首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
1.
Gd5Si1.75Ge1.75Sn0.5的结构、磁相变与磁熵变   总被引:4,自引:0,他引:4  
采用粉末XRD和振动样品磁强计研究了Gd5Si1.75Ge1.75Sn0.5合金的结构、磁相变和低场变化下的磁熵变。磁性测量结果表明,Gd5Si1.75Ge1.75Sn0.5合金的磁化强度在居里温度附近发生突变,具有一级相变的典型特征,室温具有Gd5Si2Ge2型单斜结构;合金低场磁热效应非常明显,1.8T磁场变化下,在其居里温度272K附近的最大磁熵变为16.7J.kg-1.K-1。用成本低廉的Sn取代Gd5Si2Ge2中部分Si和Ge后,Gd5Si1.75Ge1.75Sn0.5在低磁场变化下的磁熵变比金属Gd大得多并略高于Gd5Si2Ge2。  相似文献   

2.
通过X射线衍射和磁性测量等手段对金属间化合物CeFe10.5Si2.5的晶体结构、磁性以及磁熵变进行了研究. 结果表明,经过对铸态样品进行12 h退火所得的金属间化合物CeFe10.5Si2.5晶体为单相立方NaZn13型结构; 在1.5 T外磁场下居里温度TC~206 K附近的最大等温磁熵变为10.7 J·kg-1·K-1,并随着外磁场的增大而迅速增大; 从Arrott曲线中可以看出,在此化合物中没有明显的巡游电子变磁转变特性,但从低磁场下的热磁曲线可知,磁化强度在居里温度处发生陡峭的变化,这应该是该化合物获得大磁熵变的原因.  相似文献   

3.
以La Fe11.6Si1.4合金为研究对象,系统分析了该一级相变材料的居里温度(TC)、磁场诱导磁相变的临界磁场(HC)、磁化率(χ)、磁滞、磁熵变(ΔS)、制冷能力(RCP)等磁性特性。结果表明:温度诱导磁相变的居里温度和磁场诱导磁相变的临界磁场均随磁场呈线性增加,ΔTC和ΔH随磁场和温度的变化率的值分别为4.1 K·T-1和0.2 T·K-1。当合金处于纯铁磁态和顺磁态时熵变磁熵变几乎为零,但磁场诱导的磁相变,会导致某一定温度下合金磁熵变有一个突变。但合金最大熵变并不是随磁场的增加而线性增加,当磁场达到一定值后随磁场增加其值基本没有变化。不同模型计算的制冷能力均随磁场的增加而呈线性增加。在两相共存态中,同一温度下两种不同铁磁的磁化率存在差异,即因磁场诱导的铁磁态相与合金中本身的铁磁态相的磁化率存在差异,且前者小于后者,这种物理现象对深入研究温度诱导和磁场诱导磁相变的差异有一定的参考价值。  相似文献   

4.
通过X射线衍射和磁性测量研究了Tb(Co1-xSnx)2(x=0,0.025,0.050,0.075,0.100)合金的相结构和磁热性能。经分析可知Sn在TbCo2中的替代是有限的,X粉末衍射分析确定TbCo2具有MgCu2结构,其他样品由TbCo2,TbCo3和Tb5Sn3三相组成,TbCo2为主要相。随着Sn成分的增加,杂质相TbCo3和Tb5Sn3的含量增加,所有样品保持第二序磁相变。Sn的替代使磁相变的温度稍微有所提高,样品TbCo2的TC值为230 K,样品Tb(Co0.950Sn0.050)2的TC值为233 K,但Sn的成分继续增加,样品的TC值有所下降。在外加磁场2 T的作用下,样品Tb(Co1-xSnx)2(x=0,0.025,0.050,0.075)最大磁熵变值分别为3.44,2.29,1.64,1.16 J.kg-1.K-1。  相似文献   

5.
钱康  王炳武  王哲明  苏刚  高松 《化学学报》2013,71(7):1022-1028
合成了两例具有局域D4d对称性的单核钆配合物:Gd(THD)3L(THD=2,2,6,6-四甲基庚烷-3,5-二酮,L=Phen(1,10-菲罗琳,1),Bpy(2,2’-联吡啶,2).磁性研究表明其在低温下具有较大的磁热效应,在2.5 K时,0到5 T外磁场下的磁熵变(-ΔSm)分别高达17.0和18.4 J?kg-1?K-1.研究发现,化合物1,2还同时具有明显的场依赖磁弛豫性质.通过分析弛豫时间随外加磁场和温度变化的关系,发现其低场下的弛豫行为可以用共振声子俘获机制来解释,高场下的弛豫行为则可能来源于直接过程.  相似文献   

6.
通过对Dy_3NiSi_2合金样品的X射线粉末衍射谱、等温变磁磁化曲线、等磁场变温磁化曲线分析,主要研究了合金的相结构和晶格参数、磁相变属性和磁热性能。X射线衍射谱表明Dy_3NiSi_2样品几乎为Gd_3NiSi_2型正交结构(空间群:Pnma, No.62)的单相合金,其晶格常数分别为a=1.1242(3) nm,b=0.41073(3) nm,c=1.1223(9) nm。在45~300 K温度范围内, Dy_3NiSi_2合金仅展现出铁磁-顺磁相变, 0.01 T磁场下的dM/dT和0~2 T磁场下的Arrott图表明, Dy_3NiSi_2合金的磁相变为居里温度T_c=54 K的铁磁-顺磁二级磁相变。在0~2 T磁场范围内, Dy_3NiSi_2合金居里温度处的最大磁熵变为3.65 J·(kg·K)~(-1),相对制冷量为167.9 J·kg~(-1)。  相似文献   

7.
对制备的化合物La0.8Ce0.2(Fe1-xCox)11.4Si1.6(x=0.02,0.04,0.06)的相组成、巡游电子变磁转变(IEMT)特性和磁热效应(MCE)进行了研究。粉末X射线衍射结果表明,经1373 K真空退火处理7 d后,化合物La0.8Ce0.2(Fe1-xCox)11.4Si1.6(x=0.02,0.04,0.06)均为单相立方NaZn13型晶体结构。随着Co含量由x=0.02增加到x=0.06,样品的居里温度TC由207 K上升到277 K。在0~1.5 T磁场变化下,x=0.02,0.04,0.06时样品的最大磁熵变|ΔSM(T)|分别为40.17,12.60和7.65 J.kg-1.K-1,可见该化合物有巨大的磁熵变,而且随Co含量的增加最大磁熵变迅速减小。该化合物的巨大磁熵变来源于TC处的一级相变,以及在TC以上由磁场诱导IEMT,但由于Co原子对Fe原子的替代能够抑制变磁转变的发生,因此该系化合物最大磁熵变随Co含量的增加迅速减小。  相似文献   

8.
通过X射线衍射分析和超导量子干涉磁强计(SQUID)磁性测量,研究了Co替代Fe含量对居里温度在室温以上的磁制冷材料La(Fe1-xCox)11.7Al1.3(x=0.072,0.081)磁结构和磁性能的影响。La(Fe1-xCox)11.7Al1.3材料的居里温度随Co的含量增加而增加,La(Fe0.919Co0.081)11.7Al1.3的居里温度为311 K。当外场变化为1.9 T时磁熵变达到3.6 J·kg^-1·K^-1,RCP值为168.6 J·kg^-1,虽然它的磁熵变小于具有巨磁熵变的磁制冷材料,但是它在磁场为1.9 T时的制冷能力与这些材料相当。  相似文献   

9.
用不同的工艺和原料制备了3个名义成分相同的Mn1.2Fe0.8P0.48S i0.52化合物。X射线衍射结果表明,3个化合物均为Fe2P型六角结构(空间群为P-62m),并且存在少量的(Fe,Mn)3S i相。通过磁性测量发现,3个样品的居里温度有所不同,但是都在室温附近(270~290 K)。以Fe2P为原料制备的化合物具有较大的磁熵变,在1.5 T的磁场变化下其最大磁熵变为13.6 J.(kg.K)-1。以行星样品球磨机制备的化合物具有较小的热滞,最小热滞为6.7 K。这些表明不同的制备工艺和原料对化合物的居里温度、热滞和磁熵变都具有一定的影响。同时低成本的原料、简单的制备工艺、较小的热滞和较大的磁熵变使得Mn1.2Fe0.8P0.48S i0.52化合物成为一种理想的室温磁致冷候选材料。  相似文献   

10.
LaFe13-xSix (1.2≤x≤2.2)化合物的磁场诱导熵变与磁场的关系   总被引:1,自引:1,他引:0  
通过真空电弧熔炼、长时间真空热处理的方法获得了单相NaZn13-型LaFe13-xSix(1.2≤x≤2.2)化合物,并测量了它们的磁化强度与磁场和温度的关系,用Maxwell关系式计算出在不同磁场下化合物的熵变ΔS。用Landau的二级相变理论及平均场近似下的标度律,分析拟合了LaFe13-xSix化合物ΔS与H之间的关系,对于具有一级和二级相变的材料,发现均存在ΔS∝H2/3的关系,只是拟合得到的参数不同。采用熵变峰值拟合得到的参数能够反映材料中一级磁性相变的程度,研究磁场诱导的熵变与磁场的关系可以为磁制冷研制提供指导。  相似文献   

11.
In the context of the preparation of camptothecin and luotonin A analogs, the synthesis of some key keto-precursors and their use in Friedländer condensation are described. This paper also focuses on the stability of these keto intermediates and emphasizes the major differences between indolizinones and pyrroloquinazolinones series. Noteworthy is also the report of some original structures isolated as by-products of some experiments.  相似文献   

12.
KMnO4-mediated oxidative CN bond cleavage of tertiary amines producing secondary amine was introduced, which was trapped by electrophiles (acyl chloride and sulfonyl chloride) to form amides and sulfonamides. The reaction could take place at mild condition, tolerating a wide range of function groups and affording products in moderate to excellent yields.  相似文献   

13.
The review contains a concise historical account and information on the most significant researches undertaken by the staff at the A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian Academy of Sciences on the Chemistry of Heterocyclic Compounds. Dedicated to Academician of the Russian Academy of Sciences B. A. Trofimov on his 70th jubilee. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1443–1502, October, 2008.  相似文献   

14.
Different approaches for the synthesis of 1-benzyloxypyrazin-2(1H)-one derivatives from simple amino acids have been investigated. A library of 33 precursors for the preparation of N-hydroxy pyrazinones was obtained in moderate to good yields.  相似文献   

15.
A new and simple synthesis of novel N-protected methyl 5-substituted-4-hydroxypyrrole-3-carboxylates, which exist in equilibrium with their 4-oxo tautomers, has been developed in two steps starting from N-protected α-amino acids. The key intermediates are enaminones, which can also be isolated, characterized, and used for the construction of other functionalized heterocycles, before they spontaneously decompose to pyrrole products. 4-Hydroxypyrroles are prone to partial aerial oxidation but can be efficiently alkylated or reduced to stable polysubstituted pyrrolidine derivatives.  相似文献   

16.
The chemoselectivity in the intramolecular CH insertion of various diazosulfonamides has been experimentally studied. The results reveal that the aliphatic 1,4-, 1,5-, or 1,6-C(sp3)?H insertions of diazosulfonamides are not accessible, while the aromatic 1,5-C(sp2)?H insertion can be realized specifically by adjusting the diazo-adjacent group. In addition, the general chemoselectivities in the intramolecular CH insertions of diazosulfonyl compounds are summarized. Generally, diazosulfones undergo both aromatic 1,5-C(sp2)?H and aliphatic 1,5- and 1,6-C(sp3)?H insertions, while diazosulfonates undergo aliphatic 1,5- and 1,6-C(sp3)?H insertions. However, diazosulfonamides only undergo aromatic 1,5-C(sp2)?H insertion.  相似文献   

17.
The highly regioselective Buchwald–Hartwig amination at C-2 of the cheap and readily accessible reagent, 2,4-dichloropyridine with a range of anilines and heterocyclic amines is described. This new methodology is robust and provides a facile access to 4-chloro-N-phenylpyridin-2-amines on 0.25 mol scale. These intermediates undergo a further Buchwald–Hartwig amination at higher temperature to enable rapid exploration of the chemical space at C-4 and to provide a library of 2,4-bisaminopyridines.  相似文献   

18.
19.
An efficient approach to the synthesis of highly congested di, penta and hexacyclic pyrazoles as well as imidazole fragment containing novel heterocyclic molecule has been developed through a carbanion induced transformation of suitably functionalized 2H-pyran-2-ones, benzo[h]chromene and thiochromeno[4,3-b]pyrans. Due to the presence of fluorescence, we report their prime application metal sensor as off/on switching in ferric ions.  相似文献   

20.
Quercetin, the polyphenolic compound, which has the highest daily intake, is well known for its protective effects against aging diseases and has received a lot of attention for this reason. Both quercetin 3-O-β-d-glucuronide and quercetin 3′-O-β-d-glucuronide are human metabolites, which, together with their regioisomers, are required for biological as well as physical chemistry studies. We present here a novel synthetic route based on the sequential and selective protections of the hydroxyl functions of quercetin allowing selective glycosylation, followed by TEMPO-mediated oxidation to the glucuronide. This methodology enabled us to synthesize the five O-β-d-glucosides and four O-β-d-glucuronides of quercetin, including the major human metabolite, quercetin 3-O-β-d-glucuronide.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号