenediamine and liquid ammonia. The solution and the solid extract contain solely [Ge-9](4-). The remaining insoluble residue is Na4Ge4. By heating the solvate Na4Ge9(NH3)(n) releases NH3 and decomposes irreversibly at 742 K, yielding Na12Ge17 and Ge

Solid State Chemistry of Clathrate Phases: Crystal Structure, Chemical Bonding and Preparation Routes

Clathrates represent a family of inorganic materials called cage compounds. The key feature of their crystal structures is a three-dimensional (host) framework bearing large cavities (cages) with 20-28 vertices. These polyhedral cages bear-as a rule-guest species. Depending on the formal charge of the framework, clathrates are grouped in anionic, cationic and neutral. While the bonding in the framework is of (polar) covalent nature, the guest-host interaction can be ionic, covalent or even van-der Waals, depending on the chemical composition of the clathrates. The chemical composition and structural features of the cationic clathrates can be described by the enhanced Zintl concept, whereas the composition of the anionic clathrates deviates often from the Zintl counts, indicating additional atomic interactions in comparison with the ionic-covalent Zintl model. These interactions can be visualized and studied by applying modern quantum chemical approaches such as electron localizability

The Process Development and Characterization of p-type BaGaSn Thermoelectric Material

　　本實驗是分別以助熔劑法、溫度梯度法及粉末冶金法進行P型Ba8Ga16Sn30　Type -VIII晶籠化合物的製備，並探討在不同製程參數，如原始成份、溫度梯度、燒結溫度及燒結時間下材料組成、結構及熱電特性表現上的差異。藉由這些分析，以尋找出具有最好熱電表現的Ba8Ga16Sn30熱電材料的製程條件。 　　在實驗結果部分，以助熔劑法製備Ba8Ga16Sn30晶籠化合物，過程中會有多核成長的問題，以致於無法成功製備出單一Ba8Ga16Sn30晶體。而以溫度梯度法，雖可克服多核成長的影響而製備出成份穩定且尺寸較大的Ba8Ga16Sn30晶體，但樣本具有多相的樹枝狀結構，推測此結構應是組成過冷所造成的。最後在粉末冶金法中，分別嘗試在不同的燒結溫度、燒結時間與成分比例下製備Ba8Ga16Sn30晶籠化合物。在燒結溫度為450℃、燒結時間50小時、原始原子比例Ba：Ga：Sn＝8：16：30時，順利製備Ba8Ga16Sn30晶籠化合物。其化合物成分比例相當接近目標的Ba8Ga16Sn30熱電材料。當環境溫度在350℃時，材料具有最佳Seebeck係數為53.4 V/K，此時電導率為507 S/cm，熱導率為0.22 W/mK，並藉由公式計算出最佳ZT值為0.27。In this study,　the methods of self-flux, temperature gradient, and powder metallurgy were used to prepare P-type Ba8Ga16Sn30 type-VIII clathrate compounds, and the influences of various process parameters like the raw composition, temperature gradient, sintering temperature and sintering time, on the sample composition, structure and the thermo- electric properties were investigated. From these investigations, one can obtain the process condition of the optimal thermoelectric performance of Ba8Ga16Sn30 thermoelectric material. According to the results, due to the multi-nucleation of Ba8Ga16Sn30 clathrate compounds in the Ga-flux, a whole crystal of Ba8Ga16Sn30 compounds can’t be successfully prepared by means of the self-flux method. Although the temperature gradient method can inhibit the phenomenon of multi-nucleation and a bigger Ba8Ga16Sn30 clathrate cry- stal with stable composition can be prepared, a dendritic structure with eutectic phase occurs. It is suggested that the structure is evolved by constitutional undercooling. Finally, by powder metallurgy method, Ba8Ga16Sn30 clathrate compounds were successfully prepared at the sintering temperature 450oC, sintering time 50 hours, and raw com- position Ba:Ga:Sn = 8:16:30. At 350oC the sample possesses the optimal Seebeck coeifficient 53.4 V/K, while the electrical conductivity is 507 S/cm, the thermal conductivity is 0.22 W/mK, and the optimal figure of merit is 0.27.目錄 中文摘要…………………………………………………………….……i Abstract…………………………………………..……….……..……….ii 目錄………………………………………………………….…..……...iii 表目錄….………………………………………………….…….…...vii 圖目錄……………………………………………………….……..…viii 第一章 緒論 …………………………………...….………….……….1 1.1 　前言………………………………………………….……….1 1.2 　熱電材料的發展歷史…………………………….….………2 1.3 　熱電材料的應用………………………………….….………3 1.4 　研究動機及目的………………………………….….………5 第二章 理論基礎與文獻回顧 …………………………….….………8 2.1 　熱電特性簡介…………………………………….….………8 2.1.1 Seebeck效應……………………………….…..………8 2.1.2 電導率 ……………………………………….….…...10 2.1.3 功率因子 …………………………………….….…...10 2.1.4 熱導率 ……………………………………………….11 2.1.5 熱電優值 …………………………………….………12 2.2 　熱電材料…………………………………………..………..13 2.2.1 　晶籠化合物 …………………………………………...14 2.2.2 　BaGaSn晶籠化合物 …………………………….……15 2.3 　製程理論 …………………………………………………..16 2.3.1 　助熔劑法……………………………………………….16 2.3.2 　溫度梯度法………………………………….…………17 2.3.3 　粉末冶金法………………………………….…………19 2.3.4 　製程方式選擇………………………………………….21 2.4 　文獻回顧 …………………………………………..………22 第三章 實驗方法 ……………………………………………………25 3.1 　實驗流程 …………………………………………………...25 3.2 　實驗材料…………………………………….………………27 3.3 　實驗製程………………………………………………….…27 3.3.1 　真空石英封管…………………………………..………27 3.3.2 　助熔劑法……………………………………..…………29 3.3.3 　溫度梯度法………………………………………..……30 3.3.4 　粉末冶金法…………………………………….….……32 3.4 　性質分析與量測……………………………..……………...34 3.4.1 　DTA熱差分析………………………………………….34 3.4.2 　X-ray繞射結構分析……………………………………35 3.4.3 　FE-SEM顯微結構分析………………………………...36 3.4.4 　成分分析……………………………………………..…36 3.4.5 　熱電特性分析…………………………………………..37 A.　Seebeck量測………..…………………………………38 B.　電導率量測 ………..…………………………………40 C. 熱導率量測……………………………………………41 第四章 結果與討論…………………………………………………..45 4.1 　使用助熔劑法製備Ba8Ga16Sn30並在不同成分配比的比較45 4.1.1 顯微結構分析……………………………………..…45 4.1.2 成分分析……………………………………..………48 4.2 　使用溫度梯度法製備Ba8Ga16Sn30在不同溫度梯度的比較49 4.2.1 顯微結構分析………………………………………..49 4.2.2 成分分析……………………………………………..53 4.2.3 結晶結構分析………………………………..………54 4.3 　使用粉末冶金法製備Ba8Ga16Sn30在不同燒結溫度的比較56 4.3.1 DTA熱差分析………………………………………..56 4.3.2 顯微結構分析………………………………………...57 4.3.3 成分分析……………………………………..……….62 4.3.4 結晶結構分析………………………………………...64 4.4 　使用粉末冶金法製備Ba8Ga16Sn30在不同燒結時間的比較66 4.4.1 顯微結構分析…………………………..……………66 4.4.2 成分分析……………………………………………..68 4.4.3 結晶結構分析…………………………..……………69 4.4.4 熱電特性分析……………..…………………………70 4.5 　使用粉末冶金法製備Ba8Ga16Sn30在不同成分配比的比較74 4.5.1 顯微結構分析……………………………..…………74 4.5.2 成分分析…………………………………..…………76 4.5.3 結晶結構分析……………………………..…………76 4.5.4 熱電特性分析……………………………..…………77 4.5.5 再現性分析………………………………..…………80 第五章 結論………………………………………………..…………90 第六章 參考文獻…………………………………………..…………91

The early development of inorganic chlathrates

In this chapter the authors relate the discovery of the first inorganic clathrates, Na8Si46 and NaxSi136 (3 ≤ x ≤ 11), whose cage-like structures were determined by comparison with those of the two most classical gas and liquid clathrate hydrates. The main characteristics of clathrate compounds are recalled and a brief review of clathrate hydrates is given. The different polyhedral cages and their arrangements in the so-called type I structure (Na8Si46) and type II structure (NaxSi136) are described in details. The synthesis, composition and structure of other inorganic clathrates of silicon, germanium and tin with potassium, rubidium and cesium as guest atoms are reported. The crystal structure (type I or type II) and corresponding composition is closely related to the size of the guest alkali atoms. The formation of the characteristic polyhedral cages with a majority of pentagonal faces is discussed, and results from the arrangement of all the tetrahedrons in eclipsed position. The relation between clathrate structures and those of clathrasils (silica-based clathrates), Frank-Kasper alloys and fullerene forms of carbon is also discussed. The first measurements of the physical properties of inorganic clathrates are reviewed, including electrical conductivity, thermal properties, high pressure behavior, NMR and ESR investigations. The ability for the silicon, germanium and tin host lattices to form non-stoichiometric and mixed frameworks with elements of neighboring groups is briefly described, giving rise to a large variety of new inorganic clathrates with ionic guest-host interactions and semiconducting properties